try 


FORESTRY 


OF 


TEXTBOOK  OF  BOTANY 

FOR 

COLLEGES    AND    UNIVERSITIES 


BY    MEMBERS    OF    THE    BOTANICAL    STAFF    OF    THE 
•UNIVERSITY   OF    CHICAGO 

JOHN    MERLE    COULTER,    PH.D. 

PROFESSOR    OF    PLANT    MORPHOLOGY 

CHARLES    REID    BARNES,   PH.D. 

LATE    PROFESSOR    OF    PLANT    PHYSIOLOGY 

HENRY  CHANDLER    COWLES,  PH.D. 

ASSOCIATE    PROFESSOR    OF    PLANT    ECOLOGY 


VOL.    II.     ECOLOGY 


NEW  YORK  •:•  CINCINNATI  •:•  CHICAGO 

AMERICAN    BOOK    COMPANY 


COPYRIGHT,  1911,  BY 
AMERICAN   BOOK  COMPANY. 

ENTERED  AT  STATIONERS'  HALL,  LONDON,, 


A  TEXTBOOK   OF  BOTANY.      VOL.   II. 

W.P.I 


DEPt 


PREFACE 

THE  study  of  plants  has  assumed  so  many  points  of  view  that  every 
laboratory  has  developed  its  own  method  of  undergraduate  instruc- 
tion. No  laboratory  attempts  to  include  all  the  phases  of  work  that 
may  be  regarded  as  belonging  to  botany;  and  therefore  each  one 
selects  the  material  and  the  point  of  view  that  seem  to  it  to  be  the 
most  appropriate  for  its  own  purpose.  During  the  last  ten  years 
the  Hull  Botanical  Laboratory  at  the  University  of  Chicago  has  been 
developing  its  undergraduate  instruction  in  botany  to  meet  its  own 
needs.  Freed  from  the  necessity  of  laying  special  stress  upon  the 
economic  aspects  of  the  subject,  and  compelled  to  prepare  students 
for  investigation,  it  seemed  clear  that  its  selection  must  be  the  funda- 
mental facts  and  principles  of  the  science.  Its  endeavor  has  been 
to  help  the  student  to  build  up  a  coherent  and  substantial  body  of 
knowledge,  and  to  develop  an  attitude  of  mind  that  will  enable  him 
to  grapple  with  any  botanical  situation,  whether  it  be  teaching  or 
investigation.  It  has  been  thought  useful  to  present  this  point  of 
view  in  the  present  volume.  The  material  of  course  is  common 
to  all  laboratories,  but  its  selection,  its  organization,  and  its  presenta- 
tion bear  the  marks  of  individual  judgment. 

The  three  parts  of  the  book  represent  the  three  general  divisions 
of  the  subject  as  organized  at  the  Hull  Botanical  Laboratory.  They 
are  felt  to  be  the  fundamental  divisions  which  should  underlie  the 
work  of  most  subdivisions  of  botanical  investigation.  For  example, 
a  study  of  the  very  important  subject  of  plant  pathology  must  pre- 
suppose the  fundamentals  of  morphology  and  physiology ;  paleobotany 
is,  in  part,  the  application  of  morphology  and  ecology  to  fossil  plants  ; 
and  scientific  plant  breeding  rests  upon  the  foundations  laid  by 
morphology,  physiology,  and  ecology.  In  our  selection  for  under- 
graduate instruction,  therefore,  we  believe  that  there  has  been  in- 


481254 


iv  PREFACE 

eluded  the  essential  foundation  for  most  of  the  varied  work  that  is 
included  to-day  under  botany. 

We  recognize  that  the  presentation  of  the  three  great  subjects  here 
included  is  very  compact,  but  the  book  is  not  intended  for  reading 
and  recitation.  The  teacher  is  expected  to  use  it  for  suggestive 
material  and  for  its  organization ;  the  student  is  expected  to  use  it 
in  relating  his  observations  to  one  another  and  to  the  general  points 
of  view  that  -the  book  seeks  to  develop.  There  is  a  continuity  of 
presentation  in  each  part,  so  that  random  selection  may  miss  the 
largest  meaning.  For  example,  in  the  part  on  morphology,  the  thread 
upon  which  the  facts  are  strung  is  the  evolution  of  the  plant  kingdom, 
and  each  plant  introduced  has  its  peculiar  application  in  illustrating 
some  phase  of  this  evolution.  When  certain  groups  are  selected  for 
laboratory  study,  therefore,  the  intervening  text  should  be  read. 

It  is  important  to  call  attention  to  the  fact  that  the  book  has  been 
prepared  for  the  use  of  undergraduate  students.  It  does  not  repre- 
sent our  conception  of  graduate  work,  which  should  include  much 
that  is  omitted  here.  For  example,  the  graduate  student  should 
be  introduced  to  the  original  sources  of  information,  which  would 
involve  an  extensive  citation  of  literature  far  beyond  the  needs  of  the 
undergraduate.  Still  less  has  this  book  been  written  for  our  profes- 
sional colleagues,  who  will  notice  what  they  may  regard  as  glaring 
omissions.  Such  omissions  must  be  taken  to  express  a  deliberate 
judgment  as  to  what  may  be  omitted  with  the  least  damage  to  the 
undergraduate  student.  The  motive  is  to  develop  certain  general 
conceptions  that  are  felt  to  be  fundamental,  rather  than  to  present 
an  encyclopedic  collection  of  facts.  This  purpose  has  demanded 
occasionally  also  a  greater  apparent  rigidity  of  form  in  general  state- 
ments than  is  absolutely  consistent  with  all  the  facts ;  but  it  was  a 
choice  between  a  clear  and  important  conception  for  one  with  no 
perspective  and  a  contradiction  of  large  truths  by  isolated  facts,  result- 
ing in  confusion.  For  the  same  reasons,  the  extensive  terminology 
of  the  subject  has  been  kept  in  the  background  as  much  as  possible. 
Definitions  usually  are  made  an  incident  to  the  necessary  introduc- 
tion of  terms.  It  is  assumed  that  in  so  far  as  the  definite  application 
of  a  term  may  not  seem  clear,  the  student  will  find  a  compact  defini- 
tion in  the  current  dictionaries. 


PREFACE  V 

For  the  benefit  of  the  teacher  and  of  our  professional  colleagues, 
it  should  be  stated  that  much  attention  has  been  given  to  the  avoid- 
ance of  any  phraseology  that  might  involve  a  teleological  implication. 
It  has  not  been  possible  to  avoid  such  phrases  in  all  cases  without 
introducing  clumsiness  of  expression  or  breaking  the  continuity  of 
some  important  series  of  structures  or  events.  It  should  be  kept  in 
mind,  therefore,  that  all  teleological  implications  of  language  that 
remain  are  disavowed. 

It  seems  hardly  necessary  to  say  that  most  of  the  material  presented 
in  the  book  has  been  worked  over  by  classes  repeatedly.  Some  new 
matter  has  been  developed  incidentally  in  all  the  parts  in  connection 
with  ordinary  laboratory  and  field  work;  and  especially  in  Part  III 
have  many  scattered  observations  and  some  new  points  of  view  been 
included.  There  has  been  no  intention  to  include  any  formal  con- 
tribution, but  merely  to  present  in  general  outline  some  of  the  material 
worked  over  by  undergraduates,  some  of  the  results  of  investigation 
already  published  in  contributions  from  the  laboratory,  and  some  ob- 
servations and  conclusions  that  hardly  seemed  to  justify  separate  pub- 
lication. Provision  has  been  made  for  students  with  more  interest 
or  more  time  than  usual  to  get  a  somewhat  larger  view,  by  including 
in  smaller  type  further  details  of  structure,  additional  illustrative 
material,  and  suggestive  theories.  Most  of  the  illustrations  are  origi- 
nal, in  the  sense  that  they  have  been  prepared  especially  for  this 
book  or  have  appeared  in  our  own  contributions.  Those  that  have 
been  copied  or  adapted  are  credited ;  the  former  usually  being  indi- 
cated by  "from,"  the  latter  by  "after." 

The  three  authors  are  individually  responsible  only  for  their  own 
parts,  and,  while  they  had  the  advantage  of  mutual  criticism,  it  could 
not  be  expected  that  they  would  agree  absolutely  at  every  point. 
This  will  explain  any  lack  of  harmony  that  may  be  discovered  in  the 
three  parts.  A  morphologist,  a  physiologist,  and  an  ecologist  look 
at  the  same  material  from  different  angles,  and  lay  emphasis  upon 
different  features;  but  all  their  points  of  view  should  be  included 
in  any  general  consideration  of  plants.  It  is  for  this  reason,  also, 
that  the  parts  contain  a  certain  amount  of  repetition,  which  is  abso- 
lutely necessary  when  the  same  structures  or  functions  are  being 
considered  from  different  points  of  view. 


vi  PREFACE 

The  selection  and  preparation  of  the  illustrations  for  Part  I  were 
under  the  efficient  direction  of  Dr.  W.  J.  G.  LAND,  and  most  of  the 
original  drawings  of  the  book  were  made  by  Miss  ANNA  HAMILTON, 
an  artist  to  whom  great  credit  is  due.  We  owe  certain  original  illus- 
trations to  the  cooperation  of  our  colleagues,  who  are  named  in  con- 
nection with  the  figures ;  and  also  some  of  the  drawings  in  Part  III 
to  Miss  ANNA  M.  STARR.  In  addition  to  the  mutual  criticism  of  the 
authors,  Dr.  C.  J.  CHAMBERLAIN,  Dr.  WILLIAM  CROCKER,  and  Mr. 
GEORGE  D.  FULLER  made  helpful  suggestions  in  reading  the  proof. 
For  such  errors  as  remain,  after  all  our  efforts  to  eliminate  them,  the 
authors  themselves  assume  full  responsibility.  In  correcting  them,  we 
shall  welcome  the  help  of  the  wider  circle  of  users  to  whom  the  book 

now  goes. 

JOHN   M.   COULTER. 

CHARLES   R.   BARNES. 

HENRY  C.  COWLES. 
THE  UNIVERSITY  OF  CHICAGO. 


CONTENTS 
VOL.  II.,  PART  III.     ECOLOGY 

CHAPTER  PAGE 

Introduction .         .  485 

I.     Roots  and  rhizoids 491 

1.  Soil  roots  and  root  hairs .         .         .  491 

2.  Water  and  air  roots;   rhizoids     ........  509 

Water  roots    ...........  509 

Absorptive  air  roots .         .         .  511 

Anchoring  air  roots 513 

Prop  roots       .         .         .         .         .         .         .         .         .         .         .514 

Rhizoids 516 

II.     Leaves •     .         .        .         .         .         .  521 

1.  Chlorophyll  and  food  manufacture      .......  521 

Chloroplasts  and  chlorophyll  .         .         .         .         .         .         .  521 

Synthesis  of  carbohydrates .        .  525 

Starch  formation 528 

Synthesis  of  proteins       .........  528 

Anthocyan      . .  .         .  528 

2.  Structure  and  arrangement  of  chlorenchyma      .  . .        .         .  530 

3.  Relation  of  leaves  to  light '     .         .        .  539* 

4.  Air  chambers  and  stomata  .         .         .         .         .         .         .  551 

5.  Protection  from  excessive  transpiration 565. 

Significance  of  transpiration 565 

Protective  relations  of  stomata 566 

Epidermis       .         . 567 

Reduction  of  surface 577 

Leaf  orientation      ..........  57^ 

Motile  leaves       ...         .         .         .         .         .       '.'...         .  579 

Epinasty  and  hyponasty          .         . 582 

Leaf  fall 582 

Cell  sap  and  protoplasm 587 

Snow  and  dead  leaves    .         .         . 588 

6.  Variations  in  form      .         , 589 

Thalloid  plants       .         .         .         .        -        .        .        .        .        -591 

Amphibious  plants          .        . 593 

Land  plants    ...         .         .         .         .        .        .        .         .        -  598 

Asymmetry  and  anisophylly 607 

7.  Absorption          .         .         .         .    ^r  .         .         .         .         .         •        •  608 

Water  plants .         .         .         .    * 609 

Land  plants    .         .         ,        .        .        .        •         .        •        •         •  610 

Epiphytes ,j.         .  614 

Carnivorous  plants •         ,616 

vii 


viii  CONTENTS 

CHAPTER  PAGE 

(II)         8.   Secretion  and  excretion 620 

Water  exudation 620 

Secretion  of  oils,  resins,  and  mucilage 622 

Crystals  and  cystoliths 625 

9.   Accumulation  of  water  and  food 627 

IO.    Miscellaneous  structures  and  relations 636 

Run-off       .  636 

Reproduction 636 

Conductive  tissues 638 

Mechanical  tissues 639 

Leaf  tendrils        ..........  640 

Petioles 640 

Stipules 641 

Scale  leaves         .  642 

III.     Stems        . 645 

1.  Stems  as  organs  of  display 645 

Display  of  foliage  leaves .  646""** 

Lianas 651 

Epiphytes 657' 

Synthesis  and  aeration  in  stems 660 

Display  of  reproductive  organs    .......  667 

2.  Stems  as  reproductive  organs      , 667 

Rhizomes 667 

Runners      ...........  672 

Tubers,  corms,  and  bulbs 674 

Water  plants .         .  676 

3.  Conductive  tissues 678 

4.  Mechanical  tissues  .........  696 

5.  Protective  relations  of  stems 704 

Bark  ............  704 

Cork -70S 

Stem  habits 709 

Duration     ...........  717  - 

6.  Accumulation  of  water,  food,  and  waste  products  .         .         .         .718* 

Air  and  water 718 

Foods 719 

Latex 720 

Mucilage,  oils,  resins,  crystals,  tannin,  and  dyes  .         .         .  723 

7.  Variation  in  form .         .         .  725 

Elongation  in  aerial  stems - .         .  725 

Elongation  in  aquatic  stems         .......  728 

Elongation  in  stems  submerged  by  sand 729 

Dwarfing 730 

Asymmetric  stems        .         .     -    .         .         .         .         .         .         .  734 

Periodicity  in  development 735 

Origin  of  trees     .         .         .         .         .         .         .         .         .         .  737 

Spinescence 741 

Tuberization        .         .         .         .         .         .         .         .         .         .  744 

Correlation  ;   regeneration  ;   polarity 747 

JV.      Saprophytism  and  symbiosis      .         .         .         f         ,         f         .         .         .  752 


CONTENTS  ix 

CHAPTER  PAGE 

(IV)        I.    Commensalism  and  saprophytism 752 

2.  Parasitism 761 

Fungi  and  bacteria 762 

Seed  plants 769 

Grafting 776 

Galls 780 

Autoparasitism 786 

3.  Reciprocal  parasitism,  helotism,  and  endosaprophytism         .         .  786 

Root  tubercles  and  their  bacteria 787 

Mycosymbiosis 791 

Lichens 800 

Green-celled  animals 803 

V.     Reproduction  and  dispersal 805 

1.  Reproductive  behavior  in  seedless  plants       .         .         .    '    .         .  805 

Vegetative  reproduction 805 

Asexual  reproduction 809 

Sexual  reproduction 816 

2.  Flowers 825 

Role 829 

Wind  pollination 834 

Water  pollination 838 

Insect  pollination 838 

Geitonogamy     ..........  862 

Autogamy 863 

Protective  features .  869 

Origin  of  floral  structures  ........  875 

3.  Influence  of  external  factors  upon  reproductive  organs          .         .  878 

Seedless  plants 878 

Origin  of  sexuality 88 1 

Artificial  parthenogenesis 882 

Sexuality  in  the  fungi 883 

Bryophytes  and  pteridophytes 884 

Seed  plants 885 

Vegetative  and  reproductive  periods 885 

Sex  determination 895 

Flower  color 898 

Size  and  number  of  floral  organs  .         .         ...         .  900 

Flower  form 9°° 

Hybridization 9°4 

Bud  variation 9°4 

4.  Fruits  and  seeds •  9°4 

Protective  structures 9°° 

Food  accumulation *        .911 

Variation  in  relation  to  external  factors 9l6 

Dehiscence 919 

Dispersal 919 

Origin  of  seed  structures 928 

Planting  of  seeds ,  •        .928 

VI.     Germination 93° 

Vivipary •  -93° 


x  CONTENTS 

CHAPTER              f  PAGE 

(VI)  Delayed  germination 932 

External  factors  .  .  .  .  .  .  .  .  .  933 

Seedlings  .  934 

Buds  .  ....  »  .  .  .  .  .  .  936 

VII.  Plant  associations  .  .  .  .  • 939 

VIII.  Adaptation  .  .  .  .; 947 

Adaptive  response  , 947 

Fortuitous  variation 950 

Congenital  and  reaction  structures 951 

Survival  through  natural  selection 952 


rr 


PART    III  — ECOLOGY 


INTRODUCTION 

The  scope  of  ecology.  —  Ecology  is  a  science  in  its  beginnings. 
Already  it  has  a  great  body  of  data  and  theories  whose  validity  is  more 
or  less  established,  but  whose  systematic  organization  scarcely  has 
been  attempted.  Nor  is  it  possible  as  yet  to  mark  out  its  limits,  for  it 
overlaps  to  a  greater  or  less  degree  every  other  field  of  biology,  and  of 
physiography  and  geology  as  well.  Speaking  broadly,  ecology  considers 
organisms  in  relation  to  their  environment.  Somewhat  more  precisely, 
ecology  is  that  phase  of  biology  that  endeavors  to  explain  the  origin, 
variation,  and  role  of  plant  or  animal  structures,  and  the  origin  and 
variation  of  plant  or  animal  associations. 

Plant  ecology  has  a  twofold  aspect :  the  one  considers  the  individual 
organism  and  its  component  parts  as  related  to  environment ;  this, 
since  it  overlaps  morphology  and  physiology,  may  be  called  morpho- 
logical and  physiological  ecology,  or  the  ecology  of  plant  structure  and 
behavior.  The  other  aspect  considers  plants  en  masse  as  related  to 
soil  and  climate;  this,  since  it  overlaps  physiography,  may  be  called 
physiographic  ecology,  or  the  ecology  of  vegetation.1  Morphological 
and  physiological  ecology  consider  the  same  materials  as  do  morphology 
and  physiology,  but  largely  from  a  different  point  of  view.  Morphology 
deals  with  structure  and  physiology  with  behavior,  whereas  ecology  relates 
both  structure  and  behavior  to  external  conditions,  paying  attention 
chiefly  to  the  cause  and  the  significance  of  environmental  variations. 
Morphology  and  physiology  are  essentially  laboratory  sciences,  while 
ecology  is  in  the  main  a  science  of  the  field,  treating  organisms  as  they 
grow  in  nature. 

Since  ecology  overlaps  various  sciences,  it  is  less  a  simple  science  than 
a  science  complex;  its  adequate  study  presupposes  a  foundation  in  the 
basic  principles  of  physics,  chemistry,  morphology,  and  physiology, 

1  Recently  proposed  terms  for  the  ecology  of  the  individual  organism  and  for  that 
of  organisms  en  masse  are,  respectively,  autecology  and  synecology. 

485 


'  -ECOLOGY 


and,  in  the  case  of  physiographic  ecology,  of  taxonomy,  physiography, 
geology,  and  meteorology  as  well.  Partly  because  of  its  complexity 
and  partly  because  of  its  imperfect  organization,  it  is  impossible  to 
present  all  its  materials,  even  in  elementary  fashion,  within  the  com- 
pass of  such  a  book  as  this.  Consequently,  it  has  been  thought  wise  to 
center  attention  upon  morphological  and  physiological  ecology,  since 
this  aspect  is  more  in  harmony  with  the  other  parts  of  the  book  and  is 
better  suited  for  elementary  presentation  to  students  of  botany. 

Terms.  —  While  most  of  the  terms  employed  are  denned  where  first 
introduced  or  used  extensively,  a  few  are  of  such  general  employ- 
ment as  to  justify  consideration  here.  Since  water  generally  is  regarded 
as  the  most  important  factor,  the  commonest  ecological  classification 
of  plants  is  into  hydrophytes,  mesophytes,  and  xerophytes.  Hydro- 
phytes are  plants  of  water  or  of  wet  soil;  xerophytes  are  plants  of  dry 
areas,  such  as  deserts,  dry  rocks,  and  dry  sand;  and  mesophytes  are 
plants  of  soils  intermediate  as  to  moisture.  The  term  xerophyte  has 
caused  much  confusion,  because  many  plants  growing  in  soils  of  inter- 
mediate or  abundant  moisture  have  structures  resembling  those  found 
in  plants  of  dry  soils  and  climates.  The  most  conspicuous  instances  of 
such  plants  are  found  in  alpine  and  arctic  regions,  in  salt  marshes,  and 
in  peat  bogs,  and  such  terms  as  bog  xerophytes  and  salt  marsh  xerophytes 
are  in  common  usage.  The  current  theory  is  that  deserts,  rocks,  and 
sandy  areas  are  physically  dry,  that  is,  lacking  in  water  :  whereas  salt 
marshes,  peat  bogs,  and  many  alpine  and  arctic  habitats  are  physiologi- 
cally dry,  that  is,  the  water  even  if  abundant  is  unavailable.  In  arctic 
and  alpine  regions  this  is  because  of  low  temperatures,  and  in  salt 
marshes  it  is  because  of  the  high  osmotic  pressure  of  the  medium, 
while  in  the  case  of  bogs  various  theories  have  been  suggested. 

The  term  variation  is  employed  in  its  broader  sense  of  difference  or 
diversity,  rather  than  in  its  narrower  evolutionary  sense  of  "  deviation 
from  type."  Ecological  variation,  or  diversity  produced  through  the  in- 
fluence of  external  factors,  frequently  is  contrasted  with  taxonomic  varia* 
tion,  where  there  is  had  in  mind  diversity  between  allied  forms  rather 
than  actual  modification;  that  is,  the  former  is  necessarily  dynamic, 
whereas  the  latter  may  be  relatively  static.  Frequent  use  is  made  of  the 
term  external  factors,  as  opposed  to  internal  or  inherent  factors.  By 
the  former  are  understood  such  factors  as  are  outside  the  plant,  and  by 
the  latter,  such  factors  within  the  plant  as  are  hereditary.  There  is 
another  class  of  factors,  known  as  correlative  factors  or  correlations, 


INTRODUCTION 


487 


which  reside  within  the  plant,  but  are  external  to  the  organ  affected  and 
are  not  hereditary.  For  example,  the  influence  of  a  stem  upon  the 
development  of  its  lateral  branches  is  correlative;  so  far  as  the  branches 
are  concerned,  this  is  quite  as  external  as  are  such  factors  as  light  and 
humidity. 

Scarcely  second  in  importance  to  the  terms  used  are  the  terms  avoided. 
The  standpoint  of  this  book,  contrary  to  that  of  many  previous  eco- 
logical treatises,  is  that  of  mechanical  causation  rather  than  of  teleology 
and  adaptation  (see  p.  947  for  a  discussion  of  ecological  philosophy), 
hence  many  familiar  terms  are  here  avoided;  among  such  are 
words  like  adaptation,  adjustment,1  accommodation,  and  regulation,  pur- 
posive words  which  are  here  replaced  by  the  expression  advantageous 
reaction,  thus  recognizing  that  some  activities  are  indifferent  or  disad- 
vantageous. Two  of  the  commonest  and  most  insidious  words  con- 
veying teleological  implications  are  to  and  for,  in  such  sentences  as 
"  winged  seeds  are  a  mechanism /0r  dispersal"  and  "plants  close  their 
stomata  and  develop  cutin  to  check  transpiration."  Words  implying 
forethought,  such  as  reserve  and  storage,  usually  are  omitted,  as  are 
anthropomorphic  words  in  general,  particularly  those  suggestive  of 
emotion  (such  as  hydrophile,  xerophile,  heliophobe,  geophilous,  entomoph- 
ilous).  Less  objectionable,  perhaps,  but  rather  fanciful,  are  words  or 
expressions  which  imply  that  plants,  like  men,  try  to  do  things,  as 
conveyed  in  the  familiar  expression,  struggle  for  existence,  and  in 
such  words  as  success,  failure,  or  competition,  as  applied  to  plant  be- 
havior. Some  words,  in  themselves  unobjectionable,  have  been  used  so 
generally  with  teleological  implications,  that  they  mostly  are  omitted; 
for  example,  the  word  function  has  been  employed  so  often  in  the 
sense  of  purpose  that  role  is  used  wherever  feasible;  similarly,  reaction 
usually  is  employed  in  place  of  response,  though  theoretically  the  latter 
word  is  quite  as  good.  The  attempt  is  made  generally  to  use  words 
that  are  equally  applicable  in  physics  and  in  chemistry ;  especially  is 
this  attempt  worth  while,  because  ecology,  more  than  any  other  phase 
of  biology,  has  suffered  from  the  unrestricted  use  of  anthropomorphic 
similes  and  teleological  fantasies. 

It  is  realized  that  language  lags  behind  ideas,  that  we  still  speak  of 

1  Such  terms  as  adaptation  and  adjustment,  when  used  in  the  sense  of  a  state,  rather 
than  in  the  sense  of  a  process,  are  relatively  unobjectionable,  but  in  view  of  their  large 
past  use  in  the  latter  sense,  they  are  liable  to  lead  to  confusion  and  hence  are  omitted  in  all 


488  ECOLOGY 

the  heart  as  the  seat  of  the  emotions,  and  of  the  rising  and  the  setting 
of  the  sun,  so  it  is  likely  that  some  teleological  expressions  remain. 
Some  words  of  which  frequent  use  is  made,  such  as  advantage  or  efficient, 
may  to  some  appear  teleological.  Such  terms,  however,  are  employed  in 
the  sense  of  the  perpetuity  of  the  species.  Any  structure  or  reaction 
that  favors  the  extended  duration  of  an  individual  or  of  the  race  to  which 
it  belongs  is  regarded  as  advantageous,  whil  j  any  structure  or  reaction 
that  is  detrimental  to  such  duration  is  regarded  as  disadvantageous.  In 
this  sense  such  a  word  as  advantage  is  not  teleological,  for  it  might  be 
thus  employed  with  reference  to  many  inorganic  substances. 

The  arrangement  of  the  material.  —  In  the  following  pages  the  ma- 
terial is  grouped  on  the  basis  of  structure,  the  fundamental  plant  parts 
being  regarded  as  roots,  leaves,  stems,  and  reproductive  organs.  The 
same  material  might  be  grouped  under  the  head  of  behavior,  includ- 
ing such  topics  as  absorption,  conduction,  synthesis,  transpiration, 
reproduction,  and  the  like;  or  again,  the  material  might  be  grouped 
under  the  head  of  factors,  as  light,  heat,  water,  etc.  The  method  here 
used,  that  of  classification  by  structure,  seems  best  suited  to  the  present 
state  of  ecological  knowledge,  where  structure  alone  is  approximately 
sure.  In  many  cases,  organs  have  no  known  role,  and  still  more  fre- 
quently a  given  organ  has  two  or  more  roles,  thus  making  a  classi- 
fication by  behavior  difficult  and  repetitious;  furthermore,  classification 
by  behavior  places  too  much  emphasis  upon  the  employment  of  organs 
and  is  likely  to  lead  to  teleological  mysticism.  The  classification  of 
ecological  data  by  factors  is  fundamentally  sound,  but  is  at  present 
practically  impossible,  owing  to  the  paucity  of  known  facts  as  compared 
with  those  that  are  unknown. 

The  differentiation  of  the  plant  body.  —  While  the  material  here  pre- 
sented is  classified  chiefly  under  roots,  leaves,  stems,  and  reproductive 
organs,  it  is  not  assumed  that  there  are  hard  and  fast  lines  between 
these  plant  parts.  The  evolutionary  hypothesis  presupposes  transitions 
between  all  organs,  and  there  are  many  evidences  of  intergradations 
even  between  the  most  diverse  plant  parts.  In  the  lowest  plants  the 
body  is  an  undifferentiated  thallus.  In  algae  and  fungi  there  is  seen 
the  first  conspicuous  differentiation,  that  between  vegetative  and  re- 
productive organs.  In  the  thallose  liverworts  the  vegetative  part  be- 
comes, differentiated  into  a  green  food-making  aerial  portion  and  a 
colorless  rootlike  subterranean  portion;  in  the  leafy  liverworts  and  in 
mosses  the  aerial  vegetative  portion  is  differentiated  still  further  into 


INTRODUCTION  489 

stems  and  leaves.  Certain  liverworts  exhibit  all  gradations  between  thalli 
and  leafy  stems.  Even  in  the  seed  plants,  where  differentiation  into 
roots,  stems,  and  leaves  seems  relatively  fixed,  there  are  cases  where  the 
exact  nature  of  certain  organs  is  subject  to  dispute  (as  in  the  rhizophores 
of  Selaginella,  the  plant  body  of  Utricularia,  and  the  spines  of  cacti). 

The  chief  characteristics  of  roots,  leaves,  and  stems. — The  root 
generally  is  a  descending,  irregularly  branching  axis,  while  the  stem  gen- 
erally is  an  ascending,  regularly  branching  axis,  possessing  nodes  and 
internodes,  and  bearing  leaves  as  lateral  members  and  reproductive 
shoots  as  lateral  or  terminal  members;  branch  shoots  commonly  arise 
in  the  leaf  axils.  Branch  roots  arise  from  within  (endogenously),  con- 
trasting with  branch  shoots  which  arise  at  the  exterior  (exogenously). 
Young  roots  possess  a  root  cap  which  ensheathes  the  growing  tip,  and 
strands  of  xylem  and  phloem  alternate  in  the  same  circumference  (p.  683) ; 
the  central  region  is  occupied  by  conductive  tissues,  and  the  epidermis 
is  ephemeral.  Young  stems  contrast  with  young  roots  in  that  the  xylem 
and  phloem  form  continuous  or  interrupted  cylinders  about  the  pith,  the 
phloem  commonly  being  outermost;  the  epidermis  is  much  less  ephem- 
eral than  in  roots.  Stems  and  roots  commonly  are  radially  symmetrical, 
possessing  an  infinite  number  of  vertical  planes  of  symmetry,  while 
leaves  are  dorsiventrally  symmetrical,  possessing  a  single  plane  of  sym- 
metry. 

The  preceding  distinctions  between  roots,  stems,  and  leaves  are 
general  but  not  universal.  For  example,  some  roots  are  exogenous  in 
origin  and  some  stems  endogenous.  Not  all  roots  have  root  caps.  Some 
roots  bear  buds  which  a  e  able  to  develop  into  leafy  shoots;  shoots 
borne  on  roots  are  not  necessarily  subtended  by  leaves.  Old  roots  and 
stems  may  be  indistinguishable  in  structure;  many  stems  and  roots  are 
without  radial  symmetry,  and  sometimes,  there  is  but  a  single  plane  of 
symmetry,  as  in  most  leaves;  some  leaves,  however,  are  all  but  radially 
symmetrical  (p.  629).  These  and  similar  exceptions  indicate  sufficiently 
the  intergradations  between  these  organs.  Indeed,  it  is  doubtful  if  a 
single  characterization  can  be  made  that  always  holds;  occasionally  it  is 
only  by  a  majority  test  of  its  characters  that  the  nature  of  an  organ 
may  be  determined.  Sometimes  one  or  more  of  the  plant  parts  may  be 
missing,  as  the  roots  in  Salvinia,  and  as  both  leaves  and  roots  in  Wolffia; 
many  rosette  plants,  as  the  dandelion,  are  practically  stemless.  Thus, 
such  terms  as  roots,  leaves,  stems,  and  reproductive  organs  are  con- 
venient words  rather  than  fundamental  categories.  Here,  as  in  every 


490  ECOLOGY 

branch  of  knowledge,  all  schemes  of  classification  confessedly  are  artifi- 
cial, and  yet  an  orderly  arrangement  of  material  is  necessary.  Even 
where  the  various  plant  organs  are  well  differentiated,  they  are  not 
treated  rigidly  in  the  following  pages,  but  each  subtopic  is  considered 
where  it  seems  the  most  appropriate;  for  example,  food  manufacture  is 
treated  under  leaves,  conduction  and  mechanical  support  under  stems, 
and  food  accumulation  under  seeds,  while  vegetative  reproduction  is 
considered  chiefly  under  stems. 


CHAPTER   I  — ROOTS  AND   RHIZOIDS 


i.     SOIL  ROOTS  AND  ROOT  HAIRS 

Root  hairs.  —  General  remarks.  —  Most  land  plants  possess  roots  which 
branch  freely  and  penetrate  the  soil  in  all  directions.  While  the  main 
roots  of  adult  plants  often  are  large  and  stout,  the 
ultimate  branches  are  slender  and  give  rise  to  very 
delicate  organs,  the  root  hairs.  The  chief  role  of 
soil  roots  is  in  connection  with  absorption  and 
anchorage.  The  entire  root  system  is  concerned  in 
the  latter,  but  the  admission  of  water  and  salts  is 
restricted  practically  to  the  very  youngest  portions. 
The  structure  and  rdle  of  root  hairs.  —  Root  hairs 
are  extensions  of  the  epidermal  cells  of  roots  (figs. 
700-702,  705),  most  such  cells  possessing  the  ca- 
pacity of  developing  hairs,  though  many  are  with- 
out them.1  The  walls 
are  thin  and  of  cellulose, 
readily  permitting  the 
entrance  of  water  and 
solutes.  Though  root 
hairs  vary  greatly  in 


FIGS.  700,  701.  — 
Seedlings  of  mustard 
(Brassica  alba) :  700,  a 
seedling  dislodged  from 
the  soil,  showing  par- 
ticles of  earth  adhering 
to  the  root  hairs,  the  root 
tip  being  free  from  hairs 

or  attached  particles;  length  and  abundance, 
they  average  rather 
more  than  a  millimeter 


701,  a  seedling  grown 
in  moist  air,  showing  a 
primary  root  with  its 

zone  of  root  hairs,  the    in  1- ngth,  and  as  many 
younger  hairs  toward    as   three    hundred   may 

the   tip   being  progres- 


A  longitu- 
dinal section  through  the  outer 
portion  of  a  root  of  the  Wind- 


leaves  (cotyledons)  rise     millimeter.        While    the  ing  a  root  hair  (r)  arising  as 

above  the  soil,  illustrat-    entire  root  epidermis  is  an.<mtgrow^  ^  °ntSe2 

\         \f          T)CrmCclDlC   to   \Vdt"A    ctiivJ.  (YI\   miffrcitinsr  into  the  liciiri 

SACHS,  solutes,  the  chief  advan-  highly  magnified. 

i  In  various  pteridophytes  (as   Azolla)   and  monocotyls,   however,  hairs  arise   only 
from  special  small  cells  rich  in  protoplasm. 

491 


492 


ECOLOGY 


tage  derived  from  root  hairs  seems  to  be  the  increase  of  permeable  sur- 
face, which  sometimes  is  as  much  as  five  or  ten  times  that  of  a  hairless 
root  of  equal  size.  The  youngest  hairs  emerge  a  short  distance  behind  the 
tip;  farther  back  they  are  of  mature  size,  and  still  farther  back  they  are 
withered  and  dead.  Most  root  hairs  are  ephemeral  structures,  lasting 
only  a  few  days  or  weeks.  Indeed,  the  entire  epidermis  is  soon  sloughed 

off,  and  the  hypodermis  (here  called 
the  exodermis)  becomes  the  outer 
layer  of  the  root,  which  through  cut- 
inization  is  thenceforth  relatively  im- 
permeable to  water.  The  continual 
dying  of  the  older  hairs,  as  new  hairs 
develop  toward  the  tip,  gives  rise  to 
the  migration  of  root  hair  zones,  mak- 
ing absorption  possible  from  new  soil 
regions  (figs.  703,  704).  Furthermore, 
the  ever  increasing  development  of  the 
root  system  is  accompanied  by  a  con- 
tinual increase  in  hair  development, 
thereby  enlarging  the  aggregate  area 
of  absorption  and  the  total  absorption 

FIGS.  703,  704.  —  Seedlings  of  wheat    Capacity. 

When  unimpeded,  root  hairs  grow 

^  ^    showing  no 
,  ...  . 

in  703  are  hairclad  (r)  and  agglutinated   reaction  to  gravity  stimuli.     The  hairs 

to  soil  particles,  have  become  hairless  in  become  variously  gnarled  and  COn- 
704  (r'),  while  the  younger  portions  (r'')  ^  (fi  }  through  contact  with 

deeper  in  the  soil  have  hairs;  note  the 

hairless  root  tips;  the  wheat  grain  (g)  soil  particles,  with  which  an  intimate 
remains  in  the  earth,  illustrating  hy-  cementation  is  effected  by  the  trans- 

pogaean  germination  (p.  936).  -After    formatk)n  of  the  Quter  layer  into  mud_ 

lage.     So  close  is  this  attachment  that 

when  a  plant  is  pulled  from  the  ground,  considerable  earth  adheres  to 
the  root  hairs  (fig.  700),  which  are  more  apt  to  break  than  to  separate 
from  the  soil  particles;  this  indicates  the  important  part  played  by  root 
hairs  in  anchorage,  especially  in  seedlings.  The  adhesion  of  hairs  to 
soil  particles  is  of  still  greater  advantage  in  absorption,  since  most  of 
the  available  water  surrounds  the  particles  as  a  film,  in  which  are  also 
most  of  the  salts  utilized  by  plants  as  food  materials.  The  carbon 
dioxid  excreted  by  the  root  hairs  assists  in  dissolving  the  soil  salts. 


704 


(Triticum  sativum)  :   703,  a  seedling 

soon  after  germination;    704,  a  seedling  rf    , 

four  weeks  older  ;  the  root  regions,  which 


ROOTS   AND    RHIZOIDS 


493 


The  influence  of  external  factors  upon  absorption.  —  The  amount  of 
absorbed  water  decreases  as  the  soil  becomes  desiccated,  because  of  the 
increasing  concentration  of  the  soil  solutions.  Similarly,  the  absorption 
of  water  and  solutes  is  reduced  greatly  at  low  temperatures,  frozen  soils 
being  physiologically  as  dry  as  those  of  deserts.  In  both  dry  and  frozen 
soils  the  root  hairs  even  may  exude  water  instead  of  absorbing  it.  Plants 
differ  widely  in  regard  to  absorp- 
tion, s  me  carrying  on  their  ac- 
tivities in  spite  of  the  almost 
perpetually  low  temperatures  of 
polar  soils.  High  temperatures 
favor  maximum  absorption,  if 
the  water  supply  is  adequate. 
Another  important  factor  is  the 
degree  of  concentration  of  the 
medium,  strong  solutions  of 
sodium  chlorid  and  similar  salts 
greatly  retarding  absorption ; 
probably  it  is  for  this  reason  that 
salt  marsh  plants  often  have  an 
inadequate  supply  of  water.1 
Absorption  seems  also  to  be 
difficult  in  peat  bogs,  though  the 
reason  is  less  obvious.  Pos- 
sibly the  presence  of  deleterious 
substances  in  the  bog  water  is 
a  factor  of  importance. 

Soil   exhaustion  and  root   ex- 


K 


cretions. — The  migration  of  root 
hair  zones  facilitates  the  invasion 
of  new  areas  by  growing  roots. 


FIG.  705.  —  Root  hairs  of  a  lettuce  seed- 
ling (Lactuca  saliva),  which  have  developed 
in  the  soil;  note  that  the  hairs  are  somewhat 
sinuous  outgrowths  (thus  contrasting  with 
air-grown  root  hairs)  from  the  lower  portion 
(i.e.  toward  the  root  tip)  of  the  epidermal 
cells,  and  that  not  all  epidermal  cells  give 
rise  to  hairs;  note  also  the  attached  soil 
particles  (s) ;  highly  magnified. 

1  It  is  important  to  note,  however,  that  the  osmotic  pressure  of  the  cell  sap  of  plants 
varies  widely  with  the  habitat,  being  least  in  submersed  hydrophytes  and  greatest  in 
xerophytes  and  in  salt  marsh  plants.  The  sap  of  salt  marsh  plants  may  have  a 
pressure  of  twenty  atmospheres,  as  against  four  or  five  atmospheres  in  pqnd  aquatics; 
hence,  the  former  may  be  as  able  to  absorb  water  from  a  concentrated  solution  as  are 
the  latter  from  a  dilute  solution.  Many  desert  plants,  especially  shrubs,  have  a  pressure 
of  thirty-five  to  one  hundred  atmospheres,  and  thus  are  better  able  than  are  other  plants 
to  utilize  the  scant  water  of  dry  soils.  Furthermore,  it  has  been  demonstrated  that  in 
the  cell  sap  of  various  salt  water  plants,  the  osmotic  pressure  varies  considerably  from 
time  to  time,  corresponding  with  similar  variations  in  the  medium. 


494 


ECOLOGY 


The  advantage  of  such  migration  often  is  assumed,  it  being  asserted 
that  roots  exhaust  the  food  materials  in  any  given  portion  of  the  soil. 

However,  it  is  likely  that  the  amount 
of  mineral  matter  used  by  plants  is  so 
small,  and  that  the  supply  in  the  soil 
commonly  is  so  great,  that  uncultivated 
soils  rarely  if  ever  are  exhausted.  Fur- 
thermore, plants  sooner  or  later  return 
to  the  soil  the  mineral  matter  that 
they  took  from  it.  Probably  the 
"  sterility  "  of  many  soils  is  due  less 
co  the  abstraction  of  necessary  elements 
by  vegetation  than  to  the  addition  of 
deleterious  substances,  which  may  be 
definite  root  excretions,  or  which  may 
arise  through  the  decomposition  of 
organic  material  in  the  soil. 

Very  recently  it  has  been  shown  that 
the  roots  of  certain  plants  excrete  sub- 
stances which  impede  further  root  ac- 
tivity. If  this  phenomenon  proves  to 
be  general,  as  now  seems  likely,  the  in- 
vasion of  new  soil  areas  by  roots  may 
make  possible  their  escape  from  the 
substances  which  they  give  off  or  which 
arise  by  subsequent  decay.  Even  in 
the  case  of  cultivated  crops,  it  is  prob- 
able that  fertilizers  are  of  less  value  as 
sources  of  plant  food  than  in  their  action 
upon  soil  constituents  and  in  counter- 
acting the  noxious  effect  of  root  excreta 
or  of  decaying  vegetation.  Certain  root 
enzyms  are  oxidizing  agents  of  much 
importance  and  assist  in  the  destruc- 
tion of  the  deleterious  soil  compounds ; 
however,  when  these  compounds  are 


FIG.  706.  —  Seedlings  of  maize 
(Zca  Mays)  that  have  grown  in  moist 
air  just  above  water;  note  that  the 
submersed  portions  of  the  roots  are 
hairless  and  exhibit  irregular  growth 
curvatures;  the  older  leaves  (5)  en- 
sheath  the  delicate  younger  leaves; 
maize  seedlmgs  illustrate  hypogaean 
germination. 


present  in  excess,  the  oxidizing  action  becomes  lessened  and  the  addi- 
tion of  nitrates  and  of  other  fertilizer  salts  is  of  great  value.  Farmers 
have  long  believed  that  fields  occasionally  should  lie  fallow ;  the  advan- 


ROOTS   AND    RHIZOIDS  495 

tage  therefrom  would  appear  to  be  in  the  facilitation  of  the  oxidation 
and  the  removal  of  deleterious  substances.  It  seems  likely  that  sub- 
stances given  off  by  plants  of  a  particular  species  often  are  more  injurious 
to  plants  of  their  own  kind  than  to  plants  of  other  species,  a  fact  that 
may  help  explain  the  value  of  crop  rotation. 

Variations  in  the  form  and  the  development  of  root  hairs.  — Usually, 
when  a  seedling  is  transplanted,  the  hairs  die  at  once,  thus  reducing 
the  amount  of  water  admitted.  Wilting  soon  occurs,  and  it  is  only  when 
new  hairs  have  grown  that  the  plant  revives.  Root  hairs  developed  in 
moist  air  collapse  when  brought  into  a  drier  medium. 

When  maize  roots  are  grown  in  water,  hairs  commonly  are  absent, 
while  a  vigorous  growth,  apparently  exceeding  that  in  the  soil,  is 
obtained  in  moist-air  cultures  (fig.  706).  The  same  is  true,  perhaps,  of 
the  majority  of  plants,  but  there  are  many  exceptions,  as  in  wheat  and 
oat  seedlings,  though  the  latter  usually  fail  to  develop  hairs  in  water 
after  the  food  within  the  seed  is  exhausted.  Wheat  and  oat  seedlings 
also  have  hairless  roots  in  concentrated  solutions,  probably  because 
the  latter  inhibit  the  absorption  of  the  water  necessary  for  hair  develop- 
ment.1 Oxygen  is  necessary  for  the  development  of  root  hairs,  and 
it  may  be  that  their  absence  or  weak  development  in  ponds  and  swamps 
is  due  in  part  to  low  oxygen  content. 

The  exact  factors  that  determine  root  hairs  are  not  certainly  known,  but  they 
appear  to  develop  best  when  root  growth  is  retarded,  and  especially  when  retarda- 
tion is  differential.  When  roots  elongate  rapidly,  epidermal  cells  tend  to  elongate 
in  the  same  direction,  but  when  roots  elongate  slowly,  epidermal  cells  grow  trans- 
versely, developing  into  hairs.  Root  elongation  near  the  tip  probably  is  so  rapid 
that  the  epidermal  cells  grow  chiefly  in  a  longitudinal  direction.  A  short  distance 
back,  cortical  elongation  ceases,  but  the  epidermal  cells,  continuing  to  grow,  develop 
transversely  rather  than  longitudinally.  Probably  the  occurrence  of  hairs  at  the 
root  tips  of  certain  xerophytes  is  due  to  slow  root  growth.  Differential  growth 
often  leads  to  the  twisting  of  the  root  in  water  cultures,  and  hairs  may  occur  on 
the  kinked  portions. 

Maize  roots  have  hairs  in  water  if  elongation  is  mechanically  retarded;  this  may 
explain  why  some  hairless  water  roots,  as  in  Elodea,  develop  hairs  as  soon  as  they 
enter  the  soil  (maize  roots,  however,  are  hairless  in  saturated  soil).  Contact  with 
solid  bodies  also  may  favor  hair  development.  High  temperatures  favor  both 
water  entry  and  root  growth,  but  are  detrimental  to  the  development  of  root  hairs, 
further  strengthening  the  root  retardation  theory.  Sometimes  there  seems  to  be 
a  correlation  between  root  hairs  and  shoots.  An  irrigated  seedling  of  the  creo- 

1  However,  some  salt  marsh  plants,  such  as  Salicornia  and  Zostera,  have  an  abundance 
of  root  hairs,  showing  that  different  species  react  differently  to  similar  stimuli. 


496  ECOLOGY 

sole  bush  (Larrea  tridentata)  has  ample  tops  and  few  root  hairs,  while  the  reverse 
is  the  case  with  a  plant  in  dry  soil. 

Variations  in  the  occurrence  of  root  hairs.  —  While  air  roots  and  water 
roots  commonly  are  hairless,  most  soil  roots  have  hairs.  Wet  soil,  as  in 
swamps  and  beneath  ponds,  presents  a  condition  transitional  between 
water  and  mesophytic  soil;  in  such  habitats  most  roots  have  hairs,  but 
some  are  hairless.  Hairs  are  more  abundant,  even  in  the  same  species, 
in  soil  beneath  flowing  water  than  in  soil  beneath  standing  water.  The 
root  hairs  of  some  xerophytes  have  rigid  lignified  walls;  for  example, 
in  Pinus  edulis  they  are  stiff  and  brown.  In  some  succulent  xero- 
phytes (as  Opuntid)  hairs  occur  to  the  extreme  root  tip.  In  most 
plants  whose  roots  are  invested  with  fungi,  such  as  the  conifers,  oaks, 
and  many  tuberous  and  bulbous  plants,  root  hairs  are  either  almost 
or  entirely  wanting  (fig.  1106).  The  roots  of  parasites  commonly  are 
hairless,  and  in  the  green  partial  parasites  all  gradations  are  found 
between  species  with  abundant  hairs  and  those  with  none  (p.  772). 

The  structure  and  role  of  soil  roots.  —  General  features.  —  Roots  are 
more  uniform  in  their  appearance  than  are  stems  and  leaves,  and  there 
probably  is  some  connection  between  such  uniformity  and  the  com- 
paratively uniform  habitats  in  which  roots  grow.  The  root  of  the  seed- 
ling, known  as  the  primary  root,  is  a  taproot,  being  the  main*  descending 
axis  of  the  plant  (fig.  701).  At  first  this  root  with  its  hairs  represents 
the  entire  absorptive  and  anchorage  system.  Very  soon  branches 
appear,  known  as  secondary  roots,  which  differ  from  stem  branches  in 
their  irregular  position  and  in  their  endogenous  origin ;  shortly  the  root 
system  becomes  a  most  complex  affair,  owing  to  repeated  branching.  In 
many  plants  the  primary  root  persists  throughout  life,  continuing  to 
elongate  and  to  grow  in  diameter;  persistent  primary  roots  of  this  char- 
acter, which  often  are  tap  roots  as  well,  are  found  in  the  carrot,  dande- 
lion, dock,  and  in  many  trees  (fig.  708).  In  many  other  plants, 
especially  in  those  that  reproduce  by  means  of  underground  stems,  the 
primary  root  soon  dies,  its  place  being  taken  by  adventitious  roots 
(p.  503).  Roots,  particularly  those  of  trees  and  shrubs,  are  made  up 
largely  of  woody  tissue,  which  serves  as  an  avenue  of  conduction  to  and 
from  the  root  hairs;  also,  through  the  stiff  cell  walls,  these  tissues 
give  mechanical  strength  to  the  roots.  Some  of  these  wood  cells  have 
relatively  capacious  cavities  and  are  dominantly  conductive,  whereas 
others  consist  almost  wholly  of  thickened  walls  and  are  chiefly  mechan- 
ical; see  detailed  discussion  of  conductive  and  mechanical  tissues, 


ROOTS   AND    RHIZOIDS 


497 


pp.  678  and  696.  The  penetration  of  roots  into  new  soil  regions  takes 
place  without  injury  to  the  delicate  growing  root  tip,  because  the  latter 
is  protected  by  a  root  cap,  consisting  of  older  and  less  delicate  cells, 
which  slough  off  and  become  slimy,  as  they  are  pushed  ahead  of  the 
elongating  tip  (fig.  552). 

Roots  play  an  important  role  in  anchorage  and  in  nutrition.     In  many 
species  all  roots  take  part  in  both  processes,  but  there  are  some  plants 


FIG.  707. — A  sand-binding  grass  (Calamovilfa  longifoiia)  holding  a  mound  of  sand 
in  place  by  its  numerous  long  and  slender  but  tough  roots ;  to  the  left  are  exposed  roots 
and  a  trough  blown  out  by  the  wind;  note  the  small  herbs  among  the  grass  culms,  by 
which  they  are  protected;  Dune  Park,  Indiana.  —  Photograph  supplied  by  MEYERS. 

in  which  anchoring  and  nutritive  roots  are  in  sharp  contrast.  The 
former  have  a  central  vascular  strand  composed  of  thick-walled  cells  of 
small  caliber;  nutritive  roots,  however,  have  prominent  pith  and  vas- 
cular elements  that  are  relatively  thin-walled  and  of  large  caliber.  If 
growing  roots  are  subjected  to  tension,  they  develop  into  anchoring 
roots,  while  roots  not  so  subjected  develop  into  nutritive  roots  (figs.  737, 
738).  Many  cacti  have  strong  descending  anchoring  roots  and  weak 


498 


ECOLOGY 


horizontal  nutritive  roots.  The  root  systems  of  trees  differ  widely, 
the  ash,  for  example,  having  roots  of  great  length,  while  the  more 
numerous  roots  of  the  beech  are  much  shorter  and  finer. 

Roots  as  an  horage  organs.  —  In  a  mature  plant  with  a  complex  root 
system,  a  division  of  labor  is  manifest.  A  sorption  is  limited  to  the 
younger  portions  of  the  roots,  where  there  is  a  delicate  epidermis  per- 


FIGS.  708,  709.  —  Roots  of  the 
peppergrass  (Lepidium)  :  708,  an  or- 
dinary primary  root  with  laterals; 
709,  a  root  originally  similar  to  708, 
but  from  which  vigorous  lateral  roots 
have  developed,  following  the  re- 
moval of  the  primary  root. 

meab'e  to  water  and  salts.  Anchorage,  on  the  other  hand,  is  preeminently 
the  role  of  the  much-branched  root  system.  On  the  whole  there  is  a  sort 
of  correlation  between  the  developing  stems  and  roots,  large  aerial  stem 
systems  commonly  being  associated  with  extensive  root  systems.  Only 
in  certain  situations,  such  as  swamps  (see  p.  509),  or  when  there  are 
unusual  winds,  as  during  tornadoes  and  hurricanes,  are  tree  "  wind- 


ROOTS   AND   RHIZOIDS 


499 


falls  "  a  common  sight.  The  effectiveness  of  roots  as  anchorage  organs 
is  well  displayed  along  streams  and  shores,  where  erosive  forces  are 
active.  In  such  situations  the  earth  often  is  held  firmly  in  place  by 
matted  roots  and  it  is  only  by  undermining  the  grasses  and  trees,  which 
may  even  overhang  in  such  places,  that  the  erosive  forces  are  able 
finally  to  dislodge  the  plants  and  to  encroach  farther  upon  the  land.  Even 
more  striking  is  the  behavior  of  roots  in  regions  of  shifting  sands  along 
seacoasts,  where  many  grasses  and  other  plants  are  able  not  only  to 
maintain  themselves,  but  even  to  check  the  prog- 
ress of  the  sand  (fig.  707).  Such  plants  are 
known  as  sand  binders,  and  are  of  great  impor- 
tance in  preventing  the  encroachment  of  sand  upon 
villages  and  farms,  even  being  planted  for  that 
purpose  in  many  places. 

Gravity  and  the  direction  of  root  growth. — The 
tendency  cf  roots  to  grow  downward  (i.e.  to  ex- 
hibit progeotropisni)  makes  possible  a  favorable 
relation  to  absorption  and  anchorage  (figs.  691- 
693).  The  geotropic  reactions  of  roots  are  well 
shown  when  seeds  germinate  on  the  soil  surface; 
if  the  root  issues  from  the  upper  side,  it  may  curve  lins  of  maize  (Zea 

,  ,  -      0    n  ,.         .      j  Mays)  that  has   been 

through  an  arc  of  180°,  and  grow  directly  down  grown  at  the  edge  of 
into  the  soil.  But  while  the  tap  root  usually  grows  a  funnel  whose  sur- 
straight  down  in  this  fashion,  the  numerous  side  face  has  been  kePl 

,  moist ;    note   that  the 

roots  branch  out  in  almost  all  directions.  1  he  root  is  prohydrotropic, 
advantage  of  such  a  habit  is  clear  enough,  but  the  following  the  moist- 
cause  is  obscure.  If  the  growing  part  of  a  tap  e"ed  surface>  inftead 

0  of   growing    vertically 

root  is  removed,  some  of  the  stronger  side  roots     downward. 

commonly  begin  to  grow  straight  down  (figs.  708, 

709).     Apparently  there  is  something  in  the  tap  root  which  inhibits 

the  expression  of  progeotropism  by  the  side  roots.     The  removal  of  the 

tap  root  removes  this  inhibition  and  the  side  roots  change  their  growth 

direction. 

Water  and  the  direction  of  root  growth.  —  Roots  usually  grow  toward 
moisture;  that  is,  they  are  prohydrotropic.  Water  and  gravity  often  com- 
bine to  cause  downward  growth  in  roots,  but  commonly  the  water  in- 
fluence is  the  stronger  of  the  two.  When  seeds  are  planted  in  such  a 
way  that  the  source  of  water  is  at  one  side  rather  than  from  beneath, 
the  roots  grow  laterally,  not  vertically  (fig.  710)-  In  nature  similar 


FIG.  7  ro.  —  A  seed- 


5oo 


ECOLOGY 


phenomena  are  seen  along  ditches  and  irrigation  canals  and  on  the  sides 
of  vertical  cliffs.  Prohydrotropic  root  reactions  are  of  great  advantage  in 
view  of  the  dominant  role  of  water  in  the  life  of  plants.  Various  factors 
other  than  water  prevent  the  expression  of  progeotropic  tendencies.  In 
rocky  regions,  where  the  strata  are  horizontal,  the  roots  are  unable  to 
penetrate  downward.  Perhaps  the  most  notable  exception  to  the  usual 
progeotropic  reactions  are  seen  in  swamp  roots  (p.  507). 

Food  accumulation  in  roots.  —  While  stems  are  the  most  conspicuous 
food-accumulating  organs,  fleshy  roots  also  are  of  great  importance 
in  this  regard,  familiar  examples  being  afforded 
by  the  parsnip,  carrot,  turnip,  and  beet  (fig.  711; 
also  fig.  720).  Some  roots,  such  as  those  of  the 
beet,  accumulate  sugar,  while  others  accumulate 
starch  (as  in  many  orchids).  The  roots  of  many 
Compositae  accumulate  inulin  (p.  914).  Water 
is  accumulated  in  fleshy  roots  to  an  even  greater 
degree  than  are  starch  and  other  foods.  In  arid 
regions  large  fleshy  roots  are  frequent,  and  some- 
times plants  whose  stems  are  small  and  delicate 
have  roots  of  enormous  size.  Various  species  of 
Ipomoea  have  large  roots  rich  in  foods;  among 
these  is  the  sweet  potato  (/.  Batatas).  The 
man-of-the-earth  (7.  pandurata)  has  much  larger 
roots,  and  I.  leptophylla  of  the  plains  has  roots 
that  are  even  gigantic  when  compared  with  the 
relatively  small  aerial  organs. 

There  appear  to  be  several  advantages  associated 
with  food  accumulation  in  roots.  Such  organs 
seem  better  protected  from  animals  and  from  low 
temperatures  than  are  aerial  organs,  and  unques- 
tionably they  are  better  protected  from  desicca- 
tion. It  seems  more  than  a  coincidence  that  the  greatest  development 
of  large  fleshy  roots  is  in  those  regions  where  the  danger  from  drought 
is  the  greatest.  While  the  most  striking  illustrations  of  fleshy  roots  are 
found  in  arid  regions,  many  plants  of  our  woods  and  fields,  such  as 
the  dandelion,  dock,  evening  primrose,  and  various  orchids,  have  similar 
roots  that  accumulate  much  food  and  water. 


FIG.  711.  —  A  beet 
(Beta  vulgaris),  illus- 
trating a  much  en- 
larged primary  root; 
in  such  roots  large 
quantities  of  water  and 
food  accumulate. 


See  p.  911  for  a  general  consideration  of  the  significance  of  food  accumulation. 
It  may  be  noted  here  that  part  of  the  starch  that  accumulates  in  growing  rocts  seems 


ROOTS   AND    RHIZOIDS 


501 


to  cause  geotropic  curvature  (see  p.  464,  for  a  discussion  of  the  statolith  theory  of 
starch;  also  figs.  697,  698). 

The  duration  of  roots.  —  The  classification  of  plants  into  annuals, 
biennials,  and  perennials  is  based  chiefly  upon  the  length  of  life  of  roots, 
though  many  herbs  with  perennial  underground  stems  have  annual  roots. 
The  conditions  that  determine  duration  are  not  known,  though  a  little 
light  is  thrown  on  the  subject  by  the  behavior  of  annuals  and  biennials. 
A  biennial  is  a  plant  that  lives  in  two  vegetative  periods,  while  an  annual 
lives  in  but  one.  Many  plants  are 
annuals  or  biennials,  depending  in 
part  upon  the  time  of  germination ; 
for  example,  winter  wheat  is  a 
biennial,  and  spring  wheat  an  an- 
nual. Other  plants,  as  the  pepper- 
grass  and  the  shepherd's  purse,  ex- 
hibit similar  phenomena.  Probably 
most  hardy  annuals  become  bien- 
nials if  planted  in  late  summer, 
while  many  biennials  become  an- 
nuals if  planted  in  early  spring, 
and  many  more  if  started  yet 
earlier  in  a  hothouse.  When  an- 
nuals and  biennials  are  grown  in 
uniform  conditions,  as  in  the  moist 
tropics  or  in  a  moist  greenhouse, 


FIG.  712. — A  portion  of  the  creeping 
stem  of  a  water  pennywort  (Hydrocotyle) 
showing  adventitious  roots  grouped  at  the 
nodes,  each  leaf  with  its  node  and  group 


they   become  perennials    in    many      Of  roots  representing  a  potential  plant;  the 


leaves  have  long  petioles,  which  react 
readily  to  changes  in  the  direction  of  the 
incident  light. 


instances,  while  certain  plants  (as 
the  castor  bean)  that  commonly 
are  perennials  in  uniform  climates 
tend  to  become  annuals  in  periodic  climates.  Some  annuals  (as  Poa 
annua  and  Senecio  vulgaris]  become  biennials  or  perennials  when 
transferred  to  alpine  habitats.  In  some  trees  as  the  silver  poplar, 
osage  orange,  and  redwood,  new  shoots  arise  one  after  another  from 
the  old  roots,  so  that  the  tree  lives  long  after  the  first  trunk  has 
rotted  away.  Indeed,  so  far  as  root  duration  is  concerned,  some 
perennials  may  be  said  to  possess  a  capacity  for  perpetual  life. 

The  influence  of  external  factors  upon  root  form  and  develop- 
ment. —  Transplanting  in  relation  to  root  form.  —  Reference  has  been 
made  to  the  changed  direction  assumed  by  lateral  roots  when  the  tap 


502 


ECOLOGY 


root  is  removed.  The  change  in  the  general  form  of  the  root 
system  is  equally  noteworthy,  since  the  large  tap  root  is  replaced  by  a 
complex  of  much-branched  roots  of  about  equal  size  (figs.  708,  709). 
Such  a  change  in  form  usually  occurs  when  plants  (e.g.  celery  or  cab- 
bage) are  transplanted,  since  the  growing  tip  of  the  tap  root  commonly  is 
destroyed  in  the  process. 

Apart  from  the  prevention  of  overcrowding,  transplanting  may  be  of  economic 
advantage,  since  the  shallow  and  compact  root  system  thus  produced  is  better 


FIGS.  713,  714.  —  A  branch  of  the  India-rubber  tree  (Ficus  elastica),  illustrating  a 
method  of  inducing  the  development  of  adventitious  roots  to  facilitate  artificial  propa- 
gation; an  incision  is  made  in  the  stem,  which  then  is  wrapped  in  wet  moss  (713); 
shortly,  roots  develop  at  the  cut  surface  (714);  note  the  fall  of  the  sheathing  stipule 
(s)  in  714. 

fitted  to  utilize  commercial  fertilizers,  which,  for  the  most  part,  remain  in  the  super- 
ficial soil  layers;  furthermore,  the  removal  of  roots  from  the  soil  is  facilitated. 
Nursery  trees  thrive  better  than  native  trees  when  transplanted,  chiefly  because  a 
compact  root  system  is  developed  by  frequent  transplanting  ;  if  a  tap  root  is  allowed 
to  grow  to  a  considerable  depth,  successful  transplanting  becomes  difficult.  Such 
plants  as  are  used  for  their  roots  (e.g.  parsnips  and  carrots)  rarely  are  transplanted, 
since  the  process  usually  destroys  the  very  parts  desired  ;  however,  when  care  is 
exercised  in  removing  seedlings  from  the  ground,  the  tap  root  in  some  cases  con- 


ROOTS   AND   RHIZOIDS 


5°3 


tinues  to  grow  when  transplanted,  as  in  beets.  In  some  plants,  as  in  the 
radish  and  the  Windsor  bean  (Vicia  Faba),  when  the  tip  is  cut  just  below 
the  region,  of  active  growth,  the  primary  root  continues  its  activity,  ultimately 
regenerating  a  new  tip.  If  the  root  is  cut  above  this  region,  further  growth  is 
checked  and  lateral  roots  take  its  place,  as  described  above. 

Adventitious  roots. — Adventitious  roots  (i.e.  roots  arising  from  stems 
or  leaves)  reach  their  greatest  development  in  plants  with  horizontal 
stems  (fig.  712;  also  figs.  978,  983).  At  first  there  is  in  such  plants  a 
primary  tap  root  and  an  erect  stem,  but  soon  a  horizontal  stem  develops 
beneath  or  just  above  the  surface,  whereupon  adventitious  roots  issue 
from  the  nodes,  and  the  primary  root  and  stem  soon  die.  In  bulbous 
plants  a  number  of  roots  of  about  equal  size  develop  at  the  base 
(fig.  991).  If  such  a  bulb  is  removed  from  the  soil  at  maturity  and 
stored  in  a  dry  place,  these  roots  die,  but  new  adventitious  roots 
develop  rapidly,  when  there  is  access  to  water. 

While  adventitious  roots  usually  are  observed  in  plants  with  runners 
or  with  underground  stems,  many  other  plants  are 
able  to  develop  such  roots  upon  occasion.  When 
long  stems  of  the  black  raspberry  bend  over 
until  they  touch  the  ground,  adventitious  roots 
develop;  this  habit  is  made  use  of  in  artificial 
propagation,  since  a  new  shoot  develops  where  the 
stem  strikes  root.  The  same  principle  is  utilized 
in  the  artificial  propagation  of  the  rubber  plant 
(figs.  713,  714).  If  wet  moss  is  tied  about  a 
stem,  root  development  at  that  point  is  soon  in- 
cited, whereupon  the  stem  is  cut  off  below  the 
moss  and  placed  in  soil.1 

Various  swamp  plants,  such  as  the  reed  and  the  swamp 
loosestrife  (Decodon  verticillatus\  strike  root  where  the 
stem  comes  in  contact  with  wet  soil,  and  the  stems  of 
various  willows  and  dogwoods  (as  Cornus  siolonifera) 
develop  roots  in  abundance  when  placed  in  water  or 
in  moist  soil  (fig.  715).  Such  behavior  is  of  immense  ad- 
vantage in  the  case  of  plants  that  are  partly  buried  by 
accumulating  humus  or  sand,  for  as  fast  as  the  stems 
are  buried,  new  adventitious  roots  appear  at  higher  levels, 


FIG.  715.  — A  winter 
shoot  of  a  willow  (Salix), 
which  has  been  placed 
in  water ;  note  the  adven- 
titious roots  that  have  de- 
veloped at  the  nodes. 


1  A  remarkable  development  of  adventitious  roots  is  seen  on  the  trunks  of  some  tree 
ferns  (as  Dicksonia),  where  they  may  appear  in  such  abundance  as  to  enclose  the  trunk  in 
a  spongy  mass. 


5°4 


ECOLOGY 


supplementing  the  original  root  system.  Some  stems,  such  as  those  of  pines  and 
oaks,  are  unable  to  develop  roots  in  this  manner,  a  fact  that  may  account  for  the 
early  death  of  these  trees  when  partly  buried,  in  contrast  with  the  phenomenal 
success  of  poplars  and  willows  under  similar  conditions. 

Adventitious  roots  occur  less  frequently  on  leaves  than  on  stems.  The  walking 
fern  (Camptosorus  rhizophyllus)  strikes  root  at  the  leaf  tip,  much  as  does  the  rasp- 
berry at  the  stem  tip,  and  in  a  similar  way  gives  rise  to  a  new  plant.  While  not 
particularly  common  in  nature,  leaf  roots  are  induced  readily  in  a  number  of 
species  when  the  leaves  are  removed  from  the  parent  plant  and  placed  in  contact 
with  moist  soil.  In  this  way  gardeners  propagate  various  plants,  such  as  Begonia, 
Peperomia,  and  Sansevieria  (figs.  933,  934). 

Probably  moisture  is  the  chief  factor  determining  the  development  of  adventitious 
roots,  not  alone  in  water,  but  also  in  soil,  and  in  instances  like  those  noted  in  the 
rubber  plant  and  in  the  tree  ferns.  However,  given  equal  moisture,  adventitious 
roots  appear  to  develop  most  on  darkened  and  on  under  surfaces  (fig.  933);  prob- 
ably contact  with  solid  bodies  also  is  an  important  stimulative  factor. 

Root  contraction.  —  When  a  "  stemless  "  rosette  plant,  as  the  rock 
cress  or  dandelion,  grows  in  a  crevice,  the  position  of  the  rosette  in  rela- 
tion to  the  surface  is  year 
by  year  the  same  (figs.  716, 
717).  The  very  short  stem 
elongates  slightly  each  year, 
and  the  root  pulls  the  plant 
by  that  much  into  the  soil, 
so  that  the  rosette  remains 
at  a  constant  level.  This 
phenomenon  is  called  root 
contraction;  the  mechan- 
ism of  the  process  is  not 
fully  understood,  but  its 
advantage  to  the  plant  is 
very  evident.  Sometimes 
the  contractile  region  of  the 
root  becomes  less  than  two  thirds  its  original  length  (the  shortening 
may  exceed  fifteen  •  millimeters  in  Arisaema  Dracontium)',  after  con- 
traction the  root  exhibits  transverse  folds,  and  observation  shows  that 
the  contractile  tissue  is  chiefly  the  cortical  parenchyma  (fig.  720). 

In  white  clover  and  in  similar  plants,  erect  or  ascending  stems  are  pulled  down 
by  the  adventitious  roots,  which  develop  at  the  nodes  and  hold  the  stems  tightly  to 
the  soil.  The  varying  soil  levels  which  characterize  underground  stems  of  different 
species  often  are  reached  through  stem  activities  (see  figs.  718,  719).  As  crevice 


FIGS.  716,  717. —  Diagrammatic  soil  sections, 
illustrating  the  lowering  of  a  rosette  plant  (Arabis 
lyrata)  from  year  to  year  by  root  contraction:  716, 
a  two-year-old  plant  with  rosette  leaves  (r)  at  the 
surface,  and  with  remains  of  the  previous  rosette  (rr) 
deeper  down;  717,  a  three-year-old  plant,  which  has 
remains  of  a  rosette  two  years  old  (r"),  now  deeply 
buried;  these  diagrams  also  illustrate  multicipital 
stems  (p.  676). 


ROOTS   AND   RHIZOIDS 


5°5 


roots  grow  in  diameter,  the  pressure  often  becomes  sufficient  to  shatter  rocks,  so 

that  plants  contribute  materially  to  rock  disintegration. 

Propagation  by  roots.  —  As  a  rule,  roots  do  not  give  rise  to  buds,  nor  do  they 

bear  stems  and  leaves.  In  some  plants,  notably  the  silver  poplar  and  the  osage 
orange,  stems  develop  habitually  from  ordinary  soil 
roots,  resulting  in  a  spreading  colony  of  young  trees 
around  the  parent  tree.  Many  trees  and  shrubs, 
especially  various  poplars  and  willows,  manifest  this 
phenomenon  to  a  marked  degree,  if  the  roots  are 
exposed  to  the  air.  In  the  hawthorn  and  probably 
in  many  plants,  roots  can  be  induced  to  develop 
shoots,  if  the  entire  stem  system  is  removed  and  the 
roots  are  exposed  to  air  and  light  (figs.  721,  722). 
The  exact  factors  that  stimulate  stem  growth  in  these 

ii 


719 


FIGS.  718,  719,  720.  — 
The  descent  of  roots  and 
rhizomes  in  the  soil:  718,  a 
young  individual  of  Erythro- 


721 


FIGS.  721,  722.  —  Diagrammatic  sections  of  an  eroding 
clay  bluff,  illustrating  the  development  of  shoots  from 
roots:  721  represents  a  hawthorn  tree  (Crataegus)  at  the 


nium  mesachoreum,  showing     ed8e  of  the  bluff»  s  DeinS  the  original  trunk;    subsequent 


a     descending     rhizome    or 
"dropper"  (d,  see  p.  675); 


erosion   (722)   has  caused   the   destruction  of  the  trunk 
and  the  exposure  to   the  air  of  a  root   (r),  which  has 


w  ^         f  farm  _The  f 

J 


719,  an  older  descending  in-     Siven  rise  to  a  new  shoot  (*')• 

dividual,  showing   remnants         it_  ... 

r  .,     r  ,.  rather  unusual  conditions  are  not  known.     Near  the 

of  the  five  preceding  years; 

note   the  increasing  size  of     growmS  Point  wlthm  the  root  of  Habenaria  Michauxii 
the  bulb  each  year;    720,  an     there  is  organized  a  bud  that  later  develops  into  an 
individual  of  Zygadenus  Fre-     underground  stem. 
morti,  illustrating  descent  by 
root   contraction;    note    the 

contractile  roots  of  the  cur-    plants,  as  a  rule,  are  extensively  developed, 

rent  (r)  and  the  preceding  (r')     a£   least   in   comparison   With    the   shoots    (figs. 

723-725).    Some  plants  have  long  and  slender 

roots  that  reach  to  great  depths  or  that  have  considerable  lateral  exten- 
sion. In  other  cases  roots  of  immense  size  are  formed.  In  alpine 
regions  there  are  dwarfed  shoots  associated  with  roots  of  ordinary  size 
(fig.  870),  and  there  are  many  such  plants  in  dry  sandy  soil.  Per- 


5°6 


ECOLOGY 


haps  the  ease  with  which  growing 
roots  penetrate  sand  accounts  in  part 
for  their  great  length.  While  the 
initial  root  system  of  most  trees  seems 
scarcely  plastic,  later  development 
may  vary  with  the  habitat.  For  ex- 
ample, in  swamps  the  red  maple  soon 
loses  its  tap  root,  while  the  lateral  roots 
develop  extensively;  in  dry  grounds, 
however,  the  tap  root  persists  and 
most  of  the  lateral  roots  disappear. 

The  advantage  of  great  root  develop- 
ment in  xerophytes  is  clear  enough, 
since  the  more  extensive  the  root 
system,  the  greater  is  the  area  of  ab- 
sorptive surface  and  the  greater  the 
likelihood  of  root  contact  with  remote 
supplies  of  water.  The  advantage  of 
short  roots  to  swamp  plants  is  not 
clear;  indeed,  short  roots  often  fail 
to  afford  adequate  anchorage,  though 
they  may  be  sufficient  for  absorption. 
The  advantages  of  long  roots  to  xero- 
phytes are  so  obvious  that  often  they 
have  been  regarded  as  sufficiently  ex- 
water  and  showing  luxuriant  shoot  de-  plained  thereby ;  such  an  assumption 

velopment;    724  was   grown   with   an  . 

under-supply   of    water,   and  shows  a     fails   to   ^Cognize   that    plants    cannot 

reduced  root  system  and  a  shoot  yet     adapt    themselves,   but    must   react    to 

the  stimuli  which  influence  them  (p. 
947).  If  such  reactions  happen  to  be 


724 


725 


FIGS.  723,  724,  725.  —  Seedlings  of 
the  creosote  bush  {Larrea  tridentata), 
illustrating  variations  in  relative  root 
development  in  a  desert  plant:  723  is 
a  seedling  grown  with  an  excess  of 


more  reduced;  725  is  a  natural  seed- 
ling from  the  desert  and  shows  a  much 
greater  root  development  in  proportion 


to  the  shoot;  all  show  a  strong  devel-  favorable,  the  plant  thrives,  but  un- 
favorable reactions  result  in  injury  or 
in  death. 


opment  of  the  primary  root  with  but 
slight  development  of  laterals;  725  is 
drawn  on  a  much  smaller  scale  than  are 
723  and  724. — After  SPALDING  (drawn 
from  a  photographic  reproduction). 


Our  experimental  knowledge  of  roots  is 
not  yet  sufficient  to  enable  us  to  explain 
the  variations  in  their  form  and  size,  but  some  suggestions  may  be  ventured.  In  the 
first  place  it  is  not  certain  that  the  roots  of  xerophytes  actually  are  longer  l  than  those 

1  Some    recent   studies   show   that   many  cacti  have   relatively  small  root  systems 
which  are  close  to  the  surface,  and  extend  laterally  rather  than  deeply. 


ROOTS   AND    RHIZOIDS 


5°7 


of  other  plants,  though  that  claim  often  is  made.  In  the  light  of  the  experimental 
fact  that  the  optimum  development  of  roots  occurs  in  a  comparatively  moist  soil, 
it  seems  unlikely  that  plants  of  a  given  species  should  have  longer  roots  in  dry  soil 
than  elsewhere,  although  it  is  possible  that  for  certain  species  the  optimum  develop- 
ment may  be  found  where  the  percentage  of  water  in  the  soil  is  comparatively  low.1 
All  that  may  be  stated  with  certainty  is  that  the  roots  of  xerophytes,  as  compared 
with  those  of  mesophytes,  are  reduced  much  less  than  are  the  shoots,  a  phenomenon 
that  is  not  especially  difficult  to  understand.  In  arid  regions  the  aerial  organs  of 
plants  are  subject  to  excessive  transpiration,  a  process  that  retards  growth  to  an 
amazing  extent ;  the  roots  of  desert  plants,  on  the  other  hand,  are  relatively  free 
from  the  inhibitory  influence  of  transpiration. 

Whatever  may  be  true  of  roots  in  dry  soils,  there  is  no  doubt  that  roots  in 
swampy  soils  are  short,  not  only  as  compared  with  the  shoots,  but  actually  short,  as 
compared  with  roots  in  other  habitats.  Probably  the  meager  development  of 
swamp  roots  is  explained  by  such  factors  as  insufficient  oxygen,  soil  toxicity,  and 
low  temperature.  Growth  is  known  to  be  inhibited  by  lack  of  oxygen,  a  gas  in 
which  swamp  soils  are  relatively  poor.  If  root  excretions  and  products  of  plant 
decay  contain  deleterious  or  toxic  substances,  the  poor  drainage  and  oxidation  in 
swamps  would  lead  to  their  excessive  accumulation.  Low  temperature,  at  least  in 
comparison  with  that  cf  the  air,  characterizes  bog  soils,  and  it  is  known  that  low 
temperature  retards  root  development  as  well  as  absorption. 

If  further  study  should  show  that  generally  the  roots  of  xerophytes  are  not 
only  relatively  but  absolutely  more  extensive  than  are  those  of  mesophytes,  it 
is  not  to  be  concluded,  in  the  face  of  opposing  experimental  data,  that  long 
roots  are  a  reaction  to  arid  soils.  It  is  possible  that  such  xerophytes  are  by 
inheritance  long-rooted.  If  a  hemlock  and  a  red  cedar  are  grown  side  by  side  in 
similar  conditions,  the  former  has  a  meager  and  the  latter  an  extensive  root  system. 
In  the  course  of  evolution  short-  and  long-rooted  species  are  likely  to  have  originated 
in  deserts  ;  of  these  the  long-rooted  forms  are  the  more  likely  to  have  survived  the 
arid  conditions.  Similarly  both  short-  and  long-rooted  forms  are  likely  to  have 
originated  in  mesophytic  habitats,  where  survival  is  less  a  matter  of  the  root 
system  than  of  stem  and  foliage  characteristics.  Obviously  much  further  experiment 
is  necessary  before  we  may  know  to  what  extent  root  variations  are  reactions  to 
environment,  and  to  what  extent  they  are  congenital  or  characteristic  of  the  species. 

Meager  development  is  not  the  sole  characteristic  of  swamp  roots;  in 
the  tamarack  and  in  many  other  plants  the  prevalent  direction  of  root 
growth  is  horizontal  rather  than  downward.  In  some  cases  roots  grow 
directly  upward,  as  in  certain  palms  and  mangroves  and  in  Jussiaea,  or 
there  are  erect  projections  from  horizontal  roots  (known  as  knees),  as  in 
the  bald  cypress  (fig.  726).  The  exact  cause  of  these  reactions  is  not 
known,  but  it  is  probable  that  the  small  percentage  of  oxygen  in 
swamp  waters  and  the  accumulation  of  deleterious  root  excretions  and 

1  It  has  been  ascertained,  for  example,  that  the  roots  of  some  species  (as  the  cotton- 
wood)  are  longer,  slenderer,  and  nearer  the  surface  in  dry  sand  than  in  moist  clay. 


5o8 


ECOLOGY 


products  of  decaying  vegetation  influence  the  direction  as  well  as  the 
length  of  roots.  It  is  likely  that  the  inhibitory  influence  of  these  factors 
increases  with  the  depth ;  the  greater  amount  of  oxygen  in  the  surface 


FIG.  726.  —  A  cypress  swamp  at  low  water;  note  the  erect  growths  (knees)  from 
the  roots  of  the  bald  cypress  (Taxodium)  and  the  buttressed  bases  of  the  trunks; 
intermingled  with  the  cypress  (the  large  trees  in  the  foreground  with  bark  shredding 
vertically)  are  specimens  of  tupelo  (Nyssa  aquatica;  small  trees  in  the  background); 
Paragould,  Arkansas.  —  Photograph  by  MEYERS. 

layers  may  facilitate  growth,  in  part  directly,  and  in  part  indirectly, 
through  the  partial  destruction  of  deleterious  organic  compounds  by 
oxidation. 

In  a  stagnant  swamp  the  surface  layer  is  the  only  place  where  at  the  same 
time  water  and  oxygen  are  available  for  root  activity,  hence  it  would  seem  that  hori- 
zontal roots  are  best  fitted  to  thrive  in  such  habitats.  The  explanation  of  such  erect 
roots  or  root  branches  as  those  of  the  bald  cypress  and  of  the  mangroves  is 
difficult.  The  erect  growth  certainly  is  a  reaction  to  some  condition  in  the  swamp, 
since  knees  do  not  develop  when  the  bald  cypress  is  cultivated  in  uplands.  Oddly 
enough,  knees  do  not  develop  in  deep  water,  but  only  in  shallow  water  or  in  swamps. 
If  these  peculiar  structures  are  regarded  as  reactions  to  a  slight  oxygen  content, 
it  is  difficult  to  account  for  their  absence  in  deep  water,  unless  it  be  supposed  that 
the  life  conditions  there  are  too  poor  to  permit  of  growth.  The  erect  roots  of  Jus- 
siaea  are  spongy  structures  resembling  aerenchyma  (p.  553),  and  they  arise  from 
stems  instead  of  from  roots;  possibly,  like  aerenchyma,  they  develop  where  trans- 
piration is  checked. 


ROOTS   AND    RHIZOIDS 


509 


Much  has  been  said  concerning  the  advantage  of  cypress  knees  and  of  horizontal 
swamp  roots,  the  prevalent  view  being  that  respiration  is  facilitated  by  these 
structures.  The  efficiency  of  cypress  knees  as  aerating  organs  is  not  known  ; 
although  their  tissues  are  rather  loose,  the  knees  are  covered  with  bark,  and  often 
are  clothed  with  dense  layers  of  mosses  and  liverworts,  which  must  retard  the  pene- 
tration of  gases.  Furthermore,  knees  are  absent  in  deep  water,  where  aeration  is 
most  needed.  The  shallow  horizontal  root  system  of  the  tamarack  seems  disad- 
vantageous in  part,  since  it  does  not  afford  adequate  anchorage;  severe  storms 
may  overturn  these  trees  in  considerable  numbers.  Probably  horizontal  surface 
roots  in  swamps  are  of  real  advantage  in  facilitating  aeration,  though  this  view  is 
based  upon  assumption  rather  than  upon  experiment. 

The  plank  roots  of  tropical  trees,  resembling  boards  on  edge,  are  due  to  excessive 
growth  on  the  upper  sides  of  soil  roots;  the  causative  factors  are  unknown,  but  it 
is  possible  that  growth  is  freer  in  the  air  than  in  the  soil.  An  analogous  phenome- 
non, due  chiefly  to  stem  activity,  is  seen  in  various  swamp  trees,  as  the  tupelo  and 
the  bald  cypress,  and  to  some  extent  in  the  elm,  the  base  of  the  trunk  being  greatly 
enlarged  (fig.  726).  Possibly  enlarged  buttressed  trunks  and  plank  roots  are  of 
advantage  in  holding  trees  in  place  where  the  soil  roots  are  relatively  inefficient  as 
anchorage  organs. 

Correlation  of  roots  and  leaves.  —  The  ecological  behavior  of  a  plant  cannot  be 
determined  from  one  set  of  organs.  For  example,  the  hemlock  and  the  red  cedar 
have  somewhat  similar  leaves,  being  small,  thick-skinned,  and  apparently  fitted  to 
withstand  excessive  transpiration.  But  while  the  red  cedar  can  thrive  in  very  xero- 
phytic  situations,  the  hemlock  thrives  best  in  mesophytic  woods  with  such  large- 
leaved  trees  as  the  sugar  maple.  It  is  believed  that  the  differing  habitats  of  these 
two  trees  is  a  matter  of  correlation  between  roots  and  leaves,  the  red  cedar  having 
an  extensive  root  system  and  the  hemlock  one  that  is  more  meagerly  developed. 
Jt^ would  appear  that  the  ratio  between  absorption  and  transpiration  determines  the 
habitat  for  which  a  plant  is  fit.  It  should  be  emphasized,  however,  that  the  lack  of 
experimental  evidence  makes  this  theory  of  correlation  as  yet  merely  a  plausible 
hypothesis  (see  also  p.  747). 


2.    WATER  AND  AIR  ROOTS; 
RHIZOIDS 

Water  roots.  —  The  most  repre- 
sentative water  roots  are  found  in 
plants  that  are  not  attached  to  a 
substratum,  such  as  the  duckweeds 
(fig.  727)  and  the  water  hyacinth. 
Such  roots  are  not  numerous  nor 
large,  and  in  most  cases  branches  are 
few  and  root  hairs  wanting  ;  in  some 
duckweeds  there  is  but  a  single  small 


FIG.  727. — Plants  of  a  duckweed 
(Spirodela  polyrhiza)  floating  free  on 
the  water,  showing  flat  thalloid  shoots, 
from  each  of  which  there  depend 
several  water  roots.  Each  thallus  or 
frond  represents  a  single  individual  hat 
has  arisen  from  another  such  frond 
vegetatively ;  the  mother  and  the  daugh- 
ter fronds  remain  attached  for  a  time 
in  colonies. 


ECOLOGY 


root  to  a  plant.  Thus  the  modifications  induced  when  soil  roots  are 
grown  in  water  (viz.  reduction  in  size  and  in  hair  production)  are  seen 
to  be  the  usual  features  in  aquatic  roots.  The 
cause  of  reduction  and  of  hairlessness  in  water 
roots  is  unknown,  though  it  is  likely  that  the 
conclusions  reached  in  the  case  of  soil  roots  are 
applicable  here. 

In  some  water  plants,  as  Ceratophyllum,  Utricularia, 
and  Salvinia  (fig.  897),  roots  are  wanting  and  absorption 
is  confined  to 
the  leaves  and 
stems.  The 
duckweeds 
may  have  sev- 
eral roots  (as 
in  Spirodela), 
one  root  (as  in 
Lemna),  or  no 
root  (as  in 
Wolff ia);  in 
all  three  there 
is  a  thallus 
rather  than  a 
leafy  stem, 


FIG.  728.— The  tip 
of  a  water  root  of  a 
water  hyacinth  (Eich- 
hornia  speciosa),  show- 
ing the  root  pocket, 
which  fits  over  the  root 
like  a  glove  finger; 
considerably  magni- 
fied. 

and    in    the 

rootless  Wolffia,  absorption  takes  place 
in  the  same  manner  as  in  the  algae. 

In  water  roots  the  outer  layers  do 
not  become  impermeable  with  age,  so 
that  absorption  takes  place  through 
the  entire  surface  instead  of  through 
the  tips  only,  as  in  soil  roots.  Water 
roots  are  of  no  value  as  anchorage 
organs,  but  they  may  assist  in  the 
maintenance  of  equilibrium.  Many 
aquatic  roots  contain  chlorophyll,  and 


FIG.  729. — A  tropical   epiphytic  orchid 
(Epidendrum  ramosum),  showing  aerial   ab- 


it  may  be  that    food   manufacture  is     sorptive  roots  arising  adventitiously  from  the 


nodes,  and  mostly  just  above  the  leaves, 
which  exhibit  distichous  phyllotaxy  (i.e.  with 
leaves  alternating  in  two  vertical  rows,  p.  549). 


an  accessory  role  of  some  importance. 

An  interesting  feature  of  water  roots 

is  the  root  pocket,  a  structure  that  fits 

over  the  root  tip  like  a  glove  finger 

(fig.  728).     Although  root  pockets  are  much  more  conspicuous  than  are  the  root 

caps  of  soil  roots,  their  advantage  to  the  plant  is  less  evident.     Roots'  intermediate 

between  soil  and  water  roots  are  found  in  various  attached  aquatics,  such  as  Elodea 

and  Myriophyllum.     Horizontal  branches  give  rise  to  hairless  unbranched  roots 


ROOTS   AND    RHIZOIDS 


511 


resembling  water  roots,  but,  as  soon  as  they  enter  the  soil,  they  branch  freely  and 
produce  hairs. 

b 


Absorptive  air  roots.  —  Struc- 
tural features.  —  Many  plants 
(known  as  epiphytes]  grow  on  the 
branches  of  trees,  where  the  con- 
ditions for  absorption  are  much 
poorer  than  in  the  soil.  In  many 
tropical  orchids  and  aroids  (Ara- 
ceae),  the  aerial  roots  possess 
specialized  absorptive  organs 
(fig.  729).  These  roots  often  are 
silvery  white  except  for  the  green- 
ish tips,  and  their  most  distinc- 


n  c  x 


FIG.  730. — A  cross  section  of  an  aerial 
absorptive  root  of  a  tropical  epiphytic  orchid, 
showing  the  velamen  (?/),  the  exodermis  or 
outermost  cortical  layer  (#),  the  main  body 
of  the  cortex  (c),  the  endodermis  or  inner- 
most layer  of  the  cortex  (ri),  and  the  con- 
ductive region  (6).  The  velamen  (v)  repre- 
sents the  epidermis,  and  is  composed  of  dead 
cells,  which,  when  dry,  absorb  water  with 
great  rapidity;  considerably  magnified. 


tive  feature  is  the  outermost  or 
epidermal  layer,  known  as  the 
velamen,  which  usually  is  a  num- 
ber of  cells  thick  (fig.  730).  At  maturity  the  cells  are  dead  and  the  walls 
are  variously  thickened  by  reticulated  or  spirally  arranged  fibers  (fig. 
731).  The  outermost  cortical  layer,  the  exodermis,  occurs  beneath 
the  velamen  as  a  sheath  of  cells  with  walls  strongly  thickened  by  cutin 

or  cork.  Some  cells  in  this  layer, 
known  as  transfusion  cells,  remain 
with  relatively  unthickened  walls 
and  are  said  to  serve  as  passage- 
ways for  water  from  the  velamen 
to  the  cortex  (fig.  732).  The  cor- 
tical cells  resemble  those  in  soil 
roots,  except  that  they  contain  an 
abundance  of  chlorophyll,  which 
accounts  for  the  greenness  of  the 
roots  when  wet  (fig.  733). 

Role.  —  Water  can  be  taken 
up  with  rapidity  by  the  velamen 
when  dry,  the  process  being  a 
capillary  phenomenon  and  com- 
parable to  the  absorption  of  water 
by  blotting  paper,  and  quite  un- 
like absorption  by  root  hairs. 


731  "  732 

FIGS.  731,  732,  733. — Cells  from  various 
regions  of  an  aerial  absorptive  root  of  a 
tropical  epiphytic  orchid,  as  seen  in  cross 
section:  731,  a  velamen  cell,  showing  the 
characteristic  fibrous  thickening  of  the  wall; 
732,  three  exodermis  cells,  showing  con- 
siderable wall  thickening,  especially  toward 
the  cortex  (lower  side  in  figure);  733,  a  cor- 
tical cell,  showing  chloroplasts  with  included 
starch  grains ;  all  figures  highly  magnified. 


ECOLOGY 


On  the  other  hand,  the  passage  of  water  and  salts  from  the  velarrieri 
through  the '  transfusion  cells  to  the  cortex  is  a  slow  osmotic  process 
quite  comparable  to  absorption  by  root  hairs.  The  velamen  is  an 
organ  of  water  accumulation  as  well  as  an  organ  of  absorption,  and* 
it  retains  water  for  hours  and  even  for  days.  Its  significance  is  still 
more  obvious  when  it  is  realized  that  only  liquid  water  can  be  utilized 

by  plants;  orchids  even  decrease  in  weight 
in  moist  chambers  unless  watered.  Hence 
it  is  not  surprising  that  epiphytes  with  ab- 
sorptive roots  are  confined  to  warm  and 
humid  climates,  where  rain  or  dew  is  almost 
continually  available. 


u 


FIG.  734. — An  outline,  as 
seen  in  cross  section,  of  a  dor- 
si  ventral  orchid  root  (Aeranthus 
fasciola), showing  the  expanded 
upper  portion  (it),  which  con- 
tains most  of  the  chlorophyll. 
—  After  JANCZEWSKI. 


The  chlorophyll  which  is  present  in  air  roots 
probably  is  of  some  importance  in  food  manufac- 
ture. Doubtless  the  presence  of  chlorophyll  in  these 
organs  is  related  to  light,  since  various  soil  roots 
develop  chlorophyll  when  exposed  to  it.  In  Tae- 

niophyllum  and  in  similar  orchids  with  greatly  reduced  stems  and  leaves,  the 
roots,  which  are  the  chief  food-making  rgans,  are  flattened  rather  than  round, 
and  lack  the  usual  radial  structure.  The  lighted  side  has  a  thick-walled  exoder- 
mis  and  prominent  cortical  chlorophyll  but  little  or  no  velamen,  while  the 

shaded  side  has  a  strong  velamen,  a  thin- 

walled  exodermis,  and  abundant  root  hairs 
(fig.  734).  Absorptive  air  roots  play  only 
a  small  part  in  anchorage,  though  in 
some  cases,  especially  in  the  flattened  roots 
just  cited,  they  adhere  closely  to  the  tree 
branches. 


-  735-  — A  portion  of  a  progeo- 
tropic  rhizophore  of  Selaginella  apus, 
densely  clothed  with  horizontal  root 
hairs,  as  a  result  of  growth  in  a  moist 
chamber;  considerably  magnified. 


FIG.  736.  — A  portion  of  the  stem  of 
a  liana  (Philodendron  melanochrysuni) 
with  horizontal  adventitious  roots  clasp- 
ing the  trunk  of  a  tree  (Canarium),  thus 
serving  to  anchor  the  liana  to  its  sup- 
port. —  From  WENT. 


ROOTS   AND   RHIZOIDS  r^ 

Variations.  —  The  high  specialization  and  the  obvious  role  of  absorptive  air 
roots  give  the  question  of  their  origin  much  interest,  but  the  lack  of  experimental 
data  makes  the  solution  of  the  problem  very  difficult.     The  most  characteristic 
feature  of  these  roots,  the  vela- 
men,  is  possessed  by  nearly  all 
aerial  roots  and   by  almost    no 
soil  roots,  except  in  a  few  forms, 
such  as  Bletia  and  Spiranthes 
cernua;  thus  the  medium  seems 
to  have  a  rather  definite  relation 

to  the  formation  of  the  velamen .  TTr-AXJ-^^bH^--  C 
The  most  obvious  reaction  of 
air  roots  to  changed  conditions 
is  in  hair  production.  Most 
air  roots  are  hairless  ;  some  (as 
in  species  "of  A  nthurium}  com- 
monly possess  hairs,  and  root 
hairs  develop  in  a  number  of 
species  when  the  roots  are  ex- 
posed to  unusually  moist  air 
(fig-  735)-  A  curious  exception 
to  the  usual  kind  of  root  hair 
is  found  in  some  air  roots,  where 
the  hairs  are  stiff  and  rigid 
structures  of  long  duration. 


Anchoring  air  roots.  — 

Some  climbing  plants,  as 
poison  ivy  and  English  ivy, 
are  anchored  to  the  sup- 
porting trees  by  adventi- 
tious roots,  which  clasp  the 
trunk  or  penetrate  into 
bark  furrows.  Such  roots 
grow  horizontally  rather 
than  downward  (fig.  736), 


FIGS.  737,  738. — Root  cross  sections  of  Philoden- 
dron  lacerum,  a  tropical  liana:  737,  a  nutritive  root 
with  many  large  conductive  vessels  (v\  outside  of 
which  is  a  sheath  of  thick- walled  sclerotic  cells  (s), 
cortical  parenchyma  (c),  and  epidermis  (e);  738,  an 


notably  in   Certain   tropical     anchoring  root  with   much  smaller   vessels   (v)  and 
climbers,  whose  roots  are  as     with  a  much  thicker  sclerotic  sheath  (s)'t  the  nutritive 

root  is  much  the  larger,  the  two  figures  being  equally 

magnified.  —  From  WENT. 


sensitive  to  contact  stimuli 

as   are  many  tendrils.     If 

these  roots  are  at  all  geotropic,  the  bark  moisture  and  other  influences  are 

sufficient  to  overcome  gravity  and  to  induce  lateral  growth.      Climbing 

roots  commonly  are  supposed  to  be  anchoring  roots  only  and  not  absorp- 


ECOLOGY 


tive,  since  the  plants  also  possess  nutritive  soil  roots ;  these  two  kinds  of 
roots  exhibit  the  structural  contrasts  of  nutritive  and  anchoring  soil  roots, 
but  the  differences  are  more  pronounced  (figs.  737,  738).  In  the  Eng- 
lish ivy  the  anchor  roots  resemble  ordinary  adventitious  roots  in  a  state 
of  arrested  development,  and  they  become  transformed  into  such  roots 
upon  coming  into  contact  with  moist  earth. 

Most  epiphytes  are  anchored  to  the  substratum  by  roots  that  may  or  may  not  be 
absorptive.     For  example,  many  species  of  Tillandsia  are  anchored  to  branches  by 


FIG.  739.  —  The  basal  por- 
tion of  a  maize  stalk  (Zea 
Mays),  showing  prop  roots 
arising  from  the  lower  nodes; 
note  that  the  stalk  becomes 
thicker  upwards. 


FIG.  740. — A  strangling  fig  (Ficus)  that 
began  life  as  an  epiphyte,  but  which  now 
has  absorptive  ground  roots,  as  well  as  roots 
that  envelop  the  tree  (Bischofia  trifoliata)  on 
which  the  fig  started ;  Lamao  Forest  Reserve, 
Philippine  Islands. — From  WHITFORD  (Cour- 
tesy of  the  Philippine  Bureau  of  Forestry). 


wiry  roots  that  absorb  little  or  nothing  from  the  bark,  the  leaves  being  the  chief 
absorptive  organs' (p.  615).  Various  ferns  thrive  equally  well  in  the  ground  or  on 
trees,  but  in  the  latter  case  they  are  anchored  by  ordinary  absorptive  roots. 

Prop  roots.  —  In  various  monocotyls,  such  as  the  screw  pine  (Pan- 
danus),  Indian  corn,  and  certain  palms,  roots  issue  from  the  stem  at 
various  levels,  and  grow  obliquely  downward  into  the  soil;  in  the  air 


ROOTS   AND   RHIZOIDS  515 

they  are  relatively  unbranched,  but  they  branch  profusely  upon  striking 
the  soil  (fig.  739).  Since  monocotyls  possess  but  a  limited  power  of 
diametral  growth,  a  tree  like  the  screw  pine  develops  a  top-heavy  inverted 
cone  instead  of  a  stable  cone,  as  in  dicotyls,  so  that  its  prop  roots  are 
of  great  advantage  in  preserving  equilibrium.  In  the  banyan  and  in 


FIG.  741.  —  The  interior  of  a  mangrove  swamp,  showing  the  prop  roots  of  Rhizophora 
Mangle  together  with  arched  roots  that  make  a  labyrinthine  tangle  near  the  ground; 
Miami,  Florida.  —  Photograph  by  E.  W.  COWLES. 

other  species  of  Ficus,  roots  issue  from  the  horizontal  branches  and 
grow  directly  downward  to  the  ground.  The  support  given  by  such 
roots  makes  possible  the  enormous  spread  of  the  banyan.  Many 
species  of  Ficus  (sometimes  called  strangling  figs)  usually  begin  life  as 
epiphytes,  some  of  the  roots  developing  in  the  dead  bark  and  others 
descending  along  the  trunk  to  the  ground.  After  a  time  all  of  the 
nourishment  comes  from  the  ground,  and  the  supporting  tree  is  likely 
to  be  strangled  by  an  anastomosing  network  of  enveloping  roots  or 
borne  down  by  the  weight  of  the  growing  Ficus  (fig.  740)- 


ECOLOGY 


In  Washington  and  in  British  Columbia  the  hemlock  often  germinates  on 
stumps,  logs,  or  standing  trees,  and  has  a  subsequent  history  somewhat  compa- 
rable to  that  of  Ficus  in  the  tropics. 

The  banyan  habit  is  illustrated  on  a  small  scale  by  Selaginella,  especially  in  moist 
chamber  cultures,  where  there  develop  rhizophores  with  numerous  root  hairs 

(figs.  735,  896).  In  the  mangrove, 
roots  are  put  forth  much  as  in 
Ficus,  but  they  branch  profusely 
in  the  air  and  spread  out  laterally 
(fig.  741).  So  abundant  are  these 
roots  in  mangrove  swamps  that 
they  form  a  dense  network  over 
the  soil,  and  are  of  much  impor- 
tance in  supporting  the  numerous 
branches,  owing  to  the  early  death 
of  the  basal  part  of  the  primary 
trunk.  The  factors  operative  in 
the  production  of  prop  roots  are 
quite  obscure;  in  Selaginella,  water 
seems  to  be  an  important  stimulus. 

Liverwort  and  fern  rhizoids . 

—  Structure.  —  The  rhizoids 
of  liverworts  and  of  fern  pro- 
thallia  commonly  are  color- 
less, unicellular  outgrowths 
of  special  external  cells,  and 
may  or  may  not  eventually  be 
cut  off  therefrom  by  a  cell 
wall  (fig.  742).  They  closely 
resemble  root  hairs  in  struc- 
ture, but  usually  they  are  of 
much  greater  length  and  their 
protoplasm  is  more  thinly 
disseminated.  In  the  Mar- 
chantiaceae,  rhizoids  are  of 
two  kinds,  plane  rhizoids  as 
above  described  and  peg  rhi- 
zoids, in  which  the  cell  wall 
grows  out  internally  into  peg- 
like  or  antler-like  projections. 


744 


FIGS.  742,  743,  744.  —  Plants  of  a  liverwort, 
Riccia :  742,  an  individual  grown  with  soil  con- 
tact and  showing  a  luxuriant  but  thin  and  much- 
lobed  thallus  and  an  abundant  growth  of  rhizoids 
(r)j  743.  a  portion  of  the  individual  figured  in 
742,  transferred  to  water,  and  allowed  to  develop 
there  for  some  weeks;  744,  the  plant  figured  in 
743  at  a  later  stage;  note  that  in  743  and  744  the 
thallus  is  much  smaller  and  thicker  than  in  742, 
and  that  flat  ventral  scales  (v)  have  taken  the 
place  of  rhizoids  (r) ;  through  the  death  of  the  old 
part  of  the  thallus  (0)  the  new  lobes  become  de- 
tached at  the  sinuses,  resulting  in  the  vegetative 
origin  of  a  number  of  inividuals  from  one;  all 
figures  somewhat  magnified. 


The  plane  rhizoids  occur  chiefly  on  the  younger  apical  and  midrib 
portions,  while  the  peg  rhizoids  occur  on  the  older  marginal  portions. 


ROOTS   AND   RHIZOIDS 


Reactions  to  external  conditions.  —  Rhizoids  are  progeotropic,1  prohy- 
drotropic,  and  apophototropic,  thus  agreeing  with  roots  and  differing 
greatly  from  root  hairs.  In  Marchantia 
gemmae  or  fern  prothallia,  rhizoids  may  be 
induced  at  will  on  either  side  of  the  thallus 
'&  by  exposure  to  the  proper  stimuli  (moisture, 
darkness,  contact,  etc.);  after  the  rhizoids 
appear,  their  growth 
direction  may  be 
altered  by  altering 
their  relation  to  light, 
moisture,  or  gravity.1 

When  Lunularia  is 
grown  in  solutions  defi- 
cient in  nitrogen  or  in 
phosphorus,  the  rhizoids 
become  greatly  elongated, 
while  the  thallus  is  poorly 
developed,  recalling  the 
strong  roots  and  the  de- 
pauperate stems  of  xero- 
phytes.  No  rhizoids 
develop  on  Lunularia  in 
pure  water,  but  enough 
salts  are  dissolved  from 

ordinary  laboratory  vessels  to  induce  rhizoids  abun- 
dantly. Very  plastic  liverworts  are  Riccia  natans  and 
R.  lutescens,  plants  that  grow  either  in  soil  or  in  water. 
Ordinary  rhizoids  develop  in  the  soil  (fig.  742),  but 
in  the  water  their  place  is  taken  by  large  and  con- 
spicuous ventral  scales  (figs.  743-746).  Rhizoids,  like 
root  hairs,  are  better  developed  in  xerophytic  than  in 
hydrophytic  forms,  and  aquatic  species  (as  Riccia  fluitans)  commonly  are  without 
them. 


'  746  745 
FIGS.  745,  746.  —  A  ven- 
tral scale  from  the  water  form 
of  Riccia:  745,  the  tip  of 
the  scale,  showing  scattered 
mucilage  cells  (g) ;  746,  a  few 
of  the  scale  cells,  showing 
chloroplasts;  both  figures 
highly  magnified. 


FIG.  747. — A  moss 
plant  (gametophyte), 
showing  the  aerial  stem 
(s)  with  leaves  (/)  in  sev- 
eral vertical  rows;  the 
underground  organs  are 
branched  structures,  the 
rhizoids  (r). 


Rdle.  —  Liverwort  and  fern  rhizoids  obviously  are  anchorage  organs, 
and  generally  they  are  believed  to  be  organs  of  absorption  also,  although 
the  proof  therefor  is  not  so  conclusive  as  in  the  case  of  root  hairs.  Since 
the  thallus  is  close  to  the  ground  and  permeable,  rhizoid  absorption 
may  be  of  minor  value.  The  arguments  for  the  absorptive  r61e  are 
(i)  cementation  to  soil  particles  (fig.  1075)  and  modification  in  rhizoid 

1  From  recent  work  it  appears  that  the  hydrotropism  of  rhizoids  is  much  more  pro- 
nounced than  is  their  geotropism. 


ECOLOGY 


FIG.  748. — A  moss 
rhizoid  with  its 
branches;  note  the  ob- 
lique cross  walls,  im- 
mediately above  which 
branch  rhizoids  origi- 
nate at  a  definite  posi- 
tion ;  highly  magnified. 


form  as  in  root  hairs,  and  (2)  the  observed  rise  of  colored  fluids  and 
the  crystallization  within  the  rhizoids  of  the  Marchantiaceae  of  absorbed 
salts  (as  Berlin  blue).  The  special  advantage  of 
the  peg  rhizoids,  though  much  discussed,  is  not 
known. 

Moss  rhizoids.  —  Structure  and  habitat  variation. 
— Moss  rhizoids  commonly  are  brownish,  branched, 
multicellular  cell  filaments  with  oblique  cross  walls. 
Often  they  have  a  rootlike  aspect,  by  reason  of 
a  strong  central  trunk  with  small  lateral  branches 
(figs.  747,  748).  Some  mosses  (as  Funaria)  de- 
velop very  long  rhizoids  in  solutions  deficient  in 
nitrogen  or  phosphorus.  Rhizoids  develop  abun- 
dantly on  the  aerial  stems  of  some  mosses  (as  Thu- 
idium)  when  grown  in  a  moist  atmosphere;  such 
rhizoids  are  as  brown  as  those  developed  in  the 
soil  in  spite  of  the  exposure  to  light  (figs.  749, 
750).  Rhizoids  are  developed  abundantly  also  in 
xerophytic  mosses,  and  sometimes  in  mosses  that 
frequent  running  waters,  while  they  are  poorly 
developed  or  wanting  in  pond  and  swamp  mosses. 
In  dry  soil  Polytrichum  juniperinum  exhibits  a 
vigorous  development  of  soil  rhizoids,  but  in 
swamps  these  are  largely  replaced  by  aerial  rhi- 
zoids ("stem  felts").  Usually  rhizoids  are  better 
developed  in  erect  mosses  which  have  prominent 
"  vascular "  tracts,  and  which  grow  somewhat 
separately  (as  Polytrichum,  Catharinea,  and 
Mnium)  than  in  mosses  without  such  tracts,  which 
grow  in  dense  cushions  or  mats  (as  Leucobryum, 
and  Sphagnum,  which  has  no  rhizoids). 

Role.  —  Most  moss  rhizoids  clearly  are  organs 
of  anchorage,  though  the  stem  felts  common  in 
swamps  and  in  moist  woods  are  of  no  importance 
in  this  respect.  The  scantiness  of  experimental 
data  makes  it  impossible  to  speak  in  general  terms 
concerning  the  efficiency  of  the  rhizoids  in  absorption.  Probably 
mosses  like  Polytrichum,  which  grow  as  detached  individuals  and  pos- 
sess "vascular  "strands  and  extensive  rhizoids,  are  able  to  absorb  water 


750 

FIGS.  749,  7So.  — 
Aerial  rhizoids:  749,  a 
swamp  moss  as  ordi- 
narily observed;  750, 
the  same  individual  after 
several  days  in  a  moist 
chamber. 


ROOTS   AND   RHIZOIDS 


519 


and  salts  through  the  latter;  indeed,  the  rise  of  colored  liquids  -has 
been  witnessed  in  the  rhizoids  of  Polytrichum.  When  the  rhizoids  of 
Catharinea  are  severed  in  the  soil,  the  leaves  wither  precisely  as  they  do 
in  seed  plants  when  the  roots  are  cut.  Water  drops  appearing  on  cut 
stem  surfaces  of  Mnium 
have  been  supposed  to 
indicate  conduction  and 
hence  rhizoid  absorp- 
tion. In  Polytrichum 
and  in  Catharinea  the 
rhizoids  are  intertwined 
like  the  strands  of  a 
rope,  so  that  doubtless 
water  can  ascend  be- 
tween them  by  capillar- 
ity as  well  as  within 
them.  In  mat  and 
cushion  mosses  it  is  prob- 
able that  leaf  absorp- 
tion, facilitated  by  the 
ready  capillarity  made 
possible  by  close  con- 
tact between  adjoining 
shoots,  is  muchsmore  im- 
portant than  is  rhizoid 
absorption  (p.  611). 

Rhizoids  of  algae  and 
of    lichens.  —  Algae.  — 
Many  small  algae   are 

entirely  Unattached,  and          FIG.  751. — A  marine  alga  (Nereocystis),  showing  a 

move  actively  (Volvox)   rhizoid  complex  or  system  °f  hapf ra;  which  s?ves  f8 

J  '    a  holdfast  organ,  fixing  the  plant   firmly  to  a  rocky  sub- 

or  drift  passively  (Pleu-    stratum  below  sea  level;  note  the  stalk  or  stipe,  whose 
TOCOCCUS).      Other  forms    bladder-like  expansion  floats  the  leaflike  organs  at  the 

are  attached  by  the  muci-  water  surface'  ~  From  CouLTER  (Part  I}' 
lage  they  exude.  Some  filamentous  algae  (as  Ulothrix  and  Oedogonium) 
are  anchored  by  the  basal  cell,  which  may  differ  from  the  other  cells  in 
shape  and  color ;  such  forms  often  are  epiphytic  on  other  water  plants. 
Rootlike  rhizoids  occur  in  Vaucheria,  Botrydium,  and  Chara.  Large 
marine  algae  are  attached  to  rocks  by  much-branched  rhizoids  (or 


ECOLOGY 


Imptera)  of  considerable  size,   though  they  are  simple  and  undiffer- 
entiated  in  structure  (fig.  751).     Anchorage  is  thought  to  be  the  chief 

role  of  such  rhizoids,  and  it  is  an  im- 
portant role,  since  marine  algae  are 
especially  prevalent  along  rocky  coasts, 
where  wave  action  is  violent.  The 
hairlike  rhizoids  of  Bolrydium  (fig.  732) 
and  of  Chara  may  be  of  value  also 
in  absorption,  as  seems  to  have  been 
proven  in  the  case  of  Chara.  Rhizoid 
formation  in  algae  is  determined  largely 
by  contact,  especially  with  rough  sur- 
faces. 

Lichens.  —  Lichens  usually  are  at- 
tached to  the  substratum  by  rhizoids 
(or  rhizines)  which  may  have  hairlike 
or  discoid  tips.  These  rhizoids  are  of 
great  importance  as  holdfast  organs, 
since  lichens  usually  grow  in  exposed 
situations,  where  they  might  otherwise 
be  blown  away.  Lichens  are  able  to 
grow  directly  on  bare  rocks,  and  this 
is  partly  because  the  rhizoids  exude 
substances  which  corrode  the  sub- 
stratum ;  especially  is  this  true  of 
calcareous  lichens,  some  of  which 
become  almost  entirely  embedded  within  the  rock.  Possibly  lichens 
are  able  also  to  absorb  water  and  solutes  through  their  rhizoids. 

Concluding  remarks  on  roots  and  rhizoids.  —  Often  the  progress  of 
evolution  has  been  along  the  line  of  a  division  of  labor.  Such  seems  to 
have  been  the  case  in  the  evolution  of  organs  of  absorption  and  of  anchor- 
age. Among  the  algae  the  entire  plant  body  takes  part  in  the  absorp- 
tion of  water,  salts,  and  gases,  though  primitive  anchorage  organs  are 
represented  by  the  haptera.  In  the  bryophytes,  rhizoids  are  generally 
developed,  and  probably  they  are  organs  of  absorption  as  well  as  of 
anchorage,  though  the  plant  body  still  takes  an  important  part  in  ab- 
sorption. In  the  ferns  and  seed  plants,  the  roots  are  the  chief  anchorage 
organs,  and  the  young  roots  with  their  hairs  are  the  chief  organs  of  water 
absorption,  while  the  leaves  are  largely  without  absorptive  efficiency. 


FIG.  752.  —  A  plant  of  Botrydium, 
showing  the  much-branched  colorless 
subterranean  rhizoid  system  and  the 
globular  green  aerial  portion ;  the  plant 
is  a  coenocyte,  i.e.  multinucleate  but 
without  internal  cell  walls;  consider- 
ably magnified.  —  From  ROSTAFINSKI 
and  WORONIN. 


CHAPTER   II  —  LEAVES 


i.     CHLOROPHYLL   AND   FOOD  MANUFACTURE 

Introductory  statement.  —  Leaves  usually  possess  a  more  or  less  ex- 
panded portion,  the  blade,  which  mayor  may  not  be  borne  on  a  stalk, 
the  petiole.  The  blade  is  composed  of  veins  and  of  the  green  parts 
between  the  veins,  the  mesophyll;  the  latter  is  the  seat  of  food  manu- 
facture, and  the  former  are  organs  of  support  and  transportation.  In 
many  cases  leaves  are  scalelike  and  take  no  part  in  food  making, 
while,  on  the  other  hand,  stems  frequently  have  an  important  part  in 
this  process.  The  chief  foods  manufactured  by  plants  are  carbohydrates 
(such  as  the  various  sugars  and  starches),  fats,  and  proteins  (such  as 
the  albumins).  The  simplest  of 
these  foods,  the  carbohydrates,  are 
manufactured  first,  and  most  of 
our  knowledge  of  food  making 
deals  with  this  synthesis  of  car- 
bohydrates. 

Chloroplasts  and  chlorophyll.  —         FIGS.  753,  754.  —  Chloroplasts:   753,3 


The  Chloroplasts.  —  The  synthesis  cel1  from  a  moss  leaf>  sh°win8 

.         ,     ,      ,  .  i       .  ,  plasts  in   various    stages   of   division;    the 

of  carbohydrates  is  associated  with  bodies  wfthin  the  chloroplasts  are  starch 

the    green    bodies    of    plant    cells,  grains;    754,  a  cross  section  of  a  leaf   of 

the   Chloroplasts,   which   consist    of  Selaginella  Martensii,  showing  two  kinds  of 

chloroplasts,  those  in  the  lower  part  of  the 

a  colorless,  protoplasmic,  sponge-  leaf  being  small  and  resembling  the  chloro. 

like    matrix,    the    Stroma,    suffused  plasts  of   753,    while   those   in   the   upper 

Or    impregnated,    at    least    in    the  epidermis  are  large  and  solitary;    note  the 

.  .  irregular   shape   of   the  latter,  some  being 

peripheral  portions,  with  a  green  more  or  lesg  mortar.shaped,  the  maximum 

pigment,  the  chlorophyll.     In  shape,  surface   exposure   being   toward  the  light; 


chloroplasts    generally  are   some- 


note  also  the  thinness  of  the  leaf  (three  cells 
,   f          thick);  both  figures  highly  magnified. 

what  spheroidal  or  ellipsoidal  (or 

even  polygonal,  if  crowded),  the  number  in  a  cell  varying  from  few  to 
many  (fig.  753;  also  fig.  758).  However,  in  the  Conjugales  and  in  many 
other  algae  there  are  one  to  several  large  chloroplasts  in  a  cell,  which 
may  be.  tabular,  spiral,  cylindrical,  cuplike,  or  stellate  in  shape  (fig.  106). 

521 


522 


ECOLOGY 


In  Anthoceros  and  Selaginella  the  chloroplasts  in  some  cells  are  many 
and  small,  while  in  other  cells  they  are  few  and  large  (fig.  754).  Sun 
plants  commonly  have  smaller  chloroplasts  than  do  shade  and  water 
plants.  The  so-called  chromoplasts  of  carrot 
roots,  nasturtium  flowers,  etc.,  often  are  irregular 
in  shape  (fig.  755). 

The  pigments.  —  Chlorophyll  is  not  a  simple 
green  pigment,  but  it  contains,  in  addition  to  the 
green  pigment  or  chlorophyllin,  a  yellow  pigment 
known  as  xanthophyll  and  an  orange  pigment 
known  as  carotin.  Closely  related  to  xanthophyll 
and  carotin  are  most  yellow,  orange,  and  brown 
pigments  associated  with  color-bearing  bodies 
(chromatophores  or  plastids),  including  those  of 
yellow  flowers,  diatoms,  and  plants  exposed  to 
darkness  (i.e.  etiolated  plants,  whose  pigment  often 

a  p^ht^^ofto  is  called  etiolin^  Ph™°Phy11' the  brown  Pigment 
nasturtium  (Tropaeolum}*  of  the  Phaeophyceae,  is  closely  related  to  chloro- 
phyll. Chlorophyll  differs  widely  in  tint.  The 

some  (0  resembling  leaves  of  succulent  plants,  salt  plants,  and  epiphytes 
chloroplasts  in  shape,  have  a  pale  green  color  that  is  in  strong  contrast 
while  others  (»)  are  ir-  to  tne  dark  green  color  of  beech  leaves  and  of 

regular  in  outline;    these        111  n          rmj  ±-    .        c 

chromoplasts  are  yellow  shade  leaves  generally.  The  deep  green  tints  of 
or  orange  in  color;  in  shade  leaves  may  be  due  in  part  to  the  greater 
addition  there  may  be  a  concentration  of  the  chlorophyllin,  in  part  to  the 

red  pigment,  anthocyan,  .  -  , 

diffused  in  the  cell  sap      PaUClty   °f.  Xanth°- 
(position  here  indicated      phyll,  and  in  part  to 

the  number  and  size 
of  the  plastids. 
Miscellaneous   features.  —  Chloroplasts         FlG  7s6.  _Two  ch]0r0piasts 

contain  various  inclusions,  notably  Starch      of  Rhipsalis  with  several  grains  of 

grains  (fig.  756;  also  fig.733);  the  pyrenoids     starch  (J)  and  many  minute  oil 
.      ,  .       .  .         drops;    highly  magnified. — From 

of  algae  and  of  Anthoceros   are  protein     SCHIMPER 

inclusions,  and  oil  is  common,  especially 

in  dying  plastids.     Plastids  arise  by  division  from  preexisting  plastids 

(fig.  753),  and  perhaps  at  times  de  novo  in  the  cell  cytoplasm,  though 

careful  search  usually  reveals  them,  even  where  their  absence  might  be 

expected  (e.g.  in  embryos).     Although  chlorophyll  usually  occurs  only 

in   chloroplasts,  spectroscopic    tests   show  its   presence   in   the  blue- 


showing  variations  in 
the  form  of  chromoplasts, 


by  lines,  a) ;  highly  mag- 
nified. 


LEAVES  533 

green  algae1  and  in  Cuscuta,  where  ordinary  chloroplasts  probably  are 
lacking. 

The  influence  of  external  factors  upon  chlorophyll  development. 

—  Light.  —  The  general  necessity  of  light  for  chlorophyll  development  is 
shown  by  the  pallor  of  shoots  that  develop  in  the  dark  (e.g.  celery  leaves 
and  potato  sprouts).  However,  less  light  is  needed  in  some  cases  than  in 
others,  as  appears  from  the  chlorophyll  layer  beneath  the  bark  in  trees, 
and  from  the  mosses  that  grow  in  relative  darkness  in  the  mouths  of 
caves.  If  sugars  and  other  foods  are  present  in  sufficient  amount, 
conifer  seedlings,  various  ferns  and  liverworts,  and  many  algae  become 
green,  even  in  total  darkness,  and  a  number  of  angiosperm  seedlings 
become  green  in  the  dark,  if  the  fruits  in  which  the  seeds  develop  are 
produced  in  the  light.2 

On  the  other  hand,  various  algae  have  been  observed  to  lose  their  green  color  when 
grown  in  rich  nutrient  media  in  the  light,  thus  becoming  physiologically  equivalent 
to  fungi.  Cuscuta  develops  chlorophyll  in  sunlight  when  grown  on  a  starved  host  or 
in  water,  and  redwood  shoots  without  chlorophyll  have  been  observed  to  behave 
similarly  when  detached  from  the  parent  tree  and  placed  in  water.  Perhaps  an 
excess  of  food,  though  necessary  for  chlorophyll  production  in  the  dark,  may  be 
less  necessary  or  even  unfavorable  in  the  light.  Carotin  and  xanthophyll  are  less 
dependent  upon  light  than  is  chlorophyllin,  remaining  longer  when  plants  are 
placed  in  darkness,  and  appearing  sooner  when  they  are  placed  in  the  light. 

Temperature  and  salts.  —  Low  temperatures  are  more  detrimental  to  chlorophyllin 
than  to  xanthophyll,  the  latter  appearing  first  in  spring  and  remaining  latest  in 
autumn;  the  yellowing  of  evergreens  in  winter  illustrates  the  same  principle.3  The 
development  of  pallid  shoots  at  low  temperatures  has  been  observed  in  Sequoia 
and  in  Brassica.  Iron  salts  and  nitrates  are  regarded  as  favorable  for  chlorophyll 
development,  whitening  due  to  lack  of  iron  being  called  chlorosis;  common  salt 
impedes  chlorophyll  development,  and  perhaps  is  responsible  for  the  pale  color 
of  salt  marsh  plants,  as  has  been  shown  to  be  the  case  in  Salicornia.  Plants  attacked 
by  parasites  often  show  chlorophyll  impairment. 

Albescence.  —  In  many  plants,  especially  in  certain  variegated  plants  cultivated  for 
ornament  (as  Abutilon  and  C alodium),  the  absence  of  chlorophyll  is  not  obviously 
related  to  external  factors.  Yellow  spots  contain  plastids  colored  with  xanthophyll, 
and  white  spots  lack  even  plastids.  Such  plants  are  propagated  readily  by  cuttings 
and  sometimes  also  by  seed.  The  whitening  or  albescence  of  Abutilon  is  thought 
to  be  due  to  a  virus  (perhaps  an  enzym)  that  is  detrimental  to  chlorophyll  de- 

1  The  blue-green  algae,  however,  are  now  believed  to  possess  a  single  cylindrical  chro- 
matophore  in  each  cell. 

2  Recent  studies  seem  to  show  that  the  green  pigment  of  many  seeds  is  not  chlorophyll, 
though  it  becomes  chlorophyll  upon  exposure  to  light. 

3  However,  a  recent  investigation  shows    that  chlorophyll  sometimes  develops  in 
abundance  at  low  temperatures,  the  minimum  temperature  recorded  being  —  8°  C. 


524 


ECOLOGY 


velopment ;  in  any  event,  green  plants  may  become  variegated  by  grafting.  The 
boundaries  of  the  white  spots  usually  are  veins,  which  perhaps  act  as  barriers  to  the 
virus.  White  spots  exposed  for  a  time  to  darkness  and  then  to  light  become 
green,  as  though  the  virus  were  destroyed  by  darkness.  In  some  plants  (fig.  757), 
but  not  in  all,  the  albescent  leaves  or  parts  of  leaves  are  smaller  than  those  that  are 
green,  indicating  defective  food  migration  from  other  parts. 

The  movements  of  chloroplasts.  —  In  diffuse  light,  plastids  commonly 
are  close  to  the  outer  cell  walls,  and  so  arranged  as  to  expose  a  maxi- 
mum surface  to  the  light 
(epistrophe,  fig.  758).  When 

exposed  for  a  time  to  in- 

| 


tense  light,  the  plastids  for 
the  most  part  move  to  the 
side  or  rear  walls,  to  which 
they  are  closely  appressed, 


c- 


758 


759 


FIG.  757. — An  albescent  leaf 
of  Abutilon;  the  shaded  portions 
represent  the  parts  containing 
chlorophyll,  the  other  portions 
being  colorless;  note  the  greater 
development  of  those  parts  where 
the  chlorophyll  is  more  abundant. 


FIGS.  758,  759.  — Variations  in  the  form  and 
position  of  chloroplasts  (c)  in  I  socles:  758,  a 
surface  view  of  epidermal  cells  that  have  been 
exposed  to  diffuse  light;  the  position  taken 
is  that  of  epistrophe;  759,  similar  cells  that  have 
been  exposed  to  direct  and  rather  intense  sun- 
light, illustrating  apostrophe;  note  that  the 
chloroplasts  near  the  walls  differ  in  shape  from 
those  found  elsewhere;  highly  magnified. 


and  their  chief  axis  is  parallel  rather  than  perpendicular  to  the  incident 
light  (apostrophe,  fig.  759).  If  exposed  to  intense  or  long-continued 
light,  the  plastids  tend  to  become  grouped  in  the  center  of  the  cell 
(sy strophe).  These  movements  commonly  are  believed  to  be  advan- 
tageous. The  advantage  of  epistrophe  is  obvious,  since  light  is  so 
important  for  the  development  and  role  of  chlorophyll.  The  advantage 
of  apostrophe  is  less  obvious,  though  there  are  reasons  for  believing 
that  chlorophyll  is  injured  or  destroyed  in  intense  light.  The  alga 


r'. 


LEAVES  525 

Mougeotia  has  platelike  plastids  which  stand  on  edge  in  intense  light 
and  have  a  horizontal  position  in  diffuse  light. 

The  usual  view  has  been  that  plastids  are  not  actively  motile  but  drift  passively 
in  cytoplasmic  currents,  though  there  is  some  evidence  that  they  exhibit  amoe- 
boid movements,  reacting  chemotactically  to  carbon  dioxid  and  to  other  sub- 
stances unequally  distributed  in  the  cell ;  in  any  case  it  is  clear  that  they  change  their 
form  as  conditions  change  (figs.  758,  759).  Other  factors  than  light  cause  change 
of  position,  notably  changes  in  temperature  and  in  water  content.  In  Dictyota, 
apostrophe  is  caused  by  immersion  in  water  of  higher  concentration  than  sea  water, 
while  immersion  in  water  of  lower  concentration  results  in  epistrophe.  Possibly  the 
plastid  movements  commonly  referred  to  changes  in  light  intensity  may  be  due  in 
part,  at  least,  to  osmotic  changes;  at  all  events,  the  effects  of  reduced  water  supply 
and  of  high  light  intensity  seem  to  be  the  same. 

It  is  likely  that  plastid  motility  has  been  too  much  emphasized.  Intermediate  posi- 
tions are  much  more  common  than  either  apostrophe  or  epistrophe.  Sun  plants 
in  particular  have  plastids  that  are  relatively  immotile,  the  position  of  apostrophe 
(profile  position)  being  usual  in  darkness  as  well  as  in  daylight  (fig.  766).  The 
greatest  motility  is  found  in  water  plants  and  in  shade  plants ;  in  sun  plants  the 
plastids  near  the  under  surface  are  more  likely  to  change  their  position  than  are 
those  in  the  elongated  or  palisade  cells  near  the  upper  surface. 

The  synthesis  of  carbohydrates.  —  The  water  absorbed  by  the  roots 
and  the  carbon  dioxid  absorbed  by  the  leaves  combine  in  the  latter  in 
the  presence  of  sunlight  and  form  elementary  carbohydrates,  the  pro- 
cess being  accompanied  by  the  emission  of  oxygen.  Generally  the 
theory  has  obtained  that  the  first  carbohydrate  formed  is  formaldehyde, 
and  that  later  through  its  condensation  there  develop  sugars,  which  may 
migrate  as  such  through  the  plant  or  which  may  be  transformed  into 
starch  by  the  plastids;  in  the  blue-green  algae,  glycogen  is  an  early  syn- 
thetic product,  and  fatty  oils  are  the  first  visible  products  in  Vaucheria 
and  in  diatoms.  The  recent  discovery  of  formaldehyde  in  the  plastids 
of  numerous  leaves  tends  to  confirm  this  theory,  the  objection  that  it 
is  poisonous  to  plants  being  met  by  assuming  that  it  at  once  becomes 
condensed  into  harmless  carbohydrates. 

The  exact  seat  of  synthesis  is  not  surely  known,  some  investigators  holding  that 
it  is  in  the  plastid,  others  that  it  is  in  the  pigment,  and  yet  others  that  both  are  neces- 
sary. Formerly  the  chlorophyll  was  thought  to  be  a  screen  which  absorbs  rays  dele- 
terious to  plastid  activity.  A  prevalent  modern  view  regards  the  plastid  as  the  chief 
synthetic  factor,  the  chlorophyll  acting  as  a  sensitizer.  Another  view  is  that  chloro- 
phyll is  the  chief  synthetic  factor  and  that  the  plastid  furnishes  a  convenient  seat 
of  activity,  particularly  because  of  its  power  to  transform  sugar  into  starch.  Many 
investigators  regard  enzyms  as  having  an  important  part  in  the  process.  In  most 


526  ECOLOGY 

theories  light  has  been  regarded  as  the  direct  source  of  energy,  but  recently  it  has 
been  suggested  that  the  absorbed  light  rays  are  transformed  into  electricity,  which 
then  becomes  the  direct  agent  of  synthesis.  This  theory  has  been  given  notable 
support  by  the  experimental  demonstration  of  the  reduction  to  formaldehyde  of 
carbonic  acid  (formed  by  the  union  of  carbon  dioxid  and  water)  through  the  opera- 
tion of  a  silent  electric  discharge.  Still  more  recently,  formaldehyde  has  been 
synthesized  in  the  laboratory  by  the  use  of  ultra-violet  light.  The  formation 
of  formaldehyde,  accompanied  by  the  emission  of  oxygen,  has  been  observed  in  a 
chlorophyll  layer  deposited  from  solution  on  a  gelatin  plate,  which  was  exposed  to 
light,  though  it  is  to  be  noted  that  the  validity  of  this  observation  is  called  in  ques- 
tion. Some  investigators  also  have  claimed  that  the  exposure  to  sunlight  of  pulver- 
ized dead  leaves  and  glycerin  leaf  extract  results  in  the  absorption  of  carbon  dioxid, 
the  emission  of  oxygen,  and  the  formation  of  formaldehyde.  The  data  here  given 
make  increasingly  probable  the  theory  that  the  manufacture  of  food  from  carbon 
dioxid  and  water  is  not  of  necessity  a  vital  process,  though  it  must  be  confessed 
that  the  food-making  processes  of  chemical  laboratories  are  as  yet  crude  and  im- 
perfect as  compared  with  those  within  plant  cells. 

Whatever  the  office  of  chlorophyll  in  carbohydrate  synthesis,  its  importance 
is  beyond  doubt;  yet  it  is  not  indispensable,  for  carotin  and  xanthophyll  play  a 
similar  though  less  important  part,  and  in  the  brown  algae  phaeophyll  plays  the 
usual  role  of  chlorophyll.  The  blue-green  algae  apparently  do  not  have  ordinary 
chloroplasts,  yet  they  manufacture  carbohydrates,  probably  through  the  agency  of 
chlorophyll  disseminated  with  other  pigments  throughout  the  cell;  until  recently 
the  same  has  been  supposed  to  be  true  of  the  purple  bacteria,  which  now  are  re- 
garded as  dependent  organisms.  Certain  nitrifying  bacteria  (e.g.  Nitrosomonas) 
and  Bacillus  oligocarbophilus  manufacture  carbohydrates  without  the  aid  of  any 
pigment.  Some  chlorophyll,  on  the  other  hand,  seems  to  have  little  or  no  part  in 
synthesis,  as  in  some  orchids  and  in  the  ovary  of  Ornithogalum  arabicum.  In  the 
green,  partially  parasitic  Scrophulariaceae  there  have  been  thought  to  be  stages 
between  functional  chlorophyll  and  chlorophyll  with  no  synthetic  role.1 

The  influence  of   external  factors  upon  carbohydrate  synthesis.  — 

Light. —  Whatever  may  be  its  specific  role,  light  is  fundamentally 
important  in  the  early  stages  of  food-making.2  There  is  a  minimum 
intensity  of  light  below  which  synthesis  is  impossible,  and  a  certain 

1  Carbohydrate  synthesis  now  is  known  to  take  place  in  some  animals  that  contain 
chlorophyll,  notably  in  certain  species  of  flatworms  (Convoluta),  in  which  algae  living  with 
them  symbiotically  are  believed  to  be  responsible  for  the  food-making. 

2  It  has   been  discovered    that  plants    can  utilize  small   amounts  of  formaldehyde 
supplied  to  their  absorptive  organs  in  the  manufacture  of  more  complicated  carbohy- 
drates; this  process  of  condensation  may  take  place  even  in  the  dark,  appearing  to 
indicate  that  light  is  necessary  only  for  the  reduction  of  carbonic  acid  to  formaldehyde. 
Light  is  unnecessary  even  for  this  first  step  in  food-making  in  some  bacteria,  as  Bacillus 
oligocarbophilus  and  Thiobacillust  where  energy  is  derived  by  the  oxidation  of  sulfur  or 
hydrogen  sulfid,  and  in  nitrifying  bacteria,  where  energy  is  derived  by  the  oxidation  of 
ammonia.     In  one  of  these  forms,  Bacillus  pantatrophus,  formaldehyde  is  produced. 


LEAVES  527 

intensity  above  which  the  amount  of  manufactured  food  decreases,  each 
plant  having  its  optimum  light  relation.  This  optimum  usually  is  much 
lower  for  shade  plants  and  water  plants  than  for  sun  plants,  but  in  all 
cases  a  decrease  of  light  below  the  optimum  impairs  synthesis  much 
more  than  does  an  increase  of  light  above  the  optimum. 

It  is  not  clear  why  increased  light  should  ever  cause  reduced  food  production; 
possibly  it  is  because  the  enzyms  or  the  chlorophyll  itself  are  ill-adapted  to  with- 
stand intense  light,  and  it  may  be  that  the  maximum  activity  of  the  chloroplasts  is 
impaired  by  their  assumption  of  the  profile  position.  In  bright  sunlight  only  a  frac- 
tion of  the  light  available  for  synthesis  is  thus  utilized.  The  depth  at  which  green 
plants  are  found  in  the  sea  is  slight,  doubtless  because  at  a  depth  greater  than 
twenty  meters  the  light  generally  is  insufficient  to  initiate  food-making.  While  green 
algae  (e.g.  Halosphaera)  have  been  brought  up  from  great  ocean  depths,  it  is  likely 
that  they  have  sunk  from  their  place  of  development  near  the  surface.  Red  algae 
grow  at  greater  depths  than  do  most  chlorophyll-bearing  plants,  and  it  has  been 
thought  that  their  color  makes  this  possible  (see  p.  529).  Probably  sulfur  bacteria 
and  nitrifying  bacteria  are  able  to  manufacture  carbohydrates  at  great  depths  be- 
cause of  their  independence  of  light.  The  chlorophyll  layer  beneath  the  bark  of 
trees  is  known  to  manufacture  carbohydrates,  although  the  light  intensity  must 
be  low. 

The  absorption  spectrum  of  chlorophyll  shows  that  rays  toward  the  red  and  the 
blue  ends  of  the  spectrum  are  absorbed  more  completely  than  are  the  intermediate 
rays,  and  usually  it  has  been  supposed  that  the  red,  orange,  and  yellow  rays  are 
more  efficient  in  carbohydrate  synthesis  than  are  the  blue  rays,  the  green  rays 
being  the  least  efficient  of  all.1  It  has  been  held  that  the  color  of  chlorophyll  is  of 
adaptive  significance,  since  it  absorbs  the  more  useful  red  rays  rather  than  the  less 
useful  green.  This  theory  lacks  adequate  support ;  the  absorption  spectrum  of 
blood  is  quite  as  remarkable  as  is  that  of  chlorophyll,  but  the  color  has  no  adaptive 
significance.  The  color  of  blue-green  algae  may  be  of  significance,  for  it  appears 
that  in  many  forms  (e.g.  in  Oscillatoria)  developing  cells  assume  a  color  comple- 
mentary to  that  of  the  incident  light ;  the  red  alga,  Porphyra,  is  said  to  become 
green  if  grown  in  red  or  yellow  light.  It  is  not  known  whether  these  chromatic 
changes  have  any  effect  upon  the  synthesis  of  carbohydrates;  some  recent  investi- 
gators doubt  the  existence  of  such  changes. 

Temperature;  carbon  dioxid;  water.  —  In  the  laurel  cherry,  synthesis 
takes  place  only;  between  —6°  C.  and  45°  C.,  while  the  limits  of  effec- 
tive synthesis  are  still  narrower.  The  limits  and  the  optima  vary  with 
the  species,  being  lowest  in  arctic  and  alpine  forms,  where  effective  syn- 
thesis can  take  place  below  o°  C.,  though  probably  not  so  far  below  as 
sometimes  has  been  thought.  The  optimum  temperatures  usually  are 

1  From  recent  research  it  seems  probable  that  the  green  rays  are  not  utilized  at  all, 
and  that  the  blue  rays  are  equal  in  importance  to  the  red  rays,  if  of  equal  energy  value. 


528  ECOLOGY 

rather  high  (from  20°  C.  to  22°  C.  in  many  plants).  The  rate  of  carbo- 
hydrate synthesis  varies  also  with  the  carbon  dioxid  available,  the 
amount  commonly  present  being  too  low  for  optimum  synthesis.  There 
is  ground  for  believing  that  an  optimum  amount  of  carbon  dioxid  has 
been  present  in  certain  geological  ages,  which  might  account  for  the  ex- 
treme luxuriance  of  the  vegetation  then  existing.  In  dry  soil  the  water 
supply  may  be  insufficient  for  optimum  synthesis.  It  is  probable  that 
temperature,  carbon  dioxid,  and  water,  more  often  than  light,  are  limiting 
factors.  In  the  Mediterranean  region,  winter  synthesis  is  slight  because 
of  low  temperature,  while  in  summer  it  is  slight  because  of  desiccation; 
since  the  stomata  close  in  dry  weather,  desiccation  is  likely  to  result 
in  a  decrease  of  carbon  dioxid  as  well  as  in  a  decrease  of  water. 

Starch  formation.  —  The  synthetic  process  above  outlined  culminates 
in  the  formation  of  sugars,  which  readily  pass  as  solutes  from  cell  to  cell. 
During  the  day  sugar  is  manufactured  more  rapidly  than  it  can  be  trans- 
ported, and  the  excess,  for  the  most  part,  is  converted  into  starch,  which 
accumulates  in  the  chloroplasts.  During  the  night  this  starch  is  recon- 
verted into  sugar  and  removed  from  the  working  cells.  Thus  the 
accumulation  of  starch  in  the  chloroplasts  is  a  measure,  not,  as  has  been 
thought,  of  the  working  capacity  of  a  leaf,  but  of  the  excess  of  sugar- 
making  over  sugar  transportation.  Starch-making  is  a  function  of  the 
plastids,  not  only  of  the  chloroplasts  but  also  of  colorless  plastids  (leuco- 
plasts);  it  takes  place  in  the  dark  (e.g.  in  potato  tubers)  as  well  as  in 
the  light.  Starch  manufacture  is  favored  by  high  rather  than  by  low 
temperatures;  hence,  as  might  be  expected,  sugar  generally  replaces 
starch  in  winter  leaves. 

The  synthesis  of  proteins.  —  Comparatively  little  is  known  concerning  protein 
synthesis,  which  seems  to  be  a  process  common  to  all  plants.  The  chief  necessity 
is  a  supply  of  carbohydrates  and  of  available  nitrogen.  In  green  plants  the  chlo- 
rophyll-bearing cells  probably  are  the  chief  seat  of  protein  synthesis  as  of  carbo- 
hydrate synthesis,  and  the  process  seems  to  take  place  chiefly  in  the  light,  though 
neither  light  nor  chlorophyll  is  necessary  if  the  carbohydrate  supply  is  adequate. 

Anthocyan.  —  General  features.  —  Contrasting  with  chlorophyll,  xanthophyll, 
and  carotin,  which  occur  in  plastids,  are  the  red  pigments  (anthocyans)  of  many 
leaves,  which  usually  are  dissolved  in  the  cell  sap  (occasionally  occurring  as  crystals 
or  grains),  and  thus  are  disseminated  uniformly  throughout  the  cells  where  they 
occur.  They  are  most  familiar,  perhaps,  in  dying  autumn  leaves,  but  are  common 
in  young  leaves  (especially  in  spring),  and  occur  at  times  in  winter  leaves  and  in 
shade  leaves  (especially  beneath),  while  in  some  plants  (as  Coleus)  they  are  always 
present.  The  pigments  may  be  in  the  epidermis  (as  usually  in  floral  leaves  and  in 
developing  foliage  leaves),  or  they  may  occur  in  the  chlorophyll-bearing  cells  (as 


LEAVES  529 

usually  in  dying  foliage  leaves),  or  in  both,  and  the  leaf  may  appear  uniformly  col- 
ored, or  colored  chiefly  on  the  veins  or  over  the  mesophyll  (figs.  755,  8n).  The 
same  pigments  occur  in  flowers,  where  other  colors  than  red,  especially  blue,  are 
frequent.  In  alpine  regions  anthocyans  are  more  abundantly  developed  both  in 
leaves  and  in  flowers  than  they  are  in  the  lowlands. 

The  factors  concerned  in  anthocyan  production.  —  Anthocyans  commonly  are 
accompanied  by  an  excess  of  sugar,  and  premature  coloration  may  be  induced  in 
the  leaves  of  plants  by  growing  them  in  concentrated  cane-sugar  solutions.  Low 
temperatures  often  have  been  assigned  as  a  cause  of  red  coloration,  and,  since  sugar 
tends  to  accumulate  at  low  temperatures  (starch  formation  being  relatively  unusual), 
the  sugar  and  temperature  theories  of  pigmentation  seem  to  harmonize.  It  is  prob- 
able also  that  the  development  of  the  absciss  layer  (p.  582)  tends  to  impede  sugar 
migration  from  the  leaf  to  the  stem,  thus  facilitating  anthocyan  production.  Deccr- 
tication  and  probably  all  other  factors  that  retard  conduction  facilitate  the  forma- 
tion of  anthocyan;  furthermore,  drought  may  incite  brilliant  coloration,  even  in 
midsummer.  The  cause  of  spring  coloration  is  less  obvious,  but  it  may  be  associ 
ated  with  the  flow  of  sugar  into  the  developing  leaves.  Usually  those  species  that 
color  most  in  autumn  also  color  most  in  spring,  and  among  the  more  highly  colored 
plants  are  many  in  which  tannins  are  produced  in  abundance,  as  oaks  and  sumacs. 
A  current  view  of  the  composition  of  the  anthocyans  is  that  they  are  oxidized  glu- 
cosids,  formed  from  a  tannin  and  a  sugar.  Sunlight  usually  facilitates  anthocyan 
production,  perhaps  accounting  for  the  general  predominance  of  color  on  the  upper 
leaf  surface,  and  for  the  occasional  high  color  on  the  morphologically  under  surface 
of  upturned  wilted  leaves;  sunlight  also  may  account  for  the  high  coloration  in 
open  situations  as  compared  with  that  in  dense  woods.  Certain  roots,  as  those  of 
the  corn  and  the  willow,  color  in  the  sunlight.  Many  flowers  and  fruits  color  in 
complete  darkness,  if  there  is  an  adequate  food  supply. 

The  rdle  of  anthocyan.  —  Whether  leaf  anthocyan  is  of  any  advantage  to  plants  is 
very  doubtful.  Some  investigators  have  regarded  the  red  pigment  as  a  screen  that 
absorbs  those  rays  which  are  injurious  to  chlorophyll  or  which  tend  to  inhibit  the 
migration  of  carbohydrates.  Another  aspect  of  the  screen  theory  is  that  anthocyan 
absorbs  the  violet  and  ultraviolet  rays,  which  are  injurious  to  the  leaf  enzyms. 
There  is  slight  experimental  evidence  for  any  aspect  of  the  screen  theory.  A  com- 
moner view  is  that  anthocyan  absorbs  certain  rays  which  increase  the  leaf  tempera- 
ture and  hence  increase  the  efficiency  of  the  chlorophyll.  Red-leaved  varieties  of 
certain  species  (e.g.  the  copper  beech)  seem  to  have  a  slightly  higher  leaf  tempera- 
ture than  do  green-leaved  varieties,  as  is  indicated  by  the  temperature  of  solutions  in 
which  the  leaves  are  placed,  by  the  rapid  melting  of  cocoa  butter,  and  by  thermo- 
electric measurements.  These  temperature  differences,  however,  are  so  slight  as 
to  seem  inconsequential,  the  maximum  difference  observed  being  four  degrees.1 
The  red  color  of  deep-water  algae  has  been  thought  to  be  advantageous,  because 
of  its  power  to  absorb  the  blue  rays,  which  penetrate  deeper  than  do  the  others; 
red  rays  penetrate  scarcely  below  fourteen  meters,  a  depth  at  which  red  algae  are 
the  dominant  forms.  Few  of  the  theories  here  mentioned  are  more  than  guesses, 

1  Red  leaves,  as  a  rule,  contain  less  chlorophyll  than  do  green  leaves,  so  that  on 
the  whole  the  former  probably  are  the  less  efficient  working  organs. 


530 


ECOLOGY 


and  it  may  be  that  the  red  pigments  are  merely  the  indices  of  certain  chemical 
activities  that  are  quite  without  functional  significance.  Various  annuals  (e.g. 
Chenopodiunt)  assume  vivid  colors  just  as  they  are  dying,  and  any  advantage  in 
such  coloration  is  most  improbable.  It  is  not  unreasonable  to  suppose  that  plants 
may  have  many  useless  organs  and  characters,  which  are  mere  by-products  of  their 
fundamental  activities. 


2.   THE  STRUCTURE  AND  ARRANGEMENT  OF  CHLORENCHYMA 

Structural  variations  in  chlorophyll-bearing  tissues.  —  Mesophytic 
dicotyls.  —  The  tissues  that  contain  chlorophyll,  i.e.  the  chlorenchyma, 
show  considerable  diversity,  referable  in  large  part  to  environment. 
Most  mesophytic  leaves  lack  epidermal  chlorophyll  (except  in  the  guard 
cells  of  the  stomata),  but  most  of  the  internal  cells  except  those  in  the 
conductive  tract  contain  chlorophyll,  more  being  present  in  the  upper 

than  in  the  lower  half. 
The  cells  of  the  upper 
portion,  known  as  pali- 
sade cells  (or,  simply,  as 
palisades),  are  elongated 


FIG.  760.  —  A  cross  section  of  a  mesophytic  leaf, 
that  of  a  peppergrass  (Lepidium),  showing  the  upper 
epidermis  (e),  the  lower  epidermis  (er),  stomata  (s),  the 
chlorenchyma  (c)  consisting  of  closely  placed  palisade 
cells  (/>)  and  more  loosely  placed  sponge  tissue  (/), 
and  a  vascular  or  conductive  tract  (v)  with  bundle 
sheath  (b),  hadrome  or  xylem  (h),  and  leptome  or 
phloem  (/);  considerably  magnified. 


FIG.  761. — A  cross  section 
of  a  mesophytic  leaf,  that  of 
a  blue  violet  ( Viola  cucullata), 
showing  a  single  row  of  greatly 
elongated  palisade  cells  (/>), 
beneath  which  is  a  region  of 
loose  spongy  tissue  (/);  con- 
siderably magnified. 


transversely  to  the  leaf  surface  (especially  in  dicotyls) ,  and  are  arranged 
in  one  to  three  compact  layers  (figs.  760,  761).  The  cells  of  the  under 
portion,  the  spongy  parenchyma  (or,  simply,  the  sponge),  are  arranged 
loosely  and  irregularly  and  have  prominent  intervening  air  spaces, 
the  lacunae.  Both  the  palisade  cells  and  the  sponge  cells  have  thin, 
permeable,  cellulose  walls. 
Mesophytic  monocotyls;  hydrophytes  and  shade  plants.  —  The  leaves 


LEAVES 


531 


of  many  mesophytic  monocotyls,  especially  among  the  grasses  and  the 
sedges,  have  a  somewhat  compact  and  uniform  chlorenchyma  without 
palisade  cells  (fig.  7 6 2),  and  some  monocotyls  have  cells  elongated  par- 


FIG.  762.  —  A  cross  section  of  a  meso- 
phytic grass  leaf,  that  of  the  blue  grass 
(Poa  pratensis),  showing  relatively  uni- 
form chlorenchyma  (c),  an  upper  epi- 
dermis (e)  differing  considerably  from  the 
lower,  especially  in  the  large  cells  (b) 
which  are  concerned  in  leaf  movement; 
note  the  vascular  tracts  (v)  and  observe 
that  the  stomata  (s)  are  confined  to  the 
upper  surface ;  considerably  magnified. 


FIG.  763.  —  A  cross  section  of  a 
submersed  hydrophytic  leaf,  that  of  a 
pondweed  (Potamogeton  lucens),  show- 
ing the  few  cell  layers  (here  three) 
characteristic  of  a  water  leaf;  note  the 
abundance  of  chloroplasts  (c)  in  the 
epidermis  (e)  and  the  absence  of  cutin- 
ized  epidermal  walls;  highly  magni- 
fied. 


allel  to  the  leaf  surface.  In  submersed  hydrophytes  epidermal  chloro- 
phyll usually  is  abundant  (fig.  1018),  often  exceeding  in  amount  that 
contained  in  the  mesophyll,  which  may  be  loose  and  spongy  by  reason 
of  the  large  air  spaces,  or  which  may  be  reduced  to  a  single  layer  (fig. 
763).  Hydrophytes  with  emersed  leaves  are 
much  like  mesophytes,  except  for  their  large 
lacunae  (fig.  805).  Shade  plants  are  weak  in 
palisade  tissue,  but,  except  in  ferns  (fig.  754), 
they  rarely  contain  epidermal  chlorophyll.  In 
some  leaves,  especially  in  shade  plants  (such  as  :"IG-  764.  — A  cross 

'  J  section  of  the  upper  part 

Maranta  and  Asarum),  the  convex  outer  walls  Of  a  shade  leaf,  that  of  the 
of  the  epidermal  cells  converge  the  rays  of  inci-  wild  ginger  (Asarum  cana- 
dent  light;  the  reflection  of  some  of  these  rays  d*me\  Bowing  three  epi- 

?  J        dermal    cells    with  convex 

gives  a  velvety  aspect  to  the  leaf  (fig.  764).  outer  walls  which  converge 
Occasionally  (as  in  Fittonia)  isolated  epidermal  the  rays  of  incident  light; 
cells,  known  as  ocdlai,  are  more  papillate  than  considerably  magnified. 
are  their  neighbors.  It  has  been  supposed  on  somewhat  uncertain 
evidence  that  epidermal  cells  with  convex  outer  walls  are  organs  of 
light  perception.  A  more  probable  role  of  such  cells  is  the  facilita- 
tion of  synthesis  through  light  convergence.  In  the  luminous  moss, 
Schistostega,  the  chloroplasts  are  near  the  base  of  a  globose  cell,  which, 


532  ECOLOGY 

acting  as  a  lens,  converges  the  light  upon  the  plastids;  the  emission 
of  reflected  rays  causes  this  moss  and  the  alga,  Botrydium,  to  glisten 
in  the  sunlight.  The  leaves  of  water  plants  and  of  shade  plants  are 
deep  green,  which  is  due  partly  to  the  thin,  transparent  epidermis  and 
partly  to  the  peripheral  position  of  the  numerous  deep-green  chloroplasts. 
Xerophytes.  —  Xerophytes  contrast  sharply  with  hydrophytes,  dis- 
playing prominent  palisade  tissue,  often  in  many  rows  (as  in  the  olean- 
der and  in  most  cacti),  while  the  spongy  tissue  is  small  in  amount  and 
poor  in  air  spaces,  and  often  is  best  developed  toward  the  leaf  center 
(fig.  807).  Frequently  there  are  palisades  in  the  lower  half  of 'the  leaf, 
though  less  prominently  developed  than  above;  in  the  cottonwood  the 
palisades  are  developed  nearly  equally  on  the  two  sides,  and  in  many 
fleshy  vertical  leaves  there  is  a  cylinder  of  palisade  cells  about  the  color- 
less leaf  interior  (figs.  926,  927).  Xerophytic  leaves  commonly  are  thick, 
and  often  without  chlorophyll  at  the  center ;  -sometimes  also  the  epidermis 
is  very  thick,  so  that  the  peripheral  regions  are  free  from  chlorophyll  (figs. 
766,  928).  Some  succulent  xerophytes,  notably  the  Crassulaceae  (fig. 
925)  and  many  xerophytic  grasses  (fig.  835),  are  quite  without  palisade 
tissue,  but  the  compactness  of  the  chlorenchyma  distinguishes  them 
from  hydrophytes.  Xerophytic  leaves  usually  are  pale  green  in  color, 
by  reason  of  a  relatively  non-transparent  epidermis  (due  to  hairs,  wax,  or 
cutin)  or  by  reason  of  the  deep  position  of  the  chlorenchyma;  or  the 
chloroplasts  may  be  pale  in  color  or  relatively  scattered  in  position. 
Thick  leaves  with  compact  tissues  and  prominent  palisade  layers  char- 
acterize not  only  the  plants  of  ordinary  xerophytic  situations,  but  also 

those  of  peat  bogs  and  salt  marshes.  Alpine 
e  plants  have  more  palisade  tissue  than  do  arctic 

plants,  while  the  latter  have  a  more  lacunar 
r  sponge  tissue. 


Taxonomic  variations.  —  In   some   leaves,   as   in 
Sambucus  and  Lilium  (fig.  765),  there  are  arm  pali- 
FIG.  765.  —  A  cross  sec-       sadest  in  which  wall  infoldings  give  an  increased  inner 

r,01!  °LVhv  UPIT  Pa*rt  °f  ?       surface  to  the  cell;  in  pine  leaves  wall  infoldings  are 
hlyleaf  (L-iliumlongtflorum), 

showing  the  so-called  arm  remarkabl7  developed,  and  the  outer  cells  are  divided 
palisades  (£),  also  the  upper  into  palisade-like  compartments  (fig.  1039).  In  some 
epidermis  (e)  with  its  cuticle  succulents,  as  in  Portulaca  and  Begonia  (fig.  766), 
(c) ;  considerably  magnified.  there  are  funnel-shaped  palisade  cells  with  large  and 
immotile  chloroplasts  crowded  at  the  narrow  base 

near  the  conductive  tract ;  sometimes  there  is  a  festoon  of  such  cells  about  the  con- 
ductive bundle  (fig.  767),  and  occasionally  palisade  cells  curve  toward  the  bundle. 


LEAVES 


533 


In  most  of  the  lower  plants  the  chlorophyll-bearing  cells  make  up  the  body  of 
the  plant  and  are  not  delimited  into  special  tissues.  In  the  red  algae  there  are 
elongated  cells  arranged  somewhat  as  are  palisade  cells.  Most  moss  and  liver- 
wort leaves  consist  of  a  single  layer  of  green  cells,  but  in  Sphagnum  colorless  cells 
alternate  with  the  green  cells  (fig.  899);  the  leaves  of  Leucobryum  are  three  cells 
thick,  the  chlorophyll-bearing  cells  'being  centrally  placed  (fig.  900).  In  the  air 
chambers  of  some  thalloid  liverworts  (as  Marchantia,  fig.  799)  there  are  loose  cell 


FIG.  766.  —  A  cross  section  of  a 
succulent  leaf,  that  of  Begonia,  show- 
ing centrally  placed  chlorenchyma  (c), 
consisting  of  funnel-shaped  palisade 
cells  (p)  whose  chloroplasts  are  grouped 
chiefly  at  the  basal  end,  and  loosely 
arranged  sponge  cells  (/);  note  the 
heavy  epidermis  (e),  averaging  three 
cells  in  thickness  above  and  two  be- 
neath, and  quite  without  chloroplasts; 
such  an  epidermis  represents  a  peripheral 
water  tissue;  considerably  magnified. 


FIG.  767.  —  A  cross  section  of  the 
upper  part  of  a  succulent  xerophytic 
leaf,  that  of  the  purslane  (Portulaca 
oleracea),  showing  a  festoon  (f)  of 
funnel-shaped  cells  with  large  basal 
chloroplasts  grouped  about  a  small  vein 
(v),  and  an  outer  ring  of  palisade  cells 
(/>),  containing  chloroplasts  of  ordinary 
size;  note  that  the  palisades  are  not 
symmetrically  placed  but  appear  to  be 
oriented  with  respect  to  the  incident 
light;  the  epidermis  (e)  is  two  cells 
thick  and  represents  a  peripheral  water 
tissue;  highly  magnified. 


filaments,  which  contrast  with  the  compact  tissues  elsewhere;  in  the  leaf  of 
Polytrichum  similar  filaments  form  vertical  plates  hanging  loosely  from  the  leaf 
body.  Near  the  base  of  some  moss  capsules  the  chlorenchyma  is  differentiated 
into  palisade  and  sponge  tissue,  much  as  in  seed  plants  (fig.  263).  In  lichens 
the  chlorenchyma  consists  of  the  algal  layer,  which  lies  close  to  the  surface  in  shade 
forms  and  deeper  in  sun  forms  (fig.  1112). 

The  influence  of  external  factors  upon  the  form  and  orientation  of 
chlorenchyma  cells.  —  The  plasticity  of  chlorenchyma.  —  Chlorenchyma 
is  one  of  the  most  plastic  of  plant  tissues,  its  thickness,  its  compactness, 
and  even  its  differentiation  into  palisade  and  sponge  often  being  subject 


534  ECOLOGY 

to  environmental  control.  However,  there  are  many  cases  of  rigidity, 
as  in  most  monocotyls  and  in  some  dicotyls  (e.g.  in  the  Crassulaceae), 
where  no  condition  s'eems  to  induce  palisade  development;  in  various 
dicotyls  palisades  appear  to  develop  without  regard  to  external  factors. 
In  many  cases  palisades  appear  in  the  bud,  where  the  usual  palisade- 
producing  factors  can  have  no  influence.  But  even  in  these  cases  there 
often  is  variation  in  cell  length,  in  the  number  of  palisade  layers,  or 
in  compactness  of  tissue. 

Among  the  most  plastic  forms  are  the  amphibious  plants,  such  as 
Proserpinaca  and  various  buttercups,  in  which  the  submersed  leaves 
have  no  palisades,  the  entire  mesophyll  consisting  of  loose  sponge,  and 


768  769  v 

FIGS.  768,  769.  —  Leaf  cross  sections  of  the  mermaid-weed  (Proserpinaca  paluslris") : 
768,  a  section  through  a  mesophytic  air  leaf,  showing  prominent  palisades  (p)  and  epi- 
dermis (e),  and  a  stoma  (s) ;  769,  a  section  of  a  segment  of  a  water  leaf,  showing  undif- 
ferentiated  chlorenchyma  (c)  made  up  of  sponge  cells,  between  which  are  prominent  air 
spaces  (a);  the  epidermis  and  the  conductive  tract  (v,  not  shown  in  768)  are  much  re- 
duced in  the  water  form;  figures  equally  magnified. 

the  epidermis  often  containing  chlorophyll  (fig.  769) ;  the  emersed  leaves 
are  without  epidermal  chlorophyll  and  have  well-differentiated  sponge 
and  palisade  regions  (fig.  768).  When  Lactuca  scariola  is  grown  in 
intense  light,  the  leaves  assume  a  vertical  position  and  have  palisade 
layers  on  both  sides  (fig.  770);  in  diffuse  light  the  leaves  are  horizontal 
and  have  palisades  only  on  the  upper  side,  and  in  dense  shade  there  are 
no  palisades  at  all  (fig.  771).  Equally  extreme  variation  is  seen  in  Euca- 
lyptus globulus,  in  which  the  form  of  the  leaf,  as  well  as  its  position, 
changes  from  shade  to  sunlight,  the  sun  form  having  palisade  layers  both 
above  and  below,  while  the  chlorenchyma  of  the  extreme  shade  form 
consists  entirely  of  sponge.  Few  leaves  show  such  extreme  variation 
as  is  found  in  Lactuca,  Eucalyptus,  and  in  amphibious  plants,  but  there 
are  many  plants  in  which  the  upper  leaf  portion  may  have  either  pali- 
sade or  sponge,  and  many  more  in  which  the  number  of  palisade  layers, 


LEAVES 


535 


and  the  shape,  size,  and  compactness  of  the  cells  may  be  modified  con- 
siderably as  conditions  change. 

Light  and  palisade  development.  —  It  was  discovered  long  ago  that 
the  gemmae  of  Marchantia  develop  chlorenchyma  on  whichever  side  is 
exposed  to  the  light,  colorless  tissue  with  rhizoids  developing  on  the 
c  other  side;  also  that  the  chlorenchyma  develops 
equally  on  both  sides  of  a  Thuja  branch  if  ex- 
1  \\p  posed  equally  to  light,  the  palisades  commonly 
developed  on  the  upper  surface  being  due  to 
greater  light  exposure.  These  experiments,  to- 
gether with  similar  experiments  on  the  beech 
M  and  on  Lactuca  and  Eucalyptus,  as  noted  above, 
have  given  rise  to  the  prevalent  view  that  light 
determines  the  devel- 
opment of  palisade 
cells.  Furthermore, 
light  often  is  regarded 
as  the  chief  factor  con- 
cerned in  the  orienta- 
tion of  palisade  cells. 
Sometimes  palisades 
are  oblique  in  vertical 
organs,  as  in  the  leaves 
of  Typha  and  of  Iris, 
and  in  the  stems  of 
Juncus  and  of  Sali- 
cornia  (fig.  772),  thus 
giving  rise  to  the  view 
that  palisades  tend  to 
become  elongated  in  the  direction  of  the  incident  light.  However,  most 
vertical  leaves  and  stems  have  transverse  rather  than  oblique  pali- 
sades, growth  under  diverse  conditions  rarely  having  any  marked 
effect  upon  palisade  orientation.1  In  those  leaves  of  Isolepis  that 
hang  vertically  downward  there  are  oblique  palisades,  but  they  point 
down  instead  of  up,  indicating  that  their  direction  is  related  to  leaf 
structure  and  not  to  sunlight,  and  the  same  probably  is  true  pf  most 
oblique  palisades.  Furthermore,  ordinary  palisade  cells  are  transverse 


FIGS.  770,  771.  — Cross  sections  of  leaves  of  the  prickly 
lettuce  (Lactuca  scariola):  770,  a  section  through  a  leaf 
grown  in  the  sunlight  and  thus  exposed  to  strong  trans- 
piration ;  both  surfaces  have  been  exposed  directly  to  the 
sun,  and  the  chlorenchyma  is  composed  entirely  of  palisade 
cells,  though  those  at  the  center  of  the  leaf  (/>')  are  shorter 
than  are  those  near  the  periphery  (p) ;  the  cuticle  (c)  also 
is  prominent;  771,  a  section  through  a  leaf  grown  in  deep 
shade  and  thus  not  exposed  to  strong  transpiration;  the 
chlorenchyma  is  composed  entirely  of  sponge  tissue  (/), 
and  the  cuticle  (c)  is  but  slightly  developed;  the  shade 
leaf  also  is  much  thinner  than  is  the  sun  leaf;  s,  stoma; 
figures  equally  magnified. 


J  In  a  few  plants  the  more  exposed  individuals  have  the  more  oblique  palisades  (as  in 
Saxifraga  grantUata). 


536 


ECOLOGY 


FlG.  772.  —  A  longitudinal  section 
through  a  portion  of  the  stem  of 
Salicornia,  showing  oblique  palisades  (/>), 
and  also  a  "storage"  tracheid  (/);  the 


to  the  leaf  surface,  regardless  of  the 
position  of  the  leaf  in  relation  to  light. 
The  position  of  the  palisade  layers 
seems  relatively  fixed  in  most  cases 
(not  in  Lactuca  and  in  Eucalyptus),  a 
rudimentary  palisade  region  often 
being  discernible  in  the  bud;  in  sub- 
sequent development  the  thickness  of 
the  layer,  but  not  its  position,  may 
vary  as  external  conditions  vary. 
Thus  light  appears  to  affect  chiefly 
the  cell  length,  the  region  of  develop- 


arrow  is  directed  toward  the  stem  apex;    ment  and  the  orientation  of  the  pali- 

,,   stoma;   considerably  magnified.  gade    cdls_  being    due    to    Other    and 

mostly  unknown  causes.  The  influence  of  light  upon  cell  elongation 
might  be  conceived  of  as  direct  or  indirect.  In  many  but  not  in 
all  albescent  leaves  the  palisades  stop  sharply  at  the  edge  of  the 
green  tissue  (fig.  773), 
although  the  green  and 
colorless  spots  are  e^- 

thTplLade  £ 
velopment  appears  to 
be  correlated  with  the 
formation  or  with  the 
activity  of  chlorophyll 
rather  than  with  light 
directly.  Again,  the 
palisade  length  in- 
creases and  decreases 
with  the  amount  of  car- 
bon dioxid,  seeming  to 
indicate  that  palisade 
size  is  associated  with 
synthetic  activity. 

Palisades  and  trans- 
piration.— Not  all  cases 


FIG.  773. — A  cross  section  of  an  albescent  leaf  of 
Abutilon;  to  the  right  of  the  vein  (i>)  is  a  chlorophyll- 
bearing  region  with  two  rows  of  palisade  cells  (p~)  and 
about  three  layers  of  sponge  tissue  (/);  to  the  left  is  a 
colorless  portion  entirely  without  palisade  cells,  this  part 
of  the  leaf  having  remained  in  its  original  undifferentiated 
state;  the  lower  epidermis  (e~)  contains  chlorophyll,  even 
in  the  albescent  region  of  the  leaf;  note  the  lack  of 
symmetry,  due  to  differential  growth;  h,  branched  epi- 
of  palisade  development  dermal  hair.  ,f  stoma.  v>  vascular  tract.  considerably 

can  be  referred  to  light,    magnified. 


LEAVES  537 

even  indirectly.  For  example,  palisades  are  much  better  developed 
in  dry  than  in  wet  soil,  exposure  to  light  being  equal.  Again,  in 
amphibious  plants  there  usually  is  an  abrupt  change  from  air  leaves 
with  strong  pali  ades  to  water  leaves  with  no  palisades,  although  the 
change  in  light  is  gradual.1  Furthermore,  it  has  been  shown  experi- 
mentally that  palisades  develop  somewhat  better  in  dry  air  with  weak 
light  than  in  moist  air  with  intense  light.  Apparently  palisade  develop- 
ment increases  with  the  transpiration,  or,  more  precisely,  when  there 
is  an  increase  of  transpiration  in  proportion  to  absorption.  Even  where 
palisade  development  has  been  referred  to  light,  it  is  possible  that 
transpiration  is  a  factor,  since  increased  light  commonly  is  accompanied 
by  increased  transpiration.  The  maximum  development  of  palisades 
occurs  in  deserts  and  in  other  dry  exposed  situations  where  high  light 
intensity  is  coupled  with  strong  transpiration  and  relatively  low  absorp- 
tion. Strong  palisade  development  is  seen  also  in  alpine  habitats,  where 
transpiration  is  relatively  high  because  of  the  low  absorption  in  the  cold 
soil. 

The  transpiration  theory  accounts  satisfactorily  for  the  extreme  development  of 
palisades  in  plants  of  bogs  and  salt  marshes.  Salt  marsh  plants  grow  where  root  of 
formation  and  absorption  are  difficult  because  of  the  concentration  of  the  soil  water, 
hence  their  transpiration  is  high  in  proportion  to  their  absorption.  The  case  of 
bog  plants  is  more  difficult  to  understand,  though  their  poor  root  development  indi- 
cates hard  conditions  and  suggests  the  likelihood  of  low  absorption  in  proportion  to 
transpiration.  The  probable  factors  that  tend  to  reduce  absorption  in  bogs,  either 
directly  or  by  reducing  root  development,  are  low  soil  temperatures,  imperfect  aera- 
tion (especially  oxygenition),  and  the  toxicity  of  the  bog  waters ;  it  has  been  shown 
that  each  of  these  conditions  independently  is  capable  of  inducing  palisades  and 
other  xerophytic  features  in  various  species  (as  Rumex  Acetosella) .  It  has  been 
suggested  that  the  xerophytic  peculiarities  of  bog  plants  may  have  been  developed 
elsewhere,  but  the  fact  that  plants  from  without,  when  they  are  grown  in  bog  con- 
ditions, develop  weak  roots  and  small  t  ick  leaves  with  prominent  palisades  (a  com- 
bination not  characteristic  of  ordinary  xerophytes)  makes  it  probable  that  most  of 
the  bog  plants  owe  their  xerophytic  peculiarities  as  well  as  their  sparse  root  system 
to  the  bog  itself,  though  it  is  not  impossible  that  some  xerophytes  may  have  immi- 
grated from  other  habitats. 

The  precise  action  of  light  or  of  transpiration  upon  palisade  development  is  not 
known.  In  particular,  the  mechanical  reason  for  cell  elongation  is  obscure,  but  a  little 
speculation  may  be  hazarded.  Palisades  have  a  more  concentrated  cell  sap  and  hence 
a  greater  turgor  than  have  sponge  cells.  Usually  high  concentration  results  in  cell 
sphericity  (see  discussion  of  cell  form  inStigeoclonium,  p.  591),  but  palisades  depart 

1  In  the  bulrush,  however,  there  is  a  gradual  change,  palisades  occurring  even  beneath 
the  water  surface ;  light  seems  to  be  the  chief  factor  here.  .  . 


53* 


ECOLOGY 


from  rather  than  approach  sphericity.  However,  cultures  of  isolated  palisade  cells 
show  an  approach  toward  sphericity  (figs.  774,  775),  as  though  lateral  pressure 
had  been  removed  suggesting  the  possibility  that  the  close  arrangement  of  the 
palisade  cells  in  the  leaf  prevents  the  assumption  of  the  spherical  form.  In  the 
same  connection  it  is  of  interest  to  note  that  when  shade  monocotyls  are  exposed  to 
intense  light  or  to  strong  transpiration,  the  cells  which  usually  are  palisade-like  and 
elongated  parallel  to  the  surface  tend  to  approach  sphericity.  Furthermore,  leaves 
attacked  by  parasitic  fungi  show  increased  palisade 
development,  suggesting  the  possibility  that  with  the 
introduction  of  fungi  the  osmotic  pressure  of  plant 
cells  is  increased  (see  p.  746).  Where  an  increase  of 
light  or  of  carbon  dioxid  increases  the  synthesis  of 
carbohydrates,  there  results  an  increase  of  the 
osmotically  active  substances  (particularly  sugars) 
within  the  cell  and  hence  an  increase  of  the  cell 
turgor,  suggesting  that  in  the  last  analysis  the  light 
theory  and  the  transpiration  theory  of  palisade  de- 
velopment may  be  essentially  identical. 

Causes  of  variation  in  the  position  of  chloren- 
chyma.  —  The  most  significant  fact  regarding  varia- 
tion in  the  position  of  chlorenchyma  is  the  presence 
of  epidermal  chlorophyll  in  submersed  hydrophytes 
and  in  some  shade  plants,  and  its  absence  in  other 
plants,  except  in  the  guard  cells  of  stomata.  No 
adequate  cause  for  such  variation  is  known,  possibly 
because  the  problem  has  not  been  seriously  attacked. 
That  an  external  cause  is  probable,  at  least  at  times, 
is  evident  from  the  fact  that  various  amphibious 
plants  (as  species  of  Ranunculus)  exhibit  epidermal 
chlorophyll  in  the  water,  but  not  in  the  air.  Per- 
haps light  is  a  factor,  since  epidermal  chlorophyll  is  confined  essentially  to  habitats 
where  the  light  is  of  low  intensity;  deep-lying  chlorophyll  seems  to  be  impossible 
under  such  conditions.  On  the  other  hand,  if  intense  light  is  deleterious,  the  lack 
of  epidermal  chlorophyll  in  sun  plants  may  be  accounted  for,  though  its  absence 
in  most  shade  plants  would  remain  unexplained. 

The  advantages  of  differentiation  in  chlorophyll-bearing  tissues. — The 
chief  activity  associated  with  chlorenchyma  is  the  synthesis  of  carbo- 
hydrates. Probably  the  palisade  layers  form  the  chief  synthetic  tissue, 
the  spongy  tissue  having  more  to  do  with  aeration  and  hence  with  trans- 
piration (p.  552).  The  palisades  are  relatively  more  efficient  in  diffuse 
light,  because  of  .their  more  favorable  position,  while  in  intense  light  the 
efficiency  of  the  sponge  cells  increases;  in  very  intense  light  they  may 
surpass  the  palisades  in  synthetic  importance,  because  of  the  deleterious 
effect  of  light  upon  the  chlorophyll  and  the  plastids  in  the  latter.  Ex- 


775 


FIGS.  774,  775.  —  Isolated 
palisade  cells  of  Lantium  pur- 
pur  eum:  774,  two  cells  which 
have  been  in  a  nutrient  solu- 
tion for  eight  days;  the  lower 
cell  was  injured  upon  removal 
from  the  leaf,  showing  no  fur- 
ther growth;  the  upper  cell 
grew  considerably,  especially 
in  breadth;  775,  two  palisade 
cells  similarly  treated;  note 
the  great  growth  in  breadth, 
compared  with  the  growth  in 
length ;  highly  magnified.  — 
From  HABERLANDT. 


LEAVES  539 

periments  show  that  the  average  synthetic  efficiency  of  palisades  in  pro- 
portion to  sponge  varies  from  a  ratio  of  one  hundred  to  thirty-six  in 
xerophytic  leaves  to  a  ratio  of  one  hundred  to  ninety-two  in  relatively 
homogeneous  leaves  like  those  of  the  bamboo. 

Palisade  cells  commonly  are  thought  to  be  of  advantage  in  connection 
with  carbohydrate  synthesis,  partly  because  light  reception  is  favored  by 
the  absence  of  cross-partitions  and  by  the  parallelism  of  the  cells  to  inci- 
dent light,  and  partly  because  cell  elongation  makes  possible  the  motility 
of  a  relatively  large  number  of  plastids,  permitting  their  peripheral  posi- 
tion in  diffuse  light  and  their  movement  to  some  depth  in  intense  light. 
For  the  most  part,  however,  palisade  orientation  has  been  seen  to  be 
unrelated  to  light  direction;  furthermore,  light  commonly  is  present  in 
superabundance,  especially  in  the  very  plants  that  have  the  most  prom- 
inent palisades,  and  it  is  in  these  same  plants  that  plastid  motility  is 
slightest.  Another  theory  concerning  palisade  cells  is  that  they  reduce 
transpiration  because  of  their  compact  arrangement  and  the  consequent 
reduction  of  their  lacunae,  and  also  because  of  the  increased  leaf  thick- 
ness in  proportion  to  the  transpiring  surface  entailed  by  their  presence. 
This  theory  seems  quite  as  plausible  as  the  one  first  mentioned,  but 
the  quantitative  significance  of  the  palisades  in  transpiration  reduction 
is  uncertain.  A  third  theory  relates  palisade  cells  to  the  conductive 
bundles,  their  elongation  being  supposed  to  favor  the  rapid  migration 
of  the  products  of  synthesis;  there  is  no  adequate  support  for  this  theory. 
A  canvass  of  the  situation  fails  to  revealjainy  conspicuous  advantage 
inpdisajde_cells.  If  their  shape  is  due  to  crowding,  as  suggested  pre- 
viously, no  especial  advantage  is  to  be  expected.  In  any  event,  no 
theory  yet  suggested  has  enough  evidence  in  its  favor  to  give  it  particu- 
lar standing. 

3.  THE  RELATION  OF  LEAVES  TO  LIGHT 

Horizontal  leaves.  —  Growing  leaves  tend  to  present  their  surfaces, 
especially  their  upper  surfaces,  to  the  incident  light  (fig.  776);  hence, 
leaves  are  called  transversely  phototropic  or  diaphototropic  organs.  The 
simplest  light  relation  is  that  in  which  the  chlorophyll-bearing  organ 
is  more  or  less  parallel  to  a  horizontal  substratum,  as  in  liverworts  (fig. 
742),  in  foliose  lichens  (fig.  mi),  in  rosette  plants  without  petioles 
(fig.  1036),  in  floating  plants  (fig.  727),  and  in  plants  with  floating 
leaves  (fig.  777).  Even  in  such  plants  the  shifting  of  the  sun's  position 


540 


ECOLOGY 


FIG.  776  A 


FIG.  776  B 


FIG.  776.  —  Phototropism  in  the  nasturtium  (Tropaeolum):  A,  a  plant  that  has 
been  exposed  since  germination  to  ordinary  greenhouse  illumination ;  the  diaphototropic 
leaves  face  upward  in  such  a  position  that  they  receive  the  maximum  amount  of  incident 
light;  B,  the  same  plant  after  exposure  for  six  hours  to  one-sided  illumination.  — Pho- 
tographs by  FULLER. 


FIG.  777.  —  A  pond  margin  with  water  lilies  (Castalia);  note  that  the  leaves  are 
strictly  horizontal,  except  in  the  background,  where  they  are  growing  so  close  together 
that  their  edges  are  upturned;  shoreward  from  the  water  lilies  are  bulrushes  (Scirpus) 
and  cattails  (Typha);  Miller,  Indiana.  — Photograph  supplied  by  MEYERS. 


LEAVES 


541 


hour  by  hour  and  day  by  day  results  in  great  diversity  of  light  direction; 
only  in  tropical  regions  are  such  organs  strictly  transverse  to  the  dominant 
incident  light.  The  fact  that  they  are  about  equally  horizontal  at  high 
and  at  low  latitudes  shows  that  some  factor  other  than  light  determines 
their  position;  in- 
deed, rosette  leaves 
are  more  nearly  hori- 
zontal in  winter  than 
in  summer  in  spite  of 
the  slanting  rays  in 
the  former  season. 
However,  liverworts 
like  Marchantia  or 
Fegatella  clearly  are 
diaphototropic,  and 
can  be  induced  to 
develop  even  vertical 
thalli  if  the  incident 
light  is  horizontal. 

Leaves  on  erect 
stems  and  their 
branches.  --  Leaf 
orientation.  -  -  Most 
leaves  are  not  hori- 
zontal or  even  trans- 
verse to  the  prevail- 
ing incident  light  of 
the  region  where  they 
grow.  In  trees  and 
in  treelike  herbs  and 
shrubs  the  leavescom- 
monly  face  outwards 
and  upwards  on  all 
sides  (fig.  778),  each 

leaf  being  transverse  to  those  rays  that  are  dominant  for  that  par- 
ticular leaf;  leaves  on  the  north  side  of  a  tree  face  north,  where  the 
sun  is  in  the  south,  because  more  light  is  available  from  the  former 
direction.  In  a  plant  by  a  window  or  at  the  edge  of  a  forest,  all  of  the 
leaves  may  face  in  one  direction,  because  the  direct  light  that  penetrates 


FIG.  778.  —  A  cineraria  plant  (Senecio  cruentus),  illus- 
trating a  conical  habit  that  is  due  to  a  decrease  in  the 
length  and  to  a  change  in  the  orientation  of  the  petioles  from 
the  base  to  the  apex,  the  directional  variations  resulting  in 
the  assumption  by  each  blade  of  a  position  such  that  it 
receives  the  maximum  available  light.  —  Photograph  by 
FULLER. 


542 


ECOLOGY 


the  foliage  is  greater  than  the  diffuse  light  on  the  darker  side  (fig. 
776  B). 

Petioles.  —  During  development  leaves  assume  a  position  transverse 
to  the  local  incident  light,  and  this  position  is  kept  through  life.  The 
assumption  of  a  favorable  position  where  foliage  is  dense  usually  is 
due,  especially  in  dicotyls,  to  the  power  of  elongation  and  curvature 


FIG.  779.  —  A  horizontal  branch  of  the  Norway  maple  (Acer  platanoides},  illustrating 
differential  petiole  elongation ;  the  palmately  veined  leaves  are  arranged  in  one  plane  facing 
the  light,  each  leaf  being  well  placed  for  light  reception,  even  though  the  phyllotaxy  is 
decussate;  every  fourth  leaf  (a,  a')  issues  from  the  under  side  of  the  stem  and  develops  a 
long  petiole,  while  the  other  member  of  each  of  these  pairs  (6,  &')  develops  on  the  upper 
side  and  has  a  short  petiole;  each  member  of  the  intervening  pairs  (c,  C*,  d,  d')  issues 
from  the  side  of  the  stem  and  has  a  petiole  of  intermediate  length;  note  that  the  leaves 
become  progressively  smaller  and  the  petioles  progressively  shorter  toward  the  stem  tip. 
—  From  KEENER. 

possessed  by  the  growing  petioles.  On  vertical  maple  branches 
the  petioles  develop  equally,  but  on  horizontal  branches  every  fourth 
leaf  originates  on  the  under  side  and  develops  a  long  petiole,  while  the 
opposite  leaf,  originating  above,  has  a  short  petiole  (fig.  779).  The 
leaf  of  Tropaeolum  shows  even  greater  plasticity,  having  a  petiole  at- 
tached to  the  center  of  the  blade  and  capable  of  almost  unlimited  elonga- 
tion and  degree  of  curvature.  In  some  rosette  plants  (fig.  1036)  and 
in  plants  with  simple  erect  stems,  the  petioles  are  progressively  shorter 


LEAVES 


543 


upward,  so  that  the  avoidance  of  shading  is  very  striking.  In  many 
plants,  especially  in  those  in  which  petioles  are  wanting,  stem  plasticity 
may  result  similarly  in  advantageous  leaf  orientation;  stem  elongation 
is  comparable  to  petiole  elongation,  and  stem  twisting  corresponds  to 
petiole  curvature,  resulting  in  favorable  light  relations  for  all  leaves, 
including  those  that  originate  on  the  under  side  of  horizontal  stems 


781 


FIGS.  780,  781.  —  Branches  of  a  honeysuckle  (Lonicera)\  780,  an  erect  branch, 
showing  characteristic  decussate  phyllotaxy  (p.  549);  781,  a  horizontal  branch,  in  which 
stem  twisting  has  brought  the  leaves  into  a  common  plane,  obscuring  the  decussate  phyl- 
lotaxy. 


(figs.   780,  781;   also  figs.  882-884,  895;   see  discussion  of  stems  as 
organs  of  leaf  display,  p.  645). 

Leaf  mosaics.  —  Since  each  leaf  assumes  the  best  lighted  position 
possible,  there  is  in  general  an  absence  of  overlapping  in  the  leafage 
as  a  whole,  and  if  the  foliage  is  dense,  most  of  the  available  leaf  space 
becomes  occupied.  The  combined  result  of  the  absence  of  overlap  and 
of  the  maximum  occupation  of  space  often  is  spoken  of  as  a  leaf  mosaic. 
The  perfection  of  this  mosaic  sometimes  is  enhanced  by  the  fitting  of 
projecting  angles  into  reentrants  (as  in  Hedera),  by  the  reciprocal  ar- 
rangement of  unsymmetrical  leaves  (as  in  Celtis  and  in  Begonia,  fig.  895), 
or  by  the  intercalation  of  small  leaves  between  larger  leaves  (as  in  Sela- 
ginella,  fig.  896).  Excellent  mosaics  are  seen  in  climbing  plants  (as  in 


544 


ECOLOGY 


various  ivies,  fig.  782),  which  commonly  have  a  predominance  of  vertical 
leaves,  the  prevailing  incident  light  being  not  far  from  horizontal. . 

Grasslike  foliage.  —  The  leaves  of  most  grasses  and  sedges  grow  so 
close  together  that  the  assumption  of  a  position  transverse  to  incident 
light  is  mechanically  impossible.  In  a  meadow,  not  only  the  grasses, 
but  many  other  plants  as  well,  have  leaves  more  nearly  vertical  than 
horizontal  (fig.  783),  and  in  swamps  the  verticality  of  the  foliage  organs 


FIG.  782.  —  A  "  leaf  mosaic"  formed  by  leaves  of  the  Japan  ivy  (P  seder  a  tricuspidata), 
which  is  climbing  upon  a  vertical  wall;  the  leaves  occupy  most  of  the  available  space 
and  yet  have  a  minimum  of  overlap;  though  diaphototropic,  they  face  outward  and  lie* 
in  a  nearly  vertical  plane,  because  the  dominant  incident  light  is  nearer  horizontal  than 
vertical;  note  the  gradations  between  three-lobed  and  ternately  compound  leaves.  — 
Photograph  by  LAND. 

among  the  sedges,  rushes,  and  flags  is  most  striking.  Even  such  horizon- 
tal leaves  as  those  of  the  water  lilies  have  upturned  edges,  where  the 
growth  is  dense  (fig.  777).  Leaf  verticality  or  parallelism  to  the  incident 
light  results  obviously  in  minimum  lighting  for  any  individual  leaf,  but 
there  is  maximum  lighting  for  the  vegetation  as  a  whole,  since  the  more 
vertical  the  leaves,  the  more  numerous  may  they  be  in  any  given  space 
and  yet  have  sufficient  light  to  live.  Thus  the  position  that  seems  the 
worst  for  the  individual  leaf  appears  to  be  the  best  (as  well  as  mechani- 
cally unavoidable)  for  the  meadow  or  swamp  vegetation  as  a  whole, 


LEAVES 


545 


since  it  doubtless  results  in  the  greatest  food  production  possible  within 
a  given  volume  of  leafage.  In  many  swamp  plants,  verticality  is  not 
due  entirely  to  leaf  crowding;  in  various  monocotyls  (as  Typha)  the 
leaves  are  enclosed  in.  sheaths,  and  in  the  rushes  (Juncus,  Scirpus, 


FIG.  783.  —  A  colony  of  the  bur-reed  (Sparganium  eur  year  punt);  the  sunlight 
reaches  the  leaves  at  all  depths,  in  spite  of  their  dense  arrangement;  closely  placed 
vertical  leaves  permit  a  maximum  of  lighting  for  vegetation  as  a  whole,  though  the  amount 
received  by  each  leaf  is  relatively  small ;  a  water  lily  (Castalia)  is  seen  in  the  foreground. 
Miller,  Indiana.  —  Photograph  supplied  by  MEYERS. 

Eleocharis)  the  stems  rather  than  the  leaves  often  are  the  chief  foliage 
organs  (p.  666). 

Forest  undergrowth.  —  The  relation  of  light  to  foliage  is  particularly  evident  in 
forests.  The  luxuriant  undergrowth  of  open  sunny  oak  woods  often  contrasts 


546 


ECOLOGY 


strikingly  with  the  sparse  undergrowth  of  dense  shady  woods  of  beech  or  hemlock, 
the  plants  in  the  latter  consisting  largely  of  thin-leaved  mosses,  ferns,  and  other 
shade  plants  (fig.  784).  In  early  spring  our  deciduous  forests  are  well  lighted, 
and  the  undergrowth  then  displays  remarkable  activity ;  while  in  many  plants  the 
leaves  remain  alive  through  the  summer,  in  others  they  soon  die  (as  in  Claytonia, 
Dicentra,  and  Allium  tricoccum).  The  great  intensity  of  tropical  light  often  permits 


FIG.  784.  —  A  plant  of  wild  spikenard  (Aralia  racemosa),  displaying  a  kind  of  leafage 
common  in  rich  mesophytic  woods;  note  the  large,  compound  diaphototropic  leaves 
with  broad  leaflets,  which  are  very  thin  and  capable  of  enduring  considerable  shade; 
Manitou  Island,  Michigan.  —  Photograph  supplied  by  THOMPSON. 

a  dense  undergrowth  in  the  forest  shade;  where  evergreens  prevail,  the  herbage 
always  is  exempt  from  direct  insolation. 

Vertical  leaves.  —  Some  leaves  (especially  among  xerophytes)  are 
slightly  if  at  all  diaphototropic,  assuming  a  vertical  position  through 
their  growth  activity.  Lactuca  scariola,  for  example,  has  diaphototropic 
leaves  in  the  shade,  but  in  the  sunlight  the  leaves  twist  about  into  the 
profile  position  (fig.  785).  In  the  compass  plant  (Silphium  laciniatum) 
and  often  in  Lactuca  the  leaves  not  only  are  vertical,  but  also  face 
east  or  west.  In  Eucalyptus  globulus  intense  light  induces  not  only  a 
vertical  instead  of  a  horizontal  position,  but  a  change  in  leaf  form  as 
well.  Such  changes  in  reaction  accompanying  an  increase  of  light  in- 


LEAVES 


547 


tensity  are  not  at  all  easy  of  explanation.  Probably  of  similar  import 
are  the  changes  in  orientation  from  the  base  to  the  apex  in  the  leaves 
of  many  plants  (as  Verbascum  or  Nicotiana,  fig.  786);  the  intermediate 
leaves  show  gradations  between  the  large  horizontal  lower  leaves  and 
the  small  vertical  upper  leaves.  The  resultant  plant  contour  is  a  cone, 
a  shape  well  fitted  for  light  reception  Probably  such  changes  in  leaf 
orientation  are  due  partly  to  a  change  in  the  character  of  the  photo- 
tropic  reaction  as  the  light  becomes  more  intense;  but  other  explana- 
tions are  possible  (p. 
603),  and  cautious 
statement  is  necessary 
until  more  experi- 
mental data  are  avail- 
able. In  some  plants 
the  leaf  position  is 
determined  by  me- 
chanical factors;  for 
example,  banana  and 
palm  leaves,  though 
diaphototropic,  often 
hang  vertically  be- 
cause of  their  weight. 


FIG.  785.  —  A  portion  of  a  sun  plant  of  the  prickly 
lettuce  (Lactuca  scariola),  showing  the  characteristic  twisted 
leaf  bases  and  the  resultant  fixed  vertical  or  profile  posi- 
tion; occasionally,  as  here,  the  leaves  are  in  one  plane, 
facing  east  or  west,  as  in  the  compass  plant. 

Growing  Yucca  leaves 

tend  toward  verticality,  but  crowding  and  other  factors  cause  them  to 

assume  various  positions. 

The  advantages  of  leaf  reactions  to  light.  —  Diaphototropic  leaves.  — 
In  the  open,  plants  commonly  receive  much  more  light  than  they  can  use 
in  carbohydrate  synthesis,  carbon  dioxideand  temperature  being  more 
important  as  limiting  factors.  But  in  the  shade  the  amount  of  light 
may  be  insufficient,  hence  it  follows  that  there  the  leaf  arrangement  is 
of  high  importance.  Indeed,  in  dense  cultures  the  lower  leaves  of 
many  plants  soon  die,  presumably  from  lack  of  light;  the  absence  of 
leaves  on  the  lower  branches  of  forest  trees  doubtless  has  a  like  explana- 
tion. Experiments  show  that  synthesis  in  a  given  leaf  is  reduced 
greatly  when  the  incident  light  penetrates  another  leaf,  and  that  it  practi- 
cally ceases  if  it  penetrates  two  leaves.  Hence  the  avoidance  of  shading 
through  petiolar  growth  and  otherwise  is  of  great  significance.  Dia- 
phototropism  favors  maximum  light  exposure,  and  its  advantage  is 
apparent.  Less  obvious  is  the  advantage  resulting  from  the  facing  of 


548 


ECOLOGY 


leaves  toward  light;  in  so  far  as  the  upper  surface,  regardless  of  exposure, 
has  the  maximum  chlorenchyma  development,  the  advantage  is  evi- 


FIG.  786.  —  A  plant  (Nicotiana  alata),  illustrating  a  conical  habit  due  to  a  decrease 
in  the  size  and  in  the  horizontally  of  the  leaf  blades  from  the  base  to  the  apex;  note  also 
that  the  leaves  change  in  shape,  being  blunt  below  and  pointed  above,  and  that  they  are 
arranged  in  many  vertical  rows;  this  plant,  like  the  mullein,  illustrates  a  relative  maxi- 
mum of  lighting  coupled  with  a  minimum  of  shading,  and  also  a  progressive  increase 
from  the  base  to  the  apex  of  features  that  protect  from  excessive  transpiration.  —  Pho- 
tograph by  FULLER. 

dent,  but  in  a  great  many  leaves   (as  in  Lactuca  scariola,  Populus 
deltoides,  and  Eucalyptus  globulus),  the  greatest  development  of  the 


LEAVES 


549 


chlorenchyma  occurs  on  whichever  side  is  the  more  exposed  to  light. 
Surface  expansion  is  almost  universally  characteristic  of  leaves,  and 
though  perhaps  not  caused  by  light,  it  is  none  the  less  of  prime  im- 
portance in  light  reception. 

Vertical  leaves.  —  In  most  of  the  leaf  forms  above  considered  there  is 
a  tendency  toward  the  display  of  a  maximum  surface  to  incident  light, 
In  vertical  leaves  and  also  in  leafless  stems  the  exact  reverse  occurs. 
In  the  latter  the  chief  limiting  factor  is  not  insufficient  light,  but  rather 
too  great  transpiration ;  perhaps,  too,  the  excessive  light  may  be  directly 
injurious  to  the  chlorophyll  and  hence  to  synthesis.  The  peculiar 
reaction  of  the  compass  plant  has  been  supposed  to  be  particularly 
advantageous,  since  a  leaf  facing  east  or  west  misses  the  intense  zenith 
rays  and  yet  has  the  full  benefit  of  the  weaker  rays  of  morning  and 
evening.  The  situation  in  Nicotiana  and  Verbascum  is  still  more  com- 
plicated, there  being  from  the  base  to  the  apex  a  series  of  leaves  varying 
in  form  and  position  in  apparent 

correspondence  with  the  increasing  Yj/ 

light  intensity;  the  basal  leaves  seem  -       »  *A 

fitted  for  maximum  light  reception, 
and  the  apical  leaves  for  maximum 
light  avoidance.  ,. 


The  position  of  leaves  on  stems.  — 

Light  reception  is  facilitated  by  various 
features  of  leaf  structure  and  arrange- 
ment, which  have  little  or  no  causal 
connection  with  light.  While  petioled 
leaves  usually  have  considerable  plas- 
ticity in  their  orientation  and  thus  are 
relatively  free  from  disadvantages  due  to 
place  of  origin,  the  particular  orientation 
of  sessile  leaves  often  is  determined  by 
their  stem  position  or  phyllotaxy.  Leaves 
usually  are  arranged  in  cycles  (whorls)  or 
in  spirals.  A  simple  and  common  ar- 
rangement, known  as  decussate,  is  that 
in  which  two-leaved  cycles  alternate  with 
one  another,  resulting  in  four  vertical 
rows  (arthostichies)  of  leaves  (fig.  780). 
There  are  many  systems  of  spirals,  the 
simplest  being  the  distichous  or  \  ar- 
rangement (i.e.  there  are  two  orthosti- 


FIGS.  787,  788.  —  Experimental  modi- 
fication of  the  phyllotaxy  in  the  luminous 
cave  moss  (SchistostegaOsmundacea):  787, 
a  shoot  which,  after  the  development  of 
ordinary  distichous  leaves  (/),  has  been  ex- 
posed to  feeble  illumination;  the  new  leaf 
arrangement  (r)  is  spiral;  788,  a  shoot 
which  from  the  first  has  been  exposed  to 
feeble  illumination,  hence  exhibiting  spiral 
arrangement  throughout.  —  From  GOEBEL. 


55° 


ECOLOGY 


chies,  one  stem  circuit  making  a  complete  round  of  the  spiral;  fig.  729).  Progres- 
sively more  complicated  arrangements  are  £,  §,  f ,  T5y,  the  latter  meaning,  for 
example,  that  there  are  thirteen  orthostichies,  and  that  five  stem  circuits  are 
necessary  for  a  complete  round  of  the  spiral,  the  fourteenth  leaf  being  above 
the  first,  etc. 

Spiral  phyllotaxy  is  advantageous  from  the  standpoint  of  leaf  lighting,  since  it 
results  in  relative  remoteness  between  the  members  of  the  same  orthostichy;  the 
screw  pine  (Pandanus)  gives  an  admirable  illustration  of  such  arrangement,  due  to  a 
high-ranked  spiral.  A  relation  sometimes  is  claimed  to  exist  between  phyllotaxy 
and  leaf  size,  that  is,  complex  spirals  are  supposed  to  be  associated  with  small  leaves 
and  simple  spirals  with  large  leaves.  Small  leaves  often  occur  in  many  ranks  (as 
in  Yucca,  Lycopodium,  and  Polytrichum,  figs.  901,  265),  and  large  leaves  likewise; 
probably  such  relations  are  fortuitous.  The  causes  of  variations  in  phyllotaxy  are 
not  definitely  known.  A  common  theory  has  been  that  leaf  position  is  due  to  mechan- 
ical influences  exerted  in  the  bud,  such  as  the  pressure  of  older  parts  upon  those 
just  developing;  in  recent  years,  however,  many  serious  objections  to  this  view  have 
been  advanced.  The  \  system  that  commonly  obtains  in  P hyllocactus  is  changed 
to  a  \  system  when  the  plant  is  grown  in  the  dark ;  similar  changes  have  been  ob- 


FlG.  789. — A  mesophytic  forest  with  a  luxuriant  undergrowth  of  ferns  (Osmunda), 
whose  compound  leaves  permit  the  sifting  of  light  and  the  consequent  illumination  of 
subjacent  foliage;  Porter,  Indiana.  — Photograph  supplied  by  MEYERS. 


served   in   Lycopodium   and   in   Schistostega   (figs.    787,   788),  and   in   Caulerpa, 
"  leaves "  occur  only  on  the  lighted  side.      In  any  case,  no  connection  need  be 
sought  between  the  causes  and  the  advantages  of  the  various  kinds  of  phyllotaxy. 
Compound  and  small  leaves.  —  Divided  leaves,  such  as  those  of  ferns  (fig.  789), 


LEAVES 


551 


milfoils,  and  many  water  plants,  are  peculiarly  favorable  for  light  reception, 
because  the  sifting  of  the  light  between  the  leaf  divisions  enables  it  to  impinge 
upon  the  leaves  beneath.  The  aggregate  surface  exposed  in  a  day  is  much  greater 
than  in  a  colony  of  plants  with  large  simple  leaves,  because  of  the  lighting  at 
different  levels,  due  to  the  shifting  of  the  sun,  and  to  the  pliancy  of  the  leaves  in 
wind  or  water  currents.  Probably  the  amount  of  leaf  surface  lighted  at  any  given 
moment  is  also  greater  than  in  the  case  of  simple  leaves,  because  of  reflection  from 
one  leaf  surface  to  another.  Plants  with  numerous  small  leaves,  such  as  pines  and 
spruces,  have  the  same  general  effect  as  do  plants  with  divided  leaves,  and  they 
exhibit  the  same  sort  of  light-sifting.  Probably  the  lower  leaves  of  plants  with 
divided  or  small  leaves  are  less  likely  to  suffer  injury  from  shade  than  are  similar 
leaves  in  sunflowers  and  in  other  plants  with  large  entire  leaves.  In  some  plants 
(as  Monstera  and  various  oa"ks)  the  upper  leaves  are  more  divided  than  those 
beneath,  thus  facilitating  light  penetration;  in  quite  as  many  plants,  however,  the 
lower  leaves  are  more  divided  than  the  upper.  There  is  no  evidence  in  either  case 
that  light  has  any  causal  relation  to  leaf  division ;  any  advantage  that  may  come  in 
the  way  of  light  reception  is  to  be  regarded  as  purely  incidental. 

4.     AIR  CHAMBERS  AND   STOMATA 

Gaseous  exchanges  in  plants.  —  The  chief  gas  movements  in  plants 
are  associated  with  respiration,  carbohydrate  synthesis,  and  transpira- 
tion. Respiration,  involving  the 
absorption  of  oxygen  and  the 
emission  of  carbon  dioxid,  takes 
place  in  nearly  all  plants  at  all 
times,  though  it  is  slight,  or  even 
wanting,  in  "resting"  organs, 
such  as  seeds.  Carbohydrate 


FlG.  790. — A  tangential  longitudinal 
section  near  the  upper  surface  of  a  leaf 
of  the  century  plant  (Agave  americana), 
showing  palisade  cells  in  cross  section; 
note  the  small  intercellular  air  spaces; 
highly  magnified. 


FIG.  791.  —  A  cross  section  through  the 
stem  of  a  water  milfoil  (Myriophyllum), 
showing  large  and  symmetrically  arranged  air 
chambers ;  note  also  the  centrally  placed  con- 
ductive region  (v),  whose  cells  are  relatively 
undifferentiated ;  considerably  magnified. 


synthesis,  involving  the  absorption  of  carbon  dioxid  and  the  emission 
of   oxygen,   is   confined   essentially  to   chlorophyll-bearing   organs    in 


552 


ECOLOGY 


\d 


d 


the  presence  of  sunlight.  Transpiration,  involving  the  emission  of 
water  vapor  (evaporation),  occurs  in  all  aerial  organs,  being  absent  in 
the  water  and  slight  in  the  ground  or  in  very  humid  air.  Transpiration 
involves  much  the  greatest  gas  movement,  respiration  much  the  least. 
In  the  lowest  plants,  where  each  cell  is  in  contact 
with  the  surrounding  medium,  there  are  few  or  no  air 
cavities.  In  the  higher  plants,  however,  there  are 
systems  of  connecting  intercellular  air  chambers, 
communicating  with  the  outside  air  by  open- 
ings (such  as  stomata)  or  by  loose  external  tissues 
(such  as  lenticels).  Plants,  therefore,  have  an 
internal  atmosphere,  differing  somewhat  from 
that  outside,  though  tending  to  approach  it  by 
diffusion. 

The  structural  features  and  variations  of  air 
spaces. — The  lower  leaf  chlorenchyma,  the  spongy 
tissue,  is  especially  rich  in  air  spaces  or  lacunae  (figs. 
760,  761,  820),  and  a  rather  prominent  air  cavity 
underlies  each  stoma.  There  are  small  intercellular 
spaces  between  all  of  the  chlorenchyma  cells,  those 
between  the  palisade  cells  being  narrow  and  in- 
conspicuous, except  as  seen  in  cross  section  (fig. 
790).  The  thin  cell  walls  adjoining  the  air  spaces 
are  of  cellulose.  Air  chambers,  often  beautifully 
symmetrical  in  arrangement  (as  in  the  stem  of 
Myriophyttum,  fig.  791),  are  particularly  con- 
spicuous in  hydrophytes,  where  they  occur  in 
all  the  vegetative  organs,  their  total  volume  often 
being  greater  than  that  of  the  cells.  In  submersed 
leaves  the  entire  mesophyll  consists  of  spongy 
parenchyma  with  large  lacunae  (fig.  1018),  while 
in  floating  leaves  there  may  be  a  striking  contrast 
between  the  emersed  palisade  layer  and  the  submersed  sponge  (fig. 
805).  Sometimes,  as  in  the  leaf  of  Juncus  (fig.  792)  and  in  the  stem 
of  Zizania,  large  air  chambers  are  partitioned  off  by  diaphragms, 
which  often  appear  to  the  naked  eye  as  cell  walls.  Air  chambers  are 
said  to  be  less  abundant  in  plants  of  swift  streams  than  in  those  of 
ponds  and  swamps.  Xerophytes,  particularly  succulent  species,  are 
characterized  by  small  and  inconspicuous  air  spaces  (figs.  835,  926). 


FIG.  792.  —  A  part 
of  a  leaf  of  Juncus 
nodosus  with  a  portion 
cut  away,  disclosing 
capacious  air  cham- 
bers, separated  from 
each  other  by  hori- 
zontal plates,  the  dia- 
phragms (<£). 


LEAVES 


553 


a 


Alpine  plants  are  said  to  have  smaller  lacunae  than  do  the  otherwise 
similar  arctic  plants.  In  some  cases,  as  in  he  leaf  of  Allium  and 
in  the  stem  of  Equisetum,  capacious  air  chambers  develop  without 
much  reference  to  external  conditions  (fig.  1028). 

The  influence  of  external  factors  upon  the  development  of  air 
spaces.  —  The  plasticity  of  lacunar  tissues.  —  Lacunar  tissues  are  ex- 
tremely plastic,  and  commonly  have  re- 
ciprocal relations  with  palisade  tissues. 
For  example,  in  the  water  leaf  of 
Proserpinaca,  the  entire  mesophyll  is 
composed  of  lacunar  tissue  (fig.  769), 
while  the  air  leaf  is  composed  of  pali- 
sades above  and  of  lacunar  tissue  below 
(fig.  768).  In  Lactuca  scariola  there  are 
possible  all  variations  from  a  mesophyll 
composed  entirely  of  sponge  to  a  meso- 
phyll composed  essentially  of  palisades 
(figs.  770,  771).  When  the  phellogen 
or  cork  cambium  (p.  705)  of  certain 
swamp  plants  (as  Jussiaea  and  Deco- 
don)  is  submerged,  it  develops  into  a 
loose  lacunar  tissue,  known  as  aeren- 
chyma  (fig.  793),  whereas  in  the  air  it 
develops  into  cork.  In  each  case  the 
mature  tissue  is  made  up  of  cells  ar- 
ranged in  radial  rows,  but  in  aerenchyma 
capacious  air  spaces  are  interspersed 
regularly,  while  cork  is  remarkably  free 
therefrom.  Furthermore,  in  the  aeren- 
chyma the  cell  walls  are  thin  and  of 
cellulose.  The  great  development  of 
air  spaces  gives  the  stem  a  swollen  appearance,  and  frequently  the 
aerenchyma  grows  so  rapidly  as  to  break  through  the  bark,  forming 
whitish  patches;  the  so-called  water  lenticels  are  similar  scattered  patches 
of  whitish  tissue,  formed  under  identical  conditions  (p.  663). 

The  factors  involved.  —  The  exact  factors  causing  the  development  of  lacunar 
tissue  are  not  known,  though  it  is  evident  that  the  dominating  influence  is  external. 
The  essential  feature  to  be  explained  is  the  development  of  lacunae,  since  the  cell 
shape  may  remain  much  as  in  embryonic  tissue,  contrasting  with  the  great  change 


FIG.  793.  — A  cross  section  of  the 
submersed  part  of  a  stem  of  Jussiaea 
peruviana,  showing  the  development 
of  aerenchyma  (a)  from  phellogen  (p) ; 
note  the  capacious  air  spaces ;  highly 
magnified.  —  From  SCHENCK. 


554  ECOLOGY 

in  cell  shape  without  conspicuous  lacunar  development  in  the  palisade  layers. 
Two  theories  have  been  suggested,  one  that  lacunae  are  a  reaction  to  low  oxygen 
pressures,  the  other  that  they  are  developed  where  transpiration  is  weak.  The 
oxygen  theory  has  been  employed  only  in  connection  with  the  great  develop- 
ment of  lacunae  in  water  plants,  especially  where  aerenchyma  is  formed.  The 
evidence  for  this  theory  is  slight;  differences  in  oxygen  pressure  quite  fail  to  account 
for  the  sudden  change  from  lacunar  to  compact  tissue  at  the  water  line  (as  in  the 
leaf  of  Nymphaea  and  in  stems  with  aerenchyma).  Almost  without  exception 
lacunae  vary  inversely  with  the  transpiration,  the  largest  air  chambers  being  in  the 
water  where  transpiration  is  reduced  to  zero.  Elsewhere  palisades  have  been  seen 
to  vary  directly  with  the  transpiration,  so  that  the  causative  factors  of  reciprocal 
leaf  structures  themselves  appear  to  be  reciprocal.  The  mechanics  of  the  process, 
that  is,  the  exact  method  whereby  the  reduction  of  transpiration  influences  tissues 
so  as  to  produce  large  air  spaces,  is  for  the  present  scarcely  to  be  conjectured. 

The  role  of  air  spaces.  —  Air  reservoirs.  —  Air  spaces  are  of  vital  im- 
portance in  furnishing  ready  ingress  and  egress  for  oxygen  and  carbon 
dioxid  to  and  from  the  active  cells  of  the  leaf  chlorenchyma  in  connec- 
tion with  respiration  and  carbohydrate  synthesis;  these  spaces  also 
greatly  facilitate  transpiration,  the  significance  of  which  is  to  be  con- 
sidered elsewhere.  In  ordinary  lakes  and  ponds,  oxygen  and  carbon 
dioxid  are  comparatively  abundant,  hence  the  large  air  spaces  there 
appear  to  be  without  advantage,  so  far  as  aeration  is  concerned,  unless 
the  absence  of  stomata  makes  the  entrance  and  the  exit  of  gases  relatively 
slow.  In  stagnant  swamps  and  undrained  ponds,  however,  the  oxygen 
supply  often  is  scant;  indeed,  it  is  commonly  believed  that  the  lack  of 
oxygen  accounts  for  the  quick  decay  of  algae  when  transferred  from 
running  streams  to  standing  water.  In  such  habitats,  then,  capacious 
air  spaces  may  be  of  much  value  as  oxygen  reservoirs.  Green  plants  in 
their  synthetic  processes  give  off  much  more  oxygen  than  is  utilized 
in  respiration,  so  that  the  presence  of  large  air  chambers  permits,  the 
accumulation  rather  than  the  complete  dispersion  of  this  oxygen.  It 
has  been  shown  that  in  the  water  lilies  the  maximum  oxygen  content 
of  these  air  chambers  is  at  sunset,  at  the  close  of  a  day  of  synthetic 
activity,  whereas  the  maximum  for  carbon  dioxid  occurs  at  sunrise,  by 
reason  of  the  accumulation  of  the  products  of  nocturnal  respiration. 
The  carbon  dioxid  utilized  in  synthesis  probably  is  more  abundant  in 
the  average  waters  than  in  the  air,  so  that  air  reservoirs  are  of  doubtful 
efficacy  in  connection  with  that  process.  Furthermore,  in  aerenchyma 
and  in  other  lacunar  tissues  in  many  hydrophytes  chlorophyll  is  absent. 

Buoyancy.  —  In  most  water  plants  air  chambers  serve  to  give  buoyancy 
to  the  various  organs.  The  position  assumed  in  the  water  by  floating 


LEAVES 


555 


leaves  and  by  many  submersed  and  emersed  leaves  and  stems  is  deter- 
mined by  the  volume  of  their  air  spaces;  for  example,  the  distended 
petioles  of  the  water  hyacinth  make  the 
plant  so  light  that  it  floats  with  its  leaves 
high  above  the  water.  The  air-contain- 
ing bladders  of  various  marine  algae  (as 
Fucus  and  Nereocystis,  fig.  751)  serve 
to  keep  these  plants  at  levels  favorable 
for  synthetic  activity  during  periods  of 
high  tide.  Some  fresh-water  algae  (as 
Cladophora  and  Spirogyra)  are  floated 
near  the  water  surface  by  bubbles  of 
gas,  which  become  entangled  among  the 
filaments.  Ceratophyllum,  though  usually 
not  attached,  stands  vertically  in  the 
water  by  reason  of  the  difference  in 
specific  gravity  between  the  stem  and  the 
leaves,  the  latter  being  rich  in  air  spaces. 
During  the  autumn  the  winter  buds  of 
duckweeds  and  bladderworts  (figs.  998, 
999),  which  are  heavier  than  water,  be- 
come detached  from  the  floating  plant 
body,  and  sink  to  the  bottom  of  the 
pond;  in  the  following  spring  these  buds 
grow,  developing  large  air  spaces  which 
lessen  the  specific  gravity  sufficiently  to 
enable  them  to  come  again  to  the  sur- 
face, where  they  develop  into  ordinary 
vegetative  shoots. 

The  structure  of  stomata.  —  The  air 
chambers  of  leaves  and  of  young  stems 
communicate  with  the  outside  air  by 
means  of  stomata;  as  seen  in  surface 
view,  these  organs  consist  usually  of  a 
narrow  slit,  the  stoma  proper  (though 
the  apparatus  as  a  whole  often  is  called 
a  stoma),  flanked  by  two  kidney-shaped 
guard  cells  in  contact  at  the  ends  but 
separated  along  the  middle  (fig.  794). 


FIGS.  794,  795.  —  Stomata  from 
the  leaf  of  an  Easter  lily  (Lilium 
longiflorum):  794,  a  stoma,  as  seen 
in  surface  view,  showing  the  two 
kidney-shaped  guard  cells  (g),  which 
enclose  the  stomatal  aperture  (5), 
the  more  deeply  shaded  portion  rep- 
resenting the  central  slit;  note  the 
chloroplasts  in  the  guard  cells;  0, 
subsidiary  cells ;  795,  a  stoma,  as  seen 
in  cross  section;  the  wall  of  the  guard 
cell  (g)  next  to  the  subsidiary  cell  (b) 
is  the  dorsal  wall  (d),  the  wall  next 
to  the  central  slit  (s)  being  the  ven- 
tral wall  (/) ;  the  outer  slit  (0)  is  en- 
closed between  the  cutinized  outer 
guard-cell  ridges  (r)t  the  enlarged 
area  just  below  being  the  outer  ves- 
tibule (0') ;  below  the  central  slit  is 
the  inner  vestibule  (*),  which  here 
opens  directly  into  the  stomatal 
cavity  (c) ;  highly  magnified. 


556 


ECOLOGY 


Usually  the  guard  cells  are  adjoined  or  surrounded  by  subsidiary  cells 
(figs.  796-798),  which  sometimes  differ  from  the  other  epidermal  cells. 
Most  guard  cells  differ  from  the  adjoining  epidermal  cells  in  possessing 
chloroplasts,  starch  grains,  and  abundant  cytoplasm  with  a  prominent 
nucleus.  In  cross  section  (fig.  795)  the  guard  cells  reveal  some  com- 
plexity of  structure.  Toward  the  ventral  (slit)  side,  the  upper  parts  of 

the  walls  project  in  sharp  cutin- 
ized  ridges  that  almost  meet 
when  the  stoma  is  closed.  In 
most  cases  the  corresponding 
lower  walls  project  similarly  but 
less  prominently,  and  the  space 
between  them  is  greater.  In  the 
median  region  is  a  third  but 
rounded  pair  of  projections  with 
slightly  thickened  walls;  since 
stomatal  closure  is  effected  by  the 
meeting  of  these  median  projec- 
tions, the  very  narrow  slit  be- 
tween them  (known  as  the  central 
slit)  might  properly  be  regarded 
as  the  true  stoma.  The  narrow 
spaces  between  the  ridges  above 
and  below  are  known,  respec- 
tively, as  the  outer  and  the  inner 
slits  or  openings,  while  the  larger 


797 


FIGS.  796-798.  —  Stomata  from  the  leaf 
of  a  grass  (Poa) :  796,  a  stoma  from  the  blue 
grass  (Poa  pratensis),  as  seen  in  surface  view, 
showing  the  guard  cells  (g)  with  their  dumb- 
bell-shaped lumina,  the  chloroplasts  being 
confined  to  the  enlarged  portions;  note  the 
subsidiary  cells  (b)  with  their  prominent 
nuclei  (w);  797,  a  median  cross  section  of 
the  stoma  of  Poa  annua,  showing  the  narrow 
portion  of  the  guard-cell  lumina  (g)  and  the 
relatively  large  median  portion  of  the  sub- 
sidiary cells  (b);  798,  a  cross  section  through 


the  end  portion  of  the  stoma  of  Poa  annua, 
showing  the  enlarged  part  of  the  guard-cell 
lumina  (g),  the  terminal  portion  of  the  sub- 
sidiary cells  (6)  being  relatively  small ;  highly 
magnified.  —  797  and  798  from  HABER- 

LANDT. 


spaces  separating  them  from  the 
central  slit  are  the  outer  and  the 
inner  vestibules.  The  dorsal 
walls  are  of  cellulose  and  are 
much  thinner  than  are  the  ventral 
walls,  frequently  they  are  connected  with  the  subsidiary  cells  in  such  a 
way  that  the  walls  of  the  latter  may  swing  back  and  forth  like  hinges, 
moving  the  guard  cells  with  them  (figs.  800,  806).  Below  the  stoma 
is  a  rather  large  air  cavity. 

Structural  variations  of  stomata.  —  Taxonomic  variations.  —  Many 
stomatal  variations  have  no  obvious  relation  to  external  conditions,  as  in 
grasses  and  conifers,  where  a  certain  structural  plan  appears  to  charac- 
terize an  entire  family,  regardless  of  environment.  In  grasses  (figs. 


LEAVES 


557 


796-798)  there  are  large  subsidiary  cells  behind  the  dumbbell-shaped 
guard  cells,  in  which  the  lumina  are  narrow  at  the  center  and  enlarged 
at  the  ends.  In  the  conifers  (fig.  1039)  and  in  Equisetum,  the  guard  cells 
and  the  adjoining  epidermal  cells  are  incurved  below  the  epidermal  level, 
the  stomata  thus  lying  at  the  base  of  pits;  the  walls  are  very  thick  and 
heavily  cutinized,  while  the  outer  ridges  are  unusually  prominent.  In 
cycads  the  walls  may  be  lignified  instead  of  cutinized.  The  sporophytes 
of  mosses  and  of  Anthoceros  have  stomata  much  like  those  of  higher 
plants,  except  that  the  inner  cutin  ridges  usually  are  wanting  and  that 
the  guard  cells  vary  in  number  from  one  to  four.  In  the  gametophytes 
of  the  Marchantiaceae  there  are  capacious  air  chambers,  which  com- 


FiG.  799.  —  A  section  through  the  thallus  of  Marchantia  polymorpha,  showing  an 
air  chamber  (a),  an  air  pore  (/»)  with  its  surrounding  cells,  the  chlorenchyma  composed 
of  short  alga-like  filaments  (c),  and  a  tissue  of  closely  packed  cells  (s)  in  which  the  chloro- 
plasts  are  sparse;  the  latter  tissue  is  rich  in  water;  highly  magnified.  — From  COULTER 
(Part  I). 

municate  with  the  exterior  through  simple  air  pores  that  sometimes  are 
enclosed  by  chimney-like  tiers  of  cells  (as  in  Marchantia,  fig.  799). 

Variations  associated  with  habitat.  —  In  many  xerophytes  the  stomata 
occur  in  pits;  sometimes,  as  in  Dianthus  (fig.  800),  there  is  one  stoma  at 
the  base  of  each  pit,  while  in  other  cases  (Nerium,  Begonia)  the  stomata 
occur  in  groups.  In  Populus  pyramidalis  the  pits  of  the  upper  leaf  sur- 
face are  deeper  than  are  those  of  the  lower.  In  most  xerophytes  the 
guard  cell  walls  are  very  heavily  cutinized  (fig.  801);  sometimes  the 
walls  are  thickened  uniformly,  but  more  frequently  there  are  projecting 
ridges  of  remarkable  shapes  and  dimensions.  '  In  Nipa  fruticans,  for 
example,  the  ventral  walls  project  in  such  a  way  as  to  form  a  most 
tortuous  passageway  between  the  leaf  interior  and  the  outside  air  (fig. 
802).  Both  in  heavy  cutinization  and  in  depression  below  the  surface 


558 


ECOLOGY 


FIG.  800.  —  A  cross  section  through  a 
stoma  from  a  leaf  of  the  carnation  (Dianthus 
Caryophyllus) ;  by  reason  of  the  heavy  cu- 
tinization  of  the  outer  epidermal  wall  (w)t 
the  stoma  lies  below  the  surface  level  of  the 
leaf;  note  the  thin  places  (h)  above  and 
below  the  dorsal  wall  (d)  of  the  guard  cell 
(£)•  representing  the  so-called  hinges  which 
are  thought  to  facilitate  guard-cell  move- 
ment; the  chamber  above  the  stoma  is 
called  a  stomatal  pit;  general  lettering  as 
in  fig.  795;  highly  magnified. 


FIG.  802.  —  A  cross  sec- 
tion through  a  stoma  from 
the  under  leaf  surface  of  the 
nipa  palm  (Nipa  fruticans), 
showing  the  tortuous  pas- 
sageway (M),  which  must 
be  traversed  by  transpiring 
water  that  passes  from  the 
stomatal  cavity  (c)  to  the 
exterior  at  e;  g,  guard  cell; 
highly  magnified.  —  From 
BOBISUT. 


FIG.  80 1. — A  cross  section  through  a  stoma  from  a  leaf  of  the  India-rubber  tree 
(Ficus  elastica) ;  the  stoma  lies  below  the  surface  level  of  the  leaf  by  reason  of  its  position 
beneath  the  subsidiary  cells  (6),  which  have  projecting  ridges  (rf)  partially  enclosing  the 
pit  (p);  the  dorsal  wall  (d)  of  the  guard  cell  is  so  braced  by  the  thick  walls  of  the  ad- 
joining cells  that  guard-cell  movement  must  be  slight;  note  that  the  epidermis  (e)  con- 
sists of  three  cell  layers  and  that  the  cuticle  (z)  is  highly  developed;  general  lettering 
as  in  Fig.  795;  highly  magnified. 


LEAVES 


559 


level,  the  stomata  of  conifers  agree  with  those  of  many  xerophytes,  and 
are,  perhaps,  to  be  regarded  as  xerophytic  stomata.  The  stoma-bear- 
ing  under  surface  of  xerophytic  leaves  is  often  hairy,  contrasting  with 
the  smooth  upper  surface  (as  in  Antennaria  or  in  Populus  alba,  fig.  820), 
while  in  other  cases  hairs  may  develop  only  in  pits  (as  in  Nerium,  fig. 
807)  or  in  furrows.  Similarly,  wax  deposits  often  are  observed  on  stoma- 
bearing  surfaces,  and  waxy  or  resinous  excretions  may  even  clog  up  the 
stomata.  Occasionally  intrusive  growths,  known  as  tyloses,  due  to  the 
bulging  of  adjoining  cells  into  $ 

the  stomatal  air  cavity,  develop 
to  such  an  extent  as  almost  to 
block  up  the  air  passages  (fig. 
803) ;  especially  is  this  the  case 
in  xerophytic  leaves.  The 
stomata  of  vernal  herbs  (as 
Medeola)  are  likely  to  be  less 
protected  than  are  those  of 
trees  or  of  estival  herbs  (as 
Achillea),  the  latter  being  more 
like  those  of  xerophytes. 

In  most  hydrophytes  and  in 
some  mesophytes  (as  in  ferns) 
the  inner  cutin  ridges  of  the 
guard  cells  are  lacking;  in  a 
few  cases,  even  the  median 
ridges  are  absent.  Rarely,  ex- 
cept in  some  hydrophytes,  are  the  stomata  lifted  above  the  epidermal 
level.  The  stomata  of  cotyledons  are  relatively  uniform  in  structure, 
thus  seeming  to  correspond  with  the  uniform  conditions  under  which 
cotyledons  are  developed. 

The  influence  of  external  factors  upon  the  structure  of  stomata.  —  A  slight 
decrease  in  the  size  of  the  guard  cells  and  a  slight  increase  in  the  cutinization  of 
their  walls,  when  developing  stomata  are  exposed  to  dry  air  or  to  other  xerophytic 
conditions,  are  about  the  only  changes  that  have  been  experimentally  induced  in 
the  structure  of  stomata,  a  fact  that  seems  remarkable  in  the  light  of  the  extreme 
plasticity  of  other  superficial  cells  and  tissues.  The  extensive  habitat  variations 
above  noted  make  the  structural  rigidity  of  stomata  all  the  more  extraordinary.  As 
appears  from  the  following  paragraphs,  stomata  often  can  be  induced  or  inhibited 
at  will,  but  their  structure,  when  present,  seems  fixed ;  however,  the  paucity  of  ex- 
perimental data  makes  any  general  conclusion  hazardous. 


FIG.  803. — A  cross  section  through  a  stoma 
from  the  upper  leaf  surface  of  Pilea  elegans, 
showing  the  closure  of  the  stomatal  passageway 
by  the  protrusion  of  a  mesophyll  cell  (w)  into 
the  stomatal  cavity  (c);  such  outgrowths  are 
known  as  tyloses ;  note  the  great  thickening  of 
the  wall  (w}  just  beneath  the  stoma;  s,  guard 
cells;  p,  chlorenchyma  cells  with  chloroplasts 
(d)  and  nuclei  (ri) ;  highly  magnified.  —  From 
HABERLANDT. 


56o 


ECOLOGY 


The  arrangement  of  slomata. —  As  a  rule,  stomata  overlie  the  mesophyll 
rather  than  the  veins,  and  commonly  they  number  from  100  to  300  per 
square  millimeter.  In  most  mesophytic  herbs,  stomata  are  found  on 
both  leaf  surfaces,  but  rather  more  occur  below  than  above.  In  most 

monocotyl  leaves  the  stomata  are  in  longi- 
tudinal rows  and  have  a  common  orienta- 
tion, their  long  axes  coinciding  with  that  of 
the  leaf  (fig.  804).  In  most  dicotyl  leaves 
the  somewhat  more  numerous  stomata  are 
scattered  irregularly  and  their  long  axes 
are  oriented  in  various  directions  (fig.  911). 
In  some  broad  monocotyl  leaves  the  stomata 
are  arranged  and  oriented  irregularly,  while 
in  many-  narrow  dicotyl  leaves  they  occur 
in  rows,  thus  suggesting  that  leaf  shape 
and  venation  rather  than  systematic  position 
may  be  the  chief  determining  factors  ;  in 
Salsola  and  in  other  halophytes  the  stomata 
are  in  rows  and  are  transversely  oriented; 
in  Saxifraga  granulata  the  broad  basal 
leaves  exhibit  irregular  orientation,  while 
the  narrow  upper  leaves  have  the  regular 
orientation  characteristic  of  monocotyls. 

The  leaves  of  most  trees,  whether  meso- 
phytic or  xerophytic,  are  without  stomata 
on  the  upper  surface;  in  Juniperus  the 
stomata  are  confined  to  the  upper  surface, 
but  that  is  the  least  exposed  surface  during 
the  winter  (p.  582);  in  Populus  deltoides  the 
stomata  are  about  equally  abundant  on  the 
two  surfaces,  but  the  constant  trembling  of 
the  leaf  frequently  exposes  the  under  side 
to  the  sun.  In  grasses  the  stomata  usually 
are  confined  to  the  upper  surface,  which  is 

the  more  protected  side  in  dry  weather,  owing  to  the  infolding  of  the 
leaves  (figs.  835-837).  Except  in  a  few  instances  (p.  564)  submersed 
hydrophytes  are  without  stomata,  so  that  their  large  air  chambers 
have  no  direct  outside  connection.  In  floating  foliage  organs,  such  as 
water  lily  leaves  (fig.  805)  and  duckweeds,  the  submersed  under 


FIG.  804.  —  A  surface  view 
of  the  leaf  epidermis  of  Anthe- 
ricum,  showing  the  uniform 
orientation  and  the  regular  ar- 
rangement of  the  stomata  that 
characterize  most  monocotyls, 
the  long  axes  of  the  epidermal 
cells  being  parallel  to  the  long 
axis  of  the  leaf ;  the  straight  epi- 
dermal walls  contrast  with  the 
wavy  walls  of  many  dicotyls; 
considerably  magnified. 


LEAVES 


surface  is  without  stomata,  while  they  are  abundant  on  the  emersed 
upper  surface;  the  submersed  leaves  of  water  lilies  are  quite 
without  stomata.  Subterranean  organs  have  no  stomata,  as  a  rule, 
though  the  latter  are  present  in  some  instances.  Most  xerophytes 
have  few  or  no  stomata  on  the  upper  leaf  surface,  though  various 
conifers  (as  Juniperus)  and  most  Crassu- 
laceae  have  numerous  stomata  on  that 
side. 

The  influence  of  external  factors  upon 
the  development  and  arrangement  of 
stomata.  —  The  variations.  —  Xerophytic 
leaves,  though  smaller  than  the  meso- 
phytic  leaves  of  the  same  species,  usually 
contain  about  the  same  number  of  epi- 
dermal cells,  their  size  being  considerably 
less.  Hence  the  stomata  are  often  more 
numerous  per  unit  surface  in  xerophytes 
than  in  mesophytes,  though  in  the  aggre- 
gate no  more  numerous,  and  perhaps 
even  less  numerous.  Occasionally  an 
increase  of  atmospheric  moisture  results 
in  an  increase  of  stomata  in  proportion 
to  the  other  epidermal  cells.  In  some 
plants  (as  Asperula  tinctoria)  the  orienta- 
tion of  stomata  varies  with  the  habitat, 
mesophytic  individuals  exhibiting  irregular 
orientation,  while  xerophytic  individuals 
exhibit  longitudinal  orientation  as  do 
monocotyls.  The  most  pronounced  in- 
fluence of  external  factors  is  found  in 
amphibious  plants,  where  stomata  ordinarily  can  be  induced  or 
inhibited  at  will  by  growing  the  plants  respectively  in  air  or  in 
water.  Stomata  occur  on  both  surfaces  in  the  air  leaves,  but  on 
the  upper  surface  only  in  the  floating  leaves  of  Polygonum  amphibium 
and  Ranunculus  sceleratus,  the  air  leaves  of  the  latter  having  more 
stomata  on  the  upper  surface  in  moist  air  and  on  the  under  surface 
in  dry  air.  Whatever  the  conditions  of  germination,  the  first  leaves 
of  Proserpinaca  have  stomata,  but  they  originate  within  the  seed  in 
contact  with  air  instead  of  with  water;  subsequent  submersed  leaves 


FIG.  805.  —  A  cross  section 
through  a  leaf  of  the  yellow  water 
lily  (Nymphaea  advena),  showing 
a  strong  development  of  palisade 
tissue  (/>)  in  the  upper  half  which 
is  above  the  water  level  (w),  and 
an  unusually  loose  spongy  tissue 
with  large  lacunae  (/)  in  the  lower 
half  which  is  below  the  water 
level;  the  stomata  (s)  are  con- 
fined to  the  upper  surface;  a, 
slime  gland ;  m,  a  stellate  stereid ; 
considerably  magnified. 


562 


ECOLOGY 


have  no  stomata,  whereas  emersed  leaves  have  them  in  abundance  (figs. 
768,  769).  Subterranean  scale  leaves  usually  have  no  stomata,  yet 
when  the  primordia  of  such  leaves  are  exposed  to  light  and  air,  they 
often  develop  into  foliage  leaves  with  abundant  stomata,  while  the 
primordia  of  foliage  leaves  when  grown  in  the  soil  become  scale  leaves 
and  are  devoid  of  stomata.  Sometimes  stomatal  development  begins 
in  the  soil,  but  it  is  completed  only  in  the  light  and  air. 

The  factors  involved.  —  The  factors  that  induce  the  appearance  of  stomata  are 
not  known,  though  in  most  instances  air  is  a  necessary  medium  for  their  development. 
It  has  not  been  ascertained  which  element  of  the  air  is  the  most  important,  nor  what 
is  the  nature  of  its  influence.  There  is  some  evidence  also  that  light  tends  to  favor 
the  development  of  stomata.  Since  stomatal  activity  is  confined  to  the  air,  it  is 
possible  that  there  is  some  relation  between  gas  exchange  and  stomatal  development. 


FIG.  806.  —  A  diagrammatic  cross  section  through  a  stoma  of  Helleborus,  illustrat- 
ing guard-cell  movement ;  the  solid  lines  represent  the  open  position  and  the  dotted  lines 
the  closed  position;  closure  involves  the  movement  toward  the  central  slit  (s)  of  the 
ventral  wall  (/)  and  the  dorsal  wall  (d),  these  walls  assuming  the  positions,  /'  and  df,  re- 
spectively; the  hinge  (k)  moves  in  the  same  direction,  assuming  the  position,  h'\  the 
outer  walls  (w)  remain  immobile;  the  movements  of  the  ventral  and  the  dorsal  walls 
decrease  the  sphericity  of  the  cell  lumen  (/);  r',  inner  ridges;  general  lettering  as  in  fig. 
795 ;  highly  magnified.  —  From  SCHWENDENER. 

In  particular,  stomata  are  abundant  where  transpiration  is  vigorous,  and  absent 
where  it  is  reduced  or  wanting. 

The  mechanism  of  stoma  movement.  —  Guard-cell  movements  are  extremely 
complex  and  not  clearly  understood,  while  their  amount  and  importance  often  are 
overestimated.  They  are  best  illustrated  in  mesophytes,  the  stomata  of  most  xero- 
phytes  being  generally  in  a  state  of  partial  closure,  while  those  of  hydrophytes  and 
of  plants  with  motile  leaves,  as  well  as  the  air  pores  of  liverworts,  generally  are  open; 
even  in  mesophytes,  closure  by  no  means  implies  hermetic  sealing.  The  guard  cells, 


LEAVES  563 

as  compared  with  other  epidermal  cells,  are  vigorous  and  active,  living  even  for  weeks 
when  removed  from  the  leaf  and  placed  in  a  nutrient  medium ;  they  are  unusually 
resistant  also  to  high  temperatures  and  to  other  factors  which  generally,  are  harmful 
to  plant  cells.  The  opening  of  stomata  in  moist  air  is  thought  to  be  due  to  the  dis- 
tention  of  the  guard  cells  when  they  absorb  a  large  amount  of  water.  The  result- 
ing approach  toward  sphericity  in  the  guard  cells  and  the  stretching  of  their  thin 
dorsal  walls,  often  supplemented  by  the  action  of  hinge  walls  in  the  subsidiary  cells, 
cause  the  withdrawal  of  the  median  ridges  and  the  consequent  opening  of  the 
central  slit  (fig.  806).  Exposure  to  dry  air  is  thought  to  lessen  the  distention,  where- 
upon the  guard  cells  swing  back  to  a  position  of  closure.  In  grasses  stomatal  open- 
ing is  effected  by  the  mutual  pressure  of  the  dumbbell-shaped  ends  of  the  guard 
cells,  supplemented  by  the  buffer  cells  at  each  end,  which  resist  longitudinal  dis- 
tention (fig.  796). 

The  causes  of  the  nocturnal  closing  and  of  the  diurnal  opening  of  stomata  are 
imperfectly  understood,  though  it  has  been  held  that  the  sugar  manufactured  in 
guard  cells  in  daylight  increases  their  turgor  and  hence  produces  the  distention 
necessary  for  opening.  It  has  been  shown  that  guard  cells  exhibit  greater  turgor 
changes  than  do  the  adjoining  cells,  and  that  they  differ  sharply  from  palisade  cells 
in  that  the  maximum  of  starch  accumulation  takes  place  at  night.  In  daylight  the 
starch  is  digested,  and  in  the  guard  cells  of  Impatiens,  at  least,  the  presence  of 
sugar  actually  has  been  detected.  Some  stomata  open  and  close  periodically,  even 
in  darkness,  as  though  long  exposure  to  alternating  light  and  darkness  had  induced 
an  inherent  rhythm.  Stomata  close  at  low  temperatures,  as  in  evergreen  leaves  in 
winter;  whether  this  is  because  of  reduced  synthesis  or  because  of  lessened  water 
supply  is  not  known.  That  both  light  and  moisture  are  factors  in  guard-cell  move- 
ment, whatever  may  be  the  exact  mechanism  involved,  is  shown  (i)  by  the  fact  that 
the  stomata  in  most  leaves  close  at  night,  whatever  the  moisture  content  of  the  air, 
and  (2)  by  the  fact  that  wilted  leaves  have  closed  stomata,  whatever  the  intensity  of 
the  light. 

The  r61e  of  stomata.  —  Synthetic  and  respiratory  exchanges.  —  The 
chief  advantage  of  stomata  to  plants  is  the  facilitation  of  gas  exchange 
in  connection  with  carbohydrate  synthesis.  Experiments  have  shown 
that  synthetic  activity  is  much  reduced  if  the  stomata  are  artificially 
closed,  as  when  the  stoma-bearing  under  surface  of  a  Ficus  leaf  is 
coated  with  wax.  When  the  upper  surface  of  such  a  leaf  is  slit, 
starch  accumulates  in  the  cells  adjoining  the  incision,  indicating  the 
resumption  of  vigorous  synthetic  activity,  and  doubtless  betokening  the 
free  entrance  of  carbon  dioxid  and  the  exit  of  oxygen.  Small  as  are  the 
stomatal  openings  and  slight  as  is  their  aggregate  surface,  the  intake  of 
carbon  dioxid  by  an  ordinary  leaf  approaches  the  rapidity  with  which 
this  gas  is  absorbed  by  a  free  surface  of  caustic  alkali.  Indeed,  stomata 
might  be  considerably  fewer  or  smaller  than  they  are  without  appreciably 
impairing  synthesis.  Respiratory  gas  exchanges  are  so  slow  and  in- 


564  ECOLOGY 

volve  so  little  oxygen  and  carbon  dioxickthat  stomata  are  of  minor  sig- 
nificance therewith,  although  the  gases  involved  doubtless  pass  through 
the  stomatal  passageways.  Stomata  are  unnecessary  for  carbohydrate 
synthesis  as  well  as  for  respiration,  when  the  outer  epidermal  wall  is 
thin  and  composed  of  cellulose,  as  in  the  leaves  of  mosses  and  of  sub- 
mersed aquatics. 

Transpiration.  —  Much  the  greatest  movement  of  gas  through  the 
stomatal  openings  is  the  outward  movement  of  water  vapor,  known  as 
transpiration,  a  process  that  varies  with  the  saturation  deficit  of  the  ex- 
ternal atmosphere.  It  is  exceptional  to  find  an  atmosphere  in  which 
transpiration  ceases  (this  condition  is  almost  reached  in  some  tropical 
forests),  partly  because  atmospheres  rarely  are  completely  saturated  and 
partly  because  plant  temperatures  commonly  are  higher  than  is  the 
temperature  of  the  surrounding  air.  Transpiration  through  the  epider- 
mal walls  (cuticular  transpiration),  though  often  significant,  as  through 
the  thin  cellulose  walls  of  hydrophytes  and  shade  plants,  usually  is 
much  less  than  that  through  the  stomata,  even  when  the  latter  are  most 
tightly  closed;  the  transpiration  from  an  under  (stoma-bearing)  leaf 
surface  may  be  from  two  to  five  times  as  great  as  is  the  wholly  cuticular 
transpiration  from  a  stoma-free  upper  surface,  in  spite  of  the  more 
favorable  light  and  temperature  conditions  in  the  latter.  However, 
there  is  no  exact  relation  between  transpiration  and  the  size  or  number 
of  stomata;  a  Zea  leaf  transpires  more  per  unit  surface  with  closed 
stomata  than  a  Hartwegia  leaf  with  open  stomata,  and  some  halophytes 
transpire  more  from  the  relatively  stoma-free  upper  surface  than  from  the 
stoma-bearing  under  surface.  There  often  are  wide  variations  in  transpi- 
ration without  corresponding  stomatal  movements,  and  the  maxima  of 
transpiration  and  of  openness  of  stomata  do  not  necessarily  coincide. 

The  significance  of  stomatal  structures  and  of  guard-cell  movements  in  the  pre- 
vention of  excessive  transpiration  will  be  considered  in  the  following  section. 

Functionless  stomata.  —  Adult  submersed  leaves  may  have  stomata  in  all  phases 
of  development,  some  being  fully  formed,  some  resembling  these  except  that  the 
central  slit  never  opens,  some  having  the  air  cavity  clogged  up  with  tyloses,  some 
having  coalesced  cutin  ridges  or  undeveloped  vestibules,  while  cases  are  known 
where  there  occur  only  the  first  stages  of  guard-cell  formation  or  even  where  develop- 
ment ceases  as  soon  as  the  stoma  mother-cell  is  differentiated.  Fully  formed  stomata 
under  water,  though  quite  functionless,  are  not  harmful,  as  has  been  thought,  since 
water  does  not  enter  through  the  open  pores.  In  the  sporophytes  of  Sphagnum  and 
Andreaea  there  are  functionless  stomata,  which  lack  the  subjacent  air  cavity  and 
whose  guard  cells  do  not  split  apart.  The  most  tenable  hypothesis  concerning  the 


LEAVES  565 

above  cases  seems  to  be  that  the  ancestors  of  the  present  forms  lived  in  conditions 
favorable  for  stomatal  development,  and  that  only  the  vestiges  of  such  organs  now 
remain.  Stomata  also  occur  in  various  plants  without  chlorophyll,  on  some  sub- 
terranean organs,  and  on  anthers  and  the  interior  parts  of  carpels,  where  no  relation 
to  carbohydrate  synthesis  is  to  be  looked  for.  Such  stomata  either  are  functionless 
or  they  may  facilitate  respiration  and  transpiration.  It  may  be  noted  that  some 
subterranean  stomata  exhibit  guard-cell  movements. 


5.     PROTECTION  FROM  EXCESSIVE  TRANSPIRATION 

The  significance  of  transpiration.  —  The  importance  of  water.  —  Water 
plays  a  dominant  part  in  the  life  of  plants,  being  the  most  important 
single  factor  in  determining  the  varying  vegetation  of  ponds,  deserts, 
meadows,  rock  cliffs,  and  of  many  other  habitats.  Water  forms  a  large 
part  of  the  raw  material  from  which  plants  build  up  foods  and  tissues, 
and  all  plant  activity  depends  upon  its  presence  in  considerable  amount 
in  the  cell  sap.  Although  there  is  great  and  continual  use  of  water,  and 
although  the  supply  frequently  is  inadequate,  there  is  an  enormous  and 
increasing  loss  by  transpiration.  A  large  sunflower  plant  is  said  to 
transpire  a  liter  of  water  on  a  warm  day,  and  a  tree  transpires  many 
liters.  In  many  plants  from  200  to  400  grams  of  water  are  said  to 
evaporate  for  every  gram  of  dry  solid  matter  produced. 

Transpiration  and  the  absorption  of  salts.  —  Sometimes  it  is  held  that 
all  plant  activities  necessarily  are  beneficial,  else  they  would  have  been 
lost  in  the  progress  of  evolution.  Hence,  various  attempts  have  been 
made  to  discover  the  advantage  of  transpiration.  The  chief  theory  has 
been  that,  by  accelerating  the  movement  of  water  through  plants, 
transpiration  increases  the  amount  of  available  mineral  salts,  since  they 
enter  and  traverse  plant  tissues  in  solution  in  the  water.1  This  theory  is 
not  tenable,  inasmuch  as  salts  are  not  swept  along  in  the  water,  but 
enter  plants  and  move  from  cell  to  cell  independently  of  one  another 
and  of  the  rate  of  movement  of  the  water.  The  rate  of  entrance  of  a 
salt  depends  usually  upon  the  rate  at  which  it  is  utilized  within  the 
plant,  since  it  moves  from  a  place  of  high  to  one  of  low  concentration. 
Within  the  dead  conductive  vessels  a  rapid  movement  of  the  water  may 
facilitate  the  movement  of  salts,  but  no  such  phenomenon  can  occur  in 
the  living  cells  of  the  roots  and  leaves  (see  p.  693).  Furthermore,  this 
theory  greatly  exaggerates  the  amount  of  mineral  salts  used  in  plant 

1  It  has  been  believed  even  that  the  trembling  of  the  leaf  accounts  for  the  rapid  growth 
of  the  aspen  tree,  since  the  increased  transpiration  accelerates  water  conduction  .thereby 
supposedly  increasing  the  supply  of  salts. 


566  ECOLOGY 

growth.  They  are  necessary  only  in  very  small  amounts  and  are  readily 
obtainable.  The  most  luxuriant  vegetation  known  is  that  of  the  humid 
tropical  forests,  where  transpiration  often  is  very  slight  (sometimes 
being  almost  negligible  for  days  at  a  time),  and  there  is  no  transpiration 
in  submersed  vegetation;  yet  in  heither  of  these  instances  do  the  plants 
suffer  from  a  lack  of  salts.  Indeed,  vegetative  luxuriance  varies  in- 
versely rather  than  directly  with  the  transpiration. 

The  possible  advantages  and  the  certain  disadvantages  of  transpi- 
ration. —  The  most  probable  advantage  associated  with  transpiration  is 
in  those  plants  which  have  a  high  turgor  pressure  (Part  II,  p.  336), 
where  it  is  a  means  of  escape  for  an  excess  of  water,  the  injection  of  air 
spaces  thus  being  prevented.  It  has  been  claimed  also  that,  in  sun- 
shine, transpiration  occasions  a  constant  water  renewal,  which  serves  to 
keep  the  leaf  temperature  near  the  optimum;  leaves  exposed  to  sunshine 
in  saturated  air  may  be  some  degrees  warmer  than  are  freely  transpiring 
leaves.  These  and  other  advantages,  however,  are  to  be  regarded  as  in-* 
cidental.  Except  in  water  and  in  saturated  air,  transpiration  is  a  neces- 
sary companion  of  carbohydrate  synthesis,  since  the  very  features  (thin 
expanded  leaves,  numerous  open  stomata,  capacious  air  spaces)  that 
facilitate  the  latter  also  facilitate  the  former.  One  might  conceive  a 
leaf  so  fashioned  as  to  inhibit  transpiration,  but  such  a  structure  would 
be  valueless  in  synthesis.  Transpiration,  then,  is  necessary,  whether 
or  not  it  is,  as  sometimes  is  believed,  a  necessary  evil.  Excessive 
transpiration  is  the  greatest  danger  to  which  plants  are  exposed,  and 
the  harm  that  it  entails  certainly  far  exceeds  any  incidental  good. 

The  protective  structures  and  activities  of  stomata.  —  The  advantage 
of  closure.  —  The  movements  of  guard  cells  commonly  are  interpreted 
as  having  protective  significance,  since,  were  it  not  for  transpiration, 
stomata  might  remain  open  without  harm.  The  closure  of  stomata  by 
night  is  regarded  as  advantageous,  since  carbohydrate  synthesis  ceases 
upon  light  withdrawal,  while  sufficient  oxygen  for  respiration  is  easily 
obtained.  Closure  in  dry  weather  is  regarded  as  useful,  since  at  such 
a  time  it  is  much  more  important  that  the  water  supply  be  conserved 
than  that  synthetic  activity  be  continued.  In  winter,  when  synthesis  is 
slight,  closure  is  beneficial  because  transpiration  is  particularly  harmful 
by  reason  of  diminished  absorption. 

With  the  exception  of  light,  the  factors  that  increase  transpiration  also  are  the 
factors  that  close  the  stomata.  Light,  therefore,  would  be  a  source  of  danger,  but 
for  the  fact  that  the  wilting  it  induces  is  followed,  after  a  time,  by  stomatal  closure 


LEAVES 


567 


occasioned  by  water  withdrawal.  The  abundance  of  wilted  leaves  in  tropical  forests 
is  due  to  the  fact  that  the  stomata  remain  open  on  account  of  the  strong  light  and  the 
humid  air  ;  once  closed,  the  stomata  reopen  only  after  the  leaves  again  become 
turgescent.  Old  leaves  sometimes  lose  water  more  rapidly  than  do  young  leaves, 
because  the  stomatal  mechanism  becomes  less  perfect  with  increasing  age.  Thus, 
while  stomatal  movements  do  not  in  any  true  sense  regulate  transpiration,  through 
closure  they  reduce  its  amount  and  thus  contribute  to  the  welfare  of  the  plant. 

Protective  stomatal  structures.  —  Doubtless  transpiration  is  reduced  by 
various  stomatal  structures,  such  as  guard-cell  cutinization,  hairs,  wax 
and  resin  deposits,  and  tyloses.  The  frequent  restriction  of  stomata  to 
the  under  leaf  surface  probably  has  a  similar  effect.  Of  importance, 
too,  are  pits,  long  or  tortuous  passageways,  and  alternations  of  cutin 
ridges  with  vestibules,  all  of  which 
doubtless  retard  outgoing  water.  But 
stomatal  structures  and  activities  can- 
not stop  transpiration;  at  best  there 
is  only  retardation. 

The  advantages  and  disadvantages 
of  stomata.  —  The  great  advantage  of 
stomata  is  the  facilitation  of  synthesis, 
and  their  great  disadvantage  the  facili- 
tation of  transpiration.  Stomata  are 
most  necessary  where  they  entail  the 
most  harm,  namely,  in  xerophytes, 
where  the  heavy  cutinization  makes  the 
absorption  of  carbon  dioxid  through 
the  cuticle  almost  impossible.  Where 
stomata  entail  no  danger,  as  in  sub-  FIG.  807. -A  cross  section  through 

the  leaf  of  the  oleander  (Nerium  Olean- 

mersed    hydrophytes,    they    are    un-     jer),  showing  a  thick  three-layered  epi- 

necessary,     since     gases    pass    readily      dermis  (e)   with   a  prominent    cuticle 

through  the  non-cutinized  epidermis. 

The  epidermis.  —  General  features . — 
The  leaf  epidermis  consists  commonly 


(c),  a  striking  development  of  palisade 
tissue  (/>)  below  as  well  as  above,  a 
layer  of  sponge  tissue  (/)  near  the 
center  instead  of  near  the  lower  epi- 


of a  single  layer  of  cells  (figs.  760,     dermis'  and  a  stomatal  &  (6>  wi* 

v   °  stomata   (s)  that  are   slightly  elevated 

761,  926),  or  sometimes   of   two  or     above  the  level  of  the  pit  epidermis 

more   such   layers  (as  in  Nerium,  fig.      and  with    protective    epidermal   hairs 
807;     also     figs.     766,     801),    SO     Com-      <*>'    considerably  magnified. 

pactly  placed  that  no  spaces  intervene,  except  where  stomata  occur. 
On  both  leaf  surfaces  in  submersed  plants  (figs.  763,  1018),  and  often 


568  ECOLOGY 

on  the  upper  leaf  surfaces  in  mesophytes  and  in  xerophytes,  there  are 
no  breaks  whatever  in  the  epidermis,  which  thus  contrast  strikingly  with 
the  chlorenchyma.  In  most  dicotyls  the  cells  are  nearly  isodiametric 
(figs.  811,  812,  911),  while  in  monocotyls  they  usually  are  elongated  in 
the  same  direction  as  is  the  leaf  (figs.  796,  804).  Commonly  the  epi- 
dermal cells  of  air  leaves  differ  from  the  mesophyll  cells  in  the  absence  of 
chlorophyll  (except  in  the  guard  cells)  and  in  the  presence  of  a  cutinized 
outer  layer,  the  cuticle;  such  an  epidermis  soon  ceases  to  have  any 
role  other  than  that  of  protection.  In  submersed  plants,  however,  the 
epidermis  contains  chlorophyll  and  remains  uncutinized,  thus  taking  part 
in  absorption  and  in  synthesis.  The  epidermal  cells  of  hydrophytes  and 

^      of   mesophytes    usually  are 
,C     larger  than  are  like  cells  in 
~-cf     xerophytes,  and  the  growth 
of  mesophytes  in  xerophytic 
conditions  commonly  results 
in  a  decreased  cell  size. 
FIG.  808.  —A  cross  section  through  the  leaf  epi-        Theouter  epidermal  walls; 
dermis   of   the  century   plant    (Agave  americana),          ..    .      ..  „,, 

showing  the  cellulose  layer  (e"),  the  cuticular  layer     «*•*•«*»•  ~  The       outer 
(c'\  the  cuticle  (c),  and  a  superficial   layer  of  wax     Wall  of  the  epidermis,  OHgi- 
grains  (b)  which  constitute  a  glaucous  bloom;  highly     naNy   thin,  and   also  perme- 
able  because    composed   of 

cellulose,  in  the  adult  leaf  commonly  is  thickened  through  the  deposi- 
tion of  cutin,  a  fatty  substance  highly  impermeable  to  water.  Usually 
the  cutinized  portion  forms  a  continuous  yellowish  coat,  the  cuticle 
(figs.  807,  810),  below  which  is  the  slightly  modified  cellulose  portion  of 
the  outer  wall.  In  some  xerophytes  a  cuticular  layer  is  interposed  be- 
tween the  cellulose  and  the  cuticle,  the  wall  thickening  progressively 
inward  before  it  becomes  cutinized  (fig.  808).  In  Pinus  the  encroach- 
ing wall  finally  fills  the  entire  lumen  (fig.  1039).  In  the  grasses  and  in 
Equisetum,  in  addition  to  cutin,  silica  is  deposited  in  the  cell  walls. 
Highly  cutinized  walls  are  characteristic  of  xerophytes,  and  particularly 
of  evergreen  xerophytes,  such  as  conifers,  ericads,  and  many  broad- 
leaved  trees  of  warm  temperate  regions,  for  example,  the  live  oak  and  the 
olive.  Heavy  cutinization  characterizes  many  alpine  and  arctic  plants, 
and  also  plants  of  peat  bogs  (as  Ledum,  Andromeda,  Chamaedaphne) 
and  of  tropical  salt  marshes  (as  the  mangroves) ;  even  mesophytic  ever- 
greens, as  the  yew  and  the  hemlock,  may  have  a  prominent  cuticle. 
Many  succulent  xerophytes,  such  as  Sedum  and  Salsola,  have  a  very 


LEAVES  569 

weak  development  of  cutin.      Submersed  plants,  both  in  fresh  and  in 
salt  water,  commonly  are  free  from  cutinization. 

The  influence  of  external  factors  upon  cutinization.  —  No  plant  struc- 
ture reacts  more  readily  to  changes  in  conditions  than  does  the  cuticle, 
submergence  in  water  inhibiting  its  formation,  and  desiccation  favoring 
its  maximum  development.  In  the  air  the  thickness  of  the  cuticle 
appears  to  vary  directly  with  the  transpiration,  it  being  thinner  on  under 
than  on  upper  leaf  surfaces,  thinner  in  stomatal  pits  than  at  the  surface 
(figs.  801,  807),  and  thinner  in  protected  than  in  exposed  situations,  as 
in  the  basal  leaves  of  Tilia  in  moist  woods,  in  comparison  with  its  top 
leaves  on  a  dry  hill  (see  figs.  770,  771).  As  in  the  case  of  palisade  cells, 
cutin  formation  approaches  its  maximum,  both  where  transpiration  is 
large  in  amount,  as  in  most  xerophytes  or  even  in  exposed  hydrophytes 
like  the  bulrush,  and  where  there  is  a  high  ratio  of  transpiration  to 
absorption,  as  in  alpine  and  arctic  habitats,  in  peat  bogs,  and  in  tropi- 
cal salt  marshes.  Cutin  formation  is  increased  when  'ordinary  meso- 
phytes  (such  as  wheat)  are  grown  in  concentrated  solutions,  and  in 
the  mangrove,  cutinization  is  most  marked  in  the  saltiest  soils. 

In  at  least  one  submersed  marine  plant,  Cymodocea,  the  epidermis  is  cutinized, 
and  the  nearly  related  Zoster  a  has  no  epidermal  chlorophyll;  it  is  possible  that 
these  features  of  air  leaves  are  due  to  the  high  concentration  of  the  sea  water.  It 
should  be  noted  that  the  cuticle  is  not  always  a  plastic  structure;  in  the  conifers  it 
seems  as  rigid  and  as  unrelated  to  environment  as  are  any  of  the  internal  tissues. 

The  rdle  of  cutin.  —  Cutin  retards  the  egress  of  water  from  leaves, 
not  so  much  because  of  its  thickness,  as  because  its  fatty  character 
makes  it  relatively  impermeable  to  water.  The  transpiration  from  a 
peeled  apple  for  a  period  of  three  hours  is  twenty  times  that  from  an 
apple  with  cuticle  intact.  A  water  leaf,  exposed  to  dry  air,  withers 
almost  immediately  because  of  its  uncutinized  epidermis.  If  the 
stomatal  surface  of  a  Ficus  leaf  is  coated  with  wax,  the  loss  of  water  is 
enormously  reduced,  and  may  amount  in  one  day  to  but  one  two- 
hundredth  of  that  from  a  water  surface  of  equal  area.  The  cuticle, 
therefore,  is  a  transpiration-reducing  structure  of  high  efficiency,  and 
were  it  not  for  the  stomata,  which  entail  abundant  evaporation,  leaves 
would  be  almost  perfectly  protected  by  their  cutin  layer.  Its  value  is 
apparent,  not  only  in  dry  habitats,  but  also  in  peat  bogs  and  in  salt 
marshes,  as  well  as  in  alpine  and  arctic  conditions,  because  of  inade- 
quate absorption. 


570  ECOLOGY 

The  stiffness  of  most  evergreen  leaves  is  due  to  the  cuticle,  which  thus 
is  of  mechanical  value,  giving  protection  from  winds  and  storms  and 
also  from  fungi,  insects,  and  grazing  animals  (fig.  809).  Doubtless  the 
thick  epidermis  of  such  leaves  as  those  of  Nerium  and  Ficus  is  of 
mechanical  value  in  addition  to  its  importance  in  checking  evaporation; 

in  the  bromelias  there  is  a  thin  outer  layer 
of  cutinized  epidermal  cells  that  checks 

.^^^^^  ^"^  transpiration  and  a  thick  but  non-cutinized 
<gf  >^  inner  layer,  whose  r61e  is  chiefly  mechanical. 

FIG.  809. — A  leaf  of  a  In  some  leaves  stiffness  is  increased  further 
broad-leaved  sclerophyll,  the  by  bast  fibers,  collenchy ma  cells,  and  sclereids 
American  holly  (Ilex  opaca),  (  }  fa  adyant  f  fa  cutiniza. 

constituting  a  representative 

coriaceous  leaf  of  xerophytic  tion  to  mesophytes  (as  Taxus,  Tsuga,  and 
aspect,  its  stiffness  being  due  Ficus)  is  more  difficult  to  discern.  Possibly 

such  plants  have  limited  root  systems  and 

relatively  slight'  absorption  or  imperfect  conduction  (as  in  some  coni- 
fers), the  cuticle  thus  being  advantageous  in  reducing  transpiration. 
Possibly  the  cutin  has  no  role  of  importance  in  such  plants,  repre- 
senting a  surviving  structure  once  of  use  in  some  former  xerophytic  hab- 
itat, or,  perhaps,  a  structure  which  was  never  of  particular  advantage. 

Surface  peculiarities;  wax  and  resin  deposits.  —  Many  leaves,  espe- 
cially those  that  appear  "  glaucous,"  have  a  bluish  gray  surface  film  of 
wax,   sometimes  known   as  bloom,   which  is 
readily  removed  by  rubbing  (fig.  810).    Some- 
times these  wax  deposits  are  thick,  forming  a 
brittle  crust,  as  in  Sempervivum  and  in  the 
wax  palms,  or  layers  of  vertical  rods,  as  in        FIG.  8 10.— A  cross  section 
the  sugar  cane.    Wax  coats  are  best  developed    through  the  leaf  epidermis  of 

,  -,  ,  \          j     a  xerophytic  individual  of  the 

in  xerophytes  (e.g.  Agave,  Crassulaceae),  and  glaucbus  willow  (Salix  glauco_ 
like  cutin,  they  appear  to  be  increased  by  phyiia\  showing  just  outside 
excessive  transpiration.  In  common  with  the  cuticle  (0  a  thick  layer  of 

.       .      -  wax  grains  (6),  constituting  a 

other  xerophytic  features,  glaucous  leaves  are    g]aucous  bloom;  highly  mag. 
abundant  in  peat  bogs  and  in  maritime  situa-    nified. 
tions;  in  solutions  of  increasing  concentration 

the  layer  of  bloom  increases  in  thickness.  Thin  as  they  are,  wax 
coats  effectively  impede  transpiration,  the  mere  rubbing  of  a  glaucous 
leaf  sometimes  inducing  an  increase  of  a  third  in  the  transpired 
water.  Wax  coats  also  retard  the  heating  of  leaves.  As  with 
hairs,  but  not  with  cutin,  wax  coats  are  best  developed  on  the 


LEAVES 


571 


under  leaf  surface,  where  the  stomata  are  the  more  abundant.  The 
leaves  of  many  desert  xerophytes  (as  in  the  creosote  bush)  are  coated 
with  resin,  and  often  have  a  varnished  aspect,  shining  in  the  sun- 
light. Many  tropical  forest  leaves  also  are  shiny.  The  factors  influ- 
encing the  formation  of  resin  coats  are  unknown.  Like  wax  coats,  they 
may  retard  transpiration,  and  it  has  been  suggested  that  they  reflect 
light,  an  excess  of  which  may  injure  the  chlorophyll.  It  has  been 
shown  that  shiny  leaves  become  more  slowly  heated  in  the  sunlight 


FIG.  811.  —  A  surface  view  of  a 
part  of  the  under  epidermis  of  a  leaf  of 
Coleus,  showing  the  wavy  lateral  walls 
characteristic  of  the  un^igr  epidermis  of 
mesophytic  dicotyl  leaves;  the  shaded 
cells  contain  anthocyan;  note  the  sto- 
mata with  their  crescentic  guard  cells, 
chloroplasts,  and  central  slit;  highly 
magnified. 


FIG.  812.  —  A  surface  view  of  a  part 
of  the  upper  epidermis  of  a  leaf  of  Coleus, 
showing  the  straight  lateral  walls  character- 
istic of  the  upper  epidermis  of  mesophytic 
dicotyl  leaves;  note  the  absence  of  an- 
thocyan and  of  stomata;  magnification  as 
in  fig.  811. 


than  do  similar  leaves  that  are  not  shiny,  so  that  evaporation  probably 
is  reduced  thereby. 

Floating  leaves  (as  in  the  water  lilies)  often  have  waxy  or  resinous  sur- 
faces, which  are  highly  advantageous  in  that  they  prevent  the  wetting  of 
the  stoma-bearing  surface,  thus  facilitating  gas  exchanges.  Many  meso- 
phytic leaves  also  (as  in  the  meadow  rue)  are  not  readily  wetted,  their 
silvery  aspect  when  thrust  into  the  water  being  due  to  an  air  film  next  to 
the  waxy  surface.  Probably  few  stomatal  surfaces  are  readily  wetted, 
contrasting  thus  with  the  surfaces  of  submersed  leaves  and  with  the 
stoma-free  upper  surface  of  many  tropical  leaves. 


The  lateral  walls.  —  In  mesophytic  dicotyls  the  lateral  epidermal  walls  commonly 
are  straight  on  the  upper  leaf  surface  and  irregularly  wavy  on  the  lower  or  stoma- 


572 


ECOLOGY 


hearing  surface  (figs.  811,  812).  In  xcrophytic  and  hydrophytic  dicotyls  and  gen- 
erally in  monocotyls,  the  side  walls  of  the  epidermis  usually  are  straight  on  both 
leaf  surfaces  (fig.  804),  though  in  Maranta  and  in  various  grasses  there  are  wavy 
walls  of  striking  regularity.  In  plastic  species,  waviness  culminates  in  mesophytic 
conditions ;  increased  and  decreased  transpiration  each  result  in  relatively  straight 

lateral  walls.  Whether  wavy 
walls  have  a  role  of  impor- 
tance is  not  known,  though 
they  have  been  thought  to 
add  to  the  strength  of  the 
epidermis  and  also  to  give  a 
greater  diffusion  surface  for 
substances  passing  from  cell 
to  cell. 


FIGS.  813,  814.  —  Appressed  unicellular  epidermal 
hairs  from  a  scale  leaf  of  the  winter  bud  of  the  Norway 
maple  (Acer  platanoidcs) :  813,  a  general  view,  as  seen  in 
longitudinal  section ;  note  the  common  orientation  of  the 
hairs,  which  is  responsible  for  the  silky  aspect  of  the 
scale  leaf;  considerably  magnified;  814,  a  single  hair; 
highly  magnified. 


Structural  features  of 
epidermal  hairs. — "  Pro- 
tective" hairs  commonly 
are  stiff,  thick-walled 
structures,  which  often 

are  dead  and  air-containing  at  maturity.  They  may  be  attenuated  uni- 
cellular structures  perpendicular  to  the  leaf  surface  (as  in  Verbena  or 

in  Potentilla,  fig.  914);  more 

rarely  they  are  parallel  to  the 

leaf  surface  and  closely  ap- 

pressed,  their  common  orien- 
tation giving  the  leaf  a  silky 

aspect   (as  in   Aster  sericeus 

and  in  the  bud  scales  of  Acer 

platanoides ,    figs.   813,   814). 

Other  hairs  are  similar  but 

multicellular,        occasionally 

being    branched    (as   in    the 

mullein,  fig.  815).     A  woolly 

felt,    made    up    of    a   dense 

tangle  of  long  hairs  more  or 

less  parallel  to  the   surface, 

extends  in  various  directions 

(as  in   the   everlastings  and 

cinerarias,    figs.    816,    817). 

Stellate  hairs  divide  at  the  base  into  horizontal  branches,  as  in  various 

crucifers  and  mallows  (fig.  773).      In  Shepherdia  and  in  Elaeagnus- 


FIG.  815.  —  Branched  multicellular  hairs  from 
a  leaf  of  the  mullein  (Verbascum  Thapsus);  con- 
siderably magnified. 


LEAVES 


573 


FIGS.  816,  817.  — Multicellular  filamen- 
tous hairs    from    a   leaf   of    the    cineraria 
(Senecio    cruentus):    816,    a   general   view, 
as  seen   in   cross  section,   showing  several 
hairs  with  their  whip-like  ends,  which  spread 
out  horizontally,  forming  cham- 
bers between  the  basal  portions 
of  the  hairs ;  note  the  great  length 
of  the  hairs  in  proportion  to  the 
leaf  diameter ;  considerably  mag- 
nified; 817,  a  single  hair;  highly 
magnified. 


ings, Ledum, etc.).  Leaves 
frequently  are  hairier 
when  young  than  when 
mature,  many  of  the  hairs 
soon  breaking  at  a  more 
or  less  definite  weak  spot. 
While  some  leaves  are 
equally  hairy  on  both 
surfaces,  many  leaves  are 
hairy  mainly  or  only  on 
the  under  (stoma-bearing) 
surface  (as  in  the  silver 


the  leaf  hairs  take  the  form  of 
brown  or  silvery-gray  scales  (figs. 
8 1 8,  819).  In  scabrous  leaves 
the  surface  is  papillate  or  warty, 
as  in  many  composites. 

Variations  in  hair  distribution. 
—  "Protective"  epidermal  hairs 
are  most  abundant  in  xerophytes, 
especially  in  sandy  and  rocky 
regions  and  in  deserts,  where 
they  often  give  a  characteristic 
grayish  aspect  to  the  vegetation, 
as  in  sage-brush  deserts.  In 
alpine  and  arctic  regions  and  in 
bogs  and  salt  marshes,  hairs  are 
less  abundant  though  by  no 
means  absent  (as  in  the  everlast- 


FIGS.  818,  819.  —  Multicellular  shield-shaped  scale 
hairs  from  the  leaf  of  Elaeagnus :  818,  a  general  view  as 
seen  in  cross  section,  showing  the  hairs,  each  of  which 
consists  of  a  vertical  stalk  surmounted  by  a  horizontal 
scale;  considerably  magnified ;  819,  the  terminal  scale, 
as  seen  from  above ;  highly  magnified. 


574 


ECOLOGY 


poplar,  fig.  820).  In  many  leaves  there  is  no  obvious  relation  between 
habitat  and  hair  production,  mesophytes  frequently  and  hydrophytes 
rarely  (as  in  Pistia  and  Salvinia)  being  conspicuously  hairy  (fig.  897). 
The  influence  of  external  factors  upon  the  production  and  form  of 
epidermal  hairs.  —  Perhaps  the  most  striking  variations  in  hairiness 
within  the  same  species  are  found  in  amphibious  plants,  such  as  Jussiaea 
repens  and  Polygonum  amphibium.  Aquatic 
individuals  of  the  latter  have  smooth  leaves 
(fig.  821),  while  land  forms  have  leaves  covered 
with  numerous  stiff  and  long  hairs  (fig.  822). 
Indeed,  so  different  are  the  two  forms  of  Poly- 
gonum amphibium,  that  the  hairy  form  has  been 
regarded  as  a  different  species,  P.  Hartwrightii, 
although  it  is  possible  to  find  both  forms  on  the 
same  plant  at  the  edge  of  a  pond.  In  a  number 
of  land  species  with  mesophytic  and  xerophytic 
forms,  the  latter  are  the  more  hairy,  and  in 

FIG.  820.  —  A  cross  sec-  .   .  J, 

tion  through  a  leaf  of  the  some  plants  (as  Artemisia  canademis}  winter 
silver  poplar  (Popuius  alba),  leaves  are  more  hairy  than  are  the  summer 
showing  a  tangled  felt  of  ieaves>  Certain  species  (as  Convolvulus  sepium) 

woolly  hairs  on  the  under  ...  .  .  .     . 

and  stoma-bearing  surface,  are  much  more  hair7  m  maritime  than  in  inland 
situations.  From  analogy  with  the  excretion 
of  wax  and  the  formation  of  cutin  and  of 
palisade  cells,  hair  formation  would  appear  to 

tibule   thus    being   clearly     be  facilitated  by  increased   transpiration,  yet 

marked  off  from  the  sto-     it  ^  difficult  to  see  in  just  what  way  the  loss 

matal  cavity;    this  leaf  has 

a  representative  mesophytic    of  water  can  produce  a  complex  new  structure 
chlorenchyma  with  two  rows     like  the  hair  of  Polygonum.     Possibly  water  is 
a  factor  which  inhibits  hair  development,  trans- 
piration acting  rather  as  a  releasing  stimulus 
(see  p.  952). 


the  upper  epidermis  (e) 
being  smooth ;  note  that  the 
stoma  (s)  has  a  well-defined 
inner  ridge,  the  inner  ves- 


of  palisade  cells  (p)  and  a 
loose  sponge  tissue  (/) ;  con- 
siderably magnified. 


Some  remarkable  cases  of  hair  production  occur  on  insect  galls.  The  leaves  of 
Vitis  aestivalis  when  young  and  of  F.  Labrusca  throughout  life  are  covered  with  a 
tawny  tomentum,  while  the  leaves  of  V.  vulpina  and  V.  cordifolia  are  smooth. 
However,  insect  galls  occurring  on  the  latter  species  are  covered  with  a  hairy  coat 
like  that  of  ordinary  leaves  in  the  former  species  (fig.  823).  It  would  appear  that 
the  smooth-leaved  species  are  potentially  hairy,  needing  only  the  insect  stimulus  to 
induce  hair  production.  Similarly,  stellate  hairs,  resembling  those  usually  found 
on  hairy- leaved  oaks,  may  be  produced  on  smooth-leaved  oaks  through  insect 
activity;  in  like  manner,  thorns  may  be  produced  on  the  otherwise  smooth  leaves 


LEAVES 


575 


of  the  rose  (see  figs.  1094-1096  and  adjoining  text).  It  is  conceivable  that  the 
influence  of  gall  insects  upon  hair  production  is  essentially  comparable  to  that  of 
xerophytic  factors,  especially  if  the  insects  introduce  osmotically  active  substances 
into  the  plant  (see  p.  785). 


822 


FIGS.  821,  822. — Variation  in  Polygonum  amphibium:  821,  a  branch  from  the 
aquatic  form  with  floating  leaves  borne  on  a  submersed  stem;  note  the  lax  horizontal 
stem,  the  smooth  leaves,  and  the  inconspicuous  stipules  (0);  this  represents  the  "typical" 
P.  amphibium;  822,  a  branch  from  the  land  form  (often  erroneously  called  P.  Hart- 
•wrightii);  note  the  stout,  erect  emersed  stem,  the  stiff  hairy  leaves,  and  the  prominent 
sheathing  stipules  (0). 

Whatever  may  be  the  influence  of  desiccation  or  of  insect  activity  upon  hair 
production,  neither  these  nor  other  external  influences  appear  to  modify  hair  form 
to  any  great  extent.  In  most  species  the  shape  and  structure  of  the  hairs  seem 
inherent,  external  factors  determining  only  their  presence  or  their  absence.  In 


576 


ECOLOGY 


many  instances,  even  the  production  of  hairs  seems  unrelated  to  external  factors,  as 
possibly  in  Coreopsis  lanceolata,  where  both  hairy  and  smooth  forms  grow  in 
similar  habitats.  Perhaps  the  reference  of  structures  to  inherent  causes  is  but  an 
expression  of  ignorance  that  may  be  eliminated  upon  adequate  experimentation. 

The  role  of  epidermal  hairs.  —  Hairs  commonly  are  believed  to  have • 
an  important  role  in  the  reduction  of  transpiration,  but  the  evidence  for 

this  view  is  not  abun- 
dant. Probably  they 
are  much  inferior  in  this 
respect  to  cutin  or  even 
to  wax  coats.  The  most 
efficient  form  of  hair  pro- 
tection would  seem  to  be 
that  afforded  by  a  woolly 
felt ;  it  has  been  shown 
that  the  removal  of  such 
a  felt  in  Stachys  lanata 
results  in  an  increase  of 
twenty  to  fifty  per  cent 
in  the  transpiration. 
Evaporating  surfaces 
artificially  coated  with 
FIG.  823.  —  A  portion  of  a  branch  of  the  river  hairy  felts  have  been 

grape  (Vitis  vulpina),  illustrating  the  structural  trans-  shown  to  lose  much  less 
formation  that  a  plant  may  undergo  when  attacked  fa  fa 

by  a  gall-forming  insect;   the  attack  of  a  gall-fly  (Ceci- 

domyia  V  itis-pomum)  induces  the  development  of  en-  hair    covering.        Similar 

tirely  new  organs,  and  parts  that  otherwise  are  smooth  results    mav    be     looked 

become  woolly-pubescent.  r  , , 

for  in  scale-covered  leaves 

or  in  leaves  with  appressed  or  branched  hairs,  where  the  hairy  coat 
is  dense  enough  to  retard  the  escaping  water  vapor.  In  the  cinerarias 
the  hairs  at  first  grow  erect  and  then  horizontally,  producing  a 
chambered  layer,  while  in  Espeletia  there  are  two  or  more  such  layers; 
the  retarding  effect  of  these  layers  upon  escaping  water  vapor  is 
not  difficult  to  understand.  In  most  hairy  plants,  however,  the 
hairs  are  erect  and  more  or  less  scattered,  so  that  it  is  difficult  to 
see  how  they  can  appreciably  retard  escaping  water  vapor,  though 
their  presence  may  to  some  extent  reduce  the  evaporating  surface. 
Hairs  often  are  most  abundant  on  those  parts  that  most  need  protection, 
as  on  young  leaves  and  on  the  stoma-bearing  surface  of  adult  leaves. 


LEAVES 


577 


Hairs  have  been  supposed  to  protect  leaves  against  the  injurious' 
effects  of  heat,  light,  and  cold.  In  particular,  hairy  coats  on  the  upper 
surface  have  been  thought  to  screen  off  injurious  light  and  heat  rays,  it 
having  been  shown  that  hair-clad  leaves  become  heated  more  slowly  than 
do  smooth  leaves.  The  hairs  of  aquatics 
(as  in  Salvinid)  are  of  undoubted  service 
in  preventing  leaf  wetting,  thereby  facili- 
tating unimpeded  gas  exchange.  Stiff 
hairs,  as  in  mullein,  and  spiny  hairs,  as 
in  the  thistles,  probably  afford  some  pro- 
tection against  grazing  animals ;  in  pas- 
tures, thistles  frequently  are  untouched, 
while  other  plants  are  eaten  greedily. 

It  must  be  admitted  that  the  known  uses  of 
leaf  hairs  are  small  in  comparison  with  their 
abundant  development.  While  the  discovery  of 
advantages  now  unknown  is  possible,  it  is  much 
more  likely  that  most  such  hairs  are  of  little  or 
no  advantage.  The  idea  should  be  abandoned 
that  plants  have  the  power  to  discard  organs  that 
are  not  of  use. 

Stinging  hairs.  —  Stinging  hairs  are  found 
in  various  members  of  the  nettle  and  spurge 
families,  and  consist  commonly  of  a  large  elon- 
gated cell  inserted  in  a  cup-shaped  emergence 
(fig.  824).  The  cell  walls  are  thick  and  brittle, 
being  silicified  or  calcified,  and  the  enlarged  end 
is  turned  slightly  to  one  side  (figs.  825,  826). 
When  struck  sharply,  the  hair  ruptures  obliquely 
just  below  the  head,  leaving  a  sharp  point  that 
suggests  a  hypodermic  needle.  The  cell  con- 
tents, which  are  in  a  state  of  high  turgor,  rush 
out,  injecting  an  albuminoid  poison  into  the 
wound,  if  one  is  made  by  the  broken  hair  (fig. 
827).  Nowhere  in  plants  is  there  an  organ  more 
clearly  fitted  for  a  definite  function  than  are 
stinging  hairs,  yet  there  is  no  evidence  that  they 


FIGS.  824-82 7.  —  Stinging  hairs : 
824,  a  stinging  hair  from  the  wood- 
nettle  (Laportea,  canadensis),  a 
unicellular  structure  seated  on  a 
slight  leaf  emergence  (e) ;  note  the 
bulb-like  base  (6)  containing  the 
prominent  nucleus  («);  note  also 
the  curved  tip  (t);  considerably 
magnified;  825,  826,  the  tips  of 
similar  hairs,  showing  the  thin 
neck  where  oblique  breakage  oc- 
curs; highly  magnified;  827,  the 
tip  of  a  broken  stinging  hair  of 
Urtica  dioica,  showing  the  poison- 
ous contents  flowing  out;  highly 
magnified.  —  827  from  HABER- 

LANDT. 


are  of  any  special  advantage  to  the  plants  pos- 
sessing them.     Nothing  is  known  concerning  the  factors  underlying  their  develop- 
ment, since  they  neither  vary  appreciably  nor  grade   obviously  into   other   sorts 
of  hairs. 

The  reduction  of  transpiring  surface.  —  The  most  fundamental  dis- 
tinction between  xerophytic  and  mesophytic  leaves  is  in  the  proportion 


578 


ECOLOGY 


FIGS.  828-830. -Leaves 


of  surface  to  volume,  the  former  leaves  being  narrower  and  thicker 
than  the  latter,  and  thus  presenting  a  smaller  transpiring  surface  (figs. 
867,  868) ;  furthermore,  small  leaves  are  much  less  rapidly  heated  than 
are  large  leaves  with  equal  exposure  to  the  sun.  The  extremes  of 
divergence  in  this  respect  are  represented,  on  the  one  hand,  by  hydro- 
phytic  or  mesophytic  leaves  one  to  three  cells  thick  (as  in  mosses,  filmy 
ferns,  and  pondweeds,  fig.  763),  and,  on  the 
other  hand,  by  xerophytic  leaves  that  are 
round  in  cross  section  (figs.  926,  927).  Within 
the  same  species  (as  Lactuca  scariola)  the  xero- 
phytic form  has  narrower  and  thicker  leaves 
than  has  the  mesophytic  form.  In  some 
xerophytes  (notably  in  the  Ericaceae)  the 
leaves  are  revolute  (i.e.  with  edges  curved 
under),  a  habit  that  results  in  a  reduced  trans- 
piring surface  without  a  change  in  volume 
(figs.  828-830).  Frequently  sun  leaves  are 
concave  as  viewed  from  above,  hence  pre- 
senting a  reduced  surface  to  the  sun,  and 

of  certain  Ericaceae,  showing  contrasting  with  the  flatness  of  shade  leaves, 
xerophytic  features;  such , ,  xhe  internal  transpiring  surface  of  a  xero- 

leaves  are  stiff  and  leathery,  u^-ir-j                 JT_^I            j      ..•           t 

largely  by  reason  of  promi-  P^  leaf  1S  decreased  by  the  reduction  of 

nent  cutinization :  828,  a  leaf  air  spaces  and  by  the  consequent  compact  ar- 

of  the  Labrador  tea  (Ledum  rangement  of  the  mesophyll ;  this  feature,  like 

eroenlandicum),  viewed  from        .         ,  ,  .. 

above;  829, aleaf  of  thesame  the  changes  in  proportion  between  surface  and 
plant,  viewed  from  beneath,  volume,  can  be  induced  experimentally  (figs, 
showing  a  copious  woolly  to-  768,  769),  The  most  pronounced  reductions 

mentum  and  revolute  edges;  ,  .   .  ,.  P          ,  .  ,  . 

83o,  a  diagrammatic  cross  m  the  transpiring  surface  are  found  in  cushion 
section  of  a  revolute  leaf,  plants  and  in  plants  wholly  without  leaves; 

these  and  similar  cases  will  be  treated  under 

stems  (p.  739). 

Reduction  in  transpiration  due  to  leaf  orientation.  —  Diaphototropic 
leaves  appear  to  be  placed  in  a  position  favorable  for  maximum  synthesis, 
while  the  vertical  or  profile  position  seems  to  make  possible  a  reduction  in 
transpiration,  by  reason  of  less  direct  exposure  to  the  sun.  The  leaf 
orientation  of  Lactuca  (fig.  785)  and  of  Nicotiana  (fig.  786)  may  be  re- 
garded as  advantageous  from  the  standpoint  of  protection  from  trans- 
piration, the  latter  plant  exhibiting,  as  the  transpiration  increases,  a 
series  of  leaf  positions  varying  from  horizontal  to  vertical. 


that    of    the    bog    rosemary 
(Andromeda  glaucophylla). 


LEAVES 


579 


Motile  leaves.  —  The  phenomena  of  leaf  motility.  —  In  many  Legu- 
minosae  (as  in  the  clovers  and  locusts)  and  in  Oxalis  the  leaflets  of  the 
compound  leaves  close  by  night  (especially  in  cool  weather)  and  open  by 
day ;  such  closing  movements  have  been  called  photeolic  movements  or, 
less  correctly,  sleep  movements  or  nyctitropic  movements.  In  many 
legumes  desiccation  causes  the  leaflets  to  assume  the  closed  position  ; 

in  Mimosa  contact  also 
produces  the  same  effect 
(figs.  831,  832),  and  in 
Desmodium  gyrans  there 
are  movements  apparently 
without  external  cause 
(fig.  684).  While  most 
motile  leaves  are  com- 
pound (figs.  833,  834), 


831 


832 


FIGS.  831,  832.  —  Leaf  motility  in  the  sensi- 
tive plant  (Mimosa  pudica):  831,  an  open  leaf; 
832,  a  leaf  whose  leaflets  (/)  have  been  closed  by 
mechanical  impact;  note  also  that  the  petiole  (£) 
has  dropped;  s,  stipule;  m,  pulvinus. 


FIGS.  833,  834.  —  Leaf  motility 
in  Oxalis :  833,  open  leaves  as  seen 
by  day ;  834,  closed  leaves  as  seen 
by  night. 


some  simple  leaves  exhibit  motility,  as  in  Portulaca  oleracea  (figs.  686, 
687)  and  in  Euphorbia  polygonifolia,  those  of  the  latter  closing  along 
the  median  line  like  a  book. 

Most  leaf  movements  are  due  to  changes  in  the  turgescence  of  the 
delicate  cells  that  make  up  the  body  of  the  enlarged  base  (known  as  the 
pulvinus)  of  the  petiole  or  petiolule  (leaflet  stalk) ;  the  central  position 
of  the  delicate  conductive  bundle  and  the  absence  of  mechanical  cells 
facilitate  motility.  Closing  may  be  due  to  a  decrease  of  turgescence,  as 
in  drought,  or  to  an  increase,  which  is  greater  on  one  side  than  on  the 


580  ECOLOGY 

other,  as  in  the  night  closing  of  the  bean  leaflets.  The  rapid  closing  of 
Mimosa  by  contact  is  due  to  the  sudden  escape  of  water  from  the  cells 
into  the  adjoining  air  spaces,  while  subsequent  opening  is  due  to  its  slow 
reentrance ;  the  slower  photeolic  movements  are  made  possible  by  a 
change  in  the  permeability  of  the  cell  membranes  occasioned  by  a  change 
in  illumination.  The  leaflets  of  the  hydrophyte,  Marsilea,  close  if 
emersed,  but  not  if  floating,  possibly  because  water  contact  preserves 
the  necessary  tufgescence,  or  possibly  because  water  may  mechanically 
inhibit  closing. 

The  advantages  of  leaf  motility.  —  The  most  obvious  advantage  which** 
plants  derive  from  leaf  motility  is  protection  from  excessive  transpi- 
ration, by  reason  of  the  reduced  surface  in  the  closed  position  that  is 
assumed  as  a  result  of  desiccation.  In  some  leaves  (as  in  those  which  fold 
along  the  midribs)  the  exposed  surface  may  be  reduced  one  half,  and  it 
may  be  reduced  almost  as  much  in  a  plant  like  Mimosa,  where  the  leaflet 
segments  closely  overlap  one  another.  In  other  cases,  closing  means 
rather  a  changed  orientation,  as  in  the  drooping  leaflets  of  the  bean, 
or  in  the  erect  leaflets  of  the  beach  pea,  which  assume  a  temporary 
profile  position,  suggesting  the  permanent  profile  position  of  Lactuca 
and  Silphium. 

Night  closing  is  a  much  more  common  phenomenon  than  is  drought 
closing,  but  its  advantage  is  not  obvious.  Protection  from  cold  has 
been  suggested,  but  this  view  has  no  supporting  evidence;  motile  leaves 
often  are  open  on  cool  days  and  closed  on  warm  nights.  The  facilitation 
of  nocturnal  transpiration  also  has  been  suggested,  it  being  supposed  that 
vertically  prevents  wetting  by  dew;  but  there  is  no  apparent  advantage 
from  increased  nocturnal  transpiration.  Perhaps  nocturnal  closing  is 
quite  useless,  as  is  pretty  certainly  the  case  with  such  extraordinary 
movements  as  those  of  Desmodium  gyrans  and  the  contact  movements 
of  Mimosa. 

Leaves  are  subject  to  passive  movements  through  the  action  of  wind  or  water. 
The  leaves  of  the  reed  (Phragmites)  are  attached  to  the  stem  in  such  a  manner 
as  to  swing  around  in  the  wind  like  a  weather  vane,  their  ready  yielding  prevent- 
ing injury.  Compound  leaves,  as  in  the  coconut  and  in  many  submersed  aquat- 
ics (as  the  water  milfoil),  offer  but  little  resistance  to  wind  or  water  currents,  and 
hence  escape  injury,  while  large,  simple  leaves,  as  those  of  the  banana  (fig.  846) 
or  of  the  water  lilies,  are  shredded  when  similarly  exposed.  The  lateral  flatten- 
ing of  the  petiole  of  the  aspen  and  of  other  species  of  Populus  results,  even  in 
the  lightest  breezes,  in  an  almost  constant  trembling  of  the  leaf,  a  phenomenon  with- 
out obvious  advantage. 


LEAVES      .  581 

Infolded  and  withered  leaves.  —  Many  grass  leaves  (known  as  in-volute]  roll  in- 
ward when  exposed  to  desiccation,  owing  to  the  loss  of  water  from  large,  turgescent, 
thin-walled  cells,  which  are  arranged  in  longitudinal  rows,  and  often  are  below 
the  level  of  the  other  epidermal  cells  (figs.  835-837;  also  fig.  762).  The  surface  ex- 
posure of  a  grass  leaf  may,  by  such  a  process,  be  decreased  to  a  fraction  of  its 
original  area,  doubtless  resulting  in  a  much-reduced  transpiration,  particularly  be- 
cause the  stomata  are  on  the  infolded  surface.  Often  the  leaf  ridges  meet,  making 


FIGS.  835-837.  —  Cross  sections  of  the  involute  leaf  of  a  xerophytic  grass,  the  sea 
sand  reed  (Ammophila  arenarid):  835,  a  cross  section  (highly  magnified),  showing  chlo- 
renchyma  (c)  composed  of  cells  that  differ  from  palisades  in  form  and  from  sponge  cells 
in  compactness;  the  stomata  (s)  are  confined  to  the  upper  (here  the  more  protected) 
epidermis  (e) ;  note  in  the  sinuses  large  water-containing  cells  (6),  whose  changes  in 
water  content  account  largely  for  leaf  closing  and  opening;  v,  conductive  tracts;  n, 
bundle  sheath;  m,  mechanical  tissue;  836,  a  diagrammatic  cross  section  of  a  turgid 
open  leaf;  837,  a  diagrammatic  cross  section  of  a  desiccated  inrolled  leaf;  836  and  837 
slightly  magnified. 


closed  chambers  of  the  furrows.  The  wilting  of  leaves,  also  due  to  reduced  turges- 
cence,  may  involve  some  reduction  of  transpiring  surface,  but  its  significance  proba- 
bly is  small.  Surface  reduction  is  illustrated  in  several  lichens  (as  in  species  of 
Cladonia),  whose  thallus  edges  turn  up  when  desiccated,  exposing  the  white  under 
surface.  Various  mosses  (as  Orthotrichum),  the  rose  of  Jericho  (Anastatica),  and 
some  ferns  (as  Selaginella  lepidophylla  and  Polypodium  polypodioides}  curl  up  when 
desiccated,  exposing  a  reduced  transpiring  surface  ;  their  sudden  uncurling  when 
moistened  has  given  rise  to  the  term  resurrection  plant.  Such  movements  as  those 
exhibited  by  the  leaves  of  Poly  trie  hum,  due  to  changing  water  content  (figs.  901,  902), 


ECOLOGY 


are  of  significance  in  relation  to  transpiration,  since  the  closely  appressed  leaves  of 
desiccated  individuals  have  a  greatly  reduced  aggregate  surface. 

Epinasty  and  hyponasty.  —  If  a  leaf  exhibits  curvatures  by  reason  of  greater 
growth  at  the  upper  surface,  it  is  said  to  display  epinasty,  while  if  the  greater 
growth  is  at  the  lower  surface,  it  displays  hyponasty.  Various  plants,  as  Juniperus 
(fig.  838)  and  Sempervivum,  manifest  hyponasty  in  the  autumn,  the  erected  leaves 
becoming  closely  appressed  to  the  stem  or  to  one  another,  while  epinasty  the  fol- 
lowing spring  results  in  leaf  horizontality  and  increased  surface  exposure  (fig. 


FIGS.  838,  839.  —  Shoots  of  the  juni- 
per (Juniperus  communis),  showing  vary- 
ing leaf  orientation :  838,  a  shoot  as  seen 
in  winter;  note  the  ascending  or  erect 
leaves,  whose  orientation  probably  is  due 
to  a  preponderance  of  growth  beneath 
(hyponasty)  ;  839,  a  shoot  as  seen  in  sum- 
mer; note  the  spreading  leaves,  whose 
orientation  probably  is  due  to  a  pre- 
ponderance of  growth  above  (epinasty). 


FIGS.  840,  841.  —  Rosettes  of  the 
peppergrass  (Lepidium),  showing  vary- 
ing leaf  orientation:  840,  a  winter  ro- 
sette with  leaves  closely  appressed  to 
the  ground  through  a  preponderance  of 
epinastic  growth;  841,  the  same  rosette, 
after  a  stay  of  several  days  in  a  green- 
house ;  most  of  the  leaves  have  an  erect 
or  ascending  orientation  by  reason  of 
a  preponderance  of  hyponastic  growth. 


839).  In  other  cases,  especially  in  rosette  plants  (as  Lepidium),  the  reverse  is 
seen,  the  winter  leaves  being  horizontal  and  closely  appressed  to  the  ground 
as  a  result  of  epinasty  (fig.  840),  while  spring  hyponasty  results  in  their 
erection  (fig.  841).  In  all  cases  the  autumn  reaction  results  in  a  reduced 
surface  exposure,  and  hence  is  favorable  to  protection  from  cold  and  from 
excessive  transpiration,  while  the  spring  reaction  results  in  an  increased  sur- 
face exposure,  and  hence  facilitates  synthesis. 

Leaf  fall.  —  The  absciss  layer. — Leaves  differ  greatly  as  to  duration, 
most  cotyledons  and  many  xerophytic  leaves  living  for  only  a  few 
weeks  or  even  days,  while  evergreens  may  retain  their  leaves  for  a  year 
or  two,  or  even  for  ten  or  more  years,  as  in  the  pines  and  the  cycads. 
In  most  deciduous  trees  and  shrubs  the  leaves  remain  for  some  months, 


LEAVES 


583 


the  exact  period  varying  widely,  as  the  season  is  long  or  short.  Leaf 
fall,  especially  in  deciduous  trees  and  shrubs,  is  brought  about  by  the 
development  of  a  special  layer  of  separation  or  absciss  layer  at  the  base 
of  the  petiole,  representing  the  final  phase  of  leaf  activity.  The  cells  of 
this  layer  differ  from  the  adjoining  cells  in  their  greater  turgescence  and 
in  possessing  denser  cytoplasm  and  more  abundant  starch,  and  in  the 
relative  thinness  and  slight  lignification  of  their  walls.  Soon  the  walls 
become  mucilaginous  and  the  cells  then  disintegrate  along  the  plane  of 
separation  (fig.  842) ;  the  rupture  of  the  conductive 
tract  by  wind  or  otherwise  completes  the  process, 
the  leaf  falling  to  the  ground.  At  leaf  fall,  and 
sometimes  before,  the  wound  is  healed  by  the 
development  of  a  protective  cork  layer;  thereafter, 
the  place  of  leaf  attachment  is  marked  by  a  leaf 
scar,  whose  shape  and  structure  vary  with  the  spe- 
cies (figs.  1057-1059).  In  some  compound  leaves 
(as  in  the  hop  tree  and  the  Virginia  creeper)  absciss 
layers  may  develop  first  at  the  base  of  the  leaflets, 
leaving  the  stems  with  a  number  of  bare  petioles. 
Sometimes  the  absciss  layer  is  imperfectly  if  at  all 
developed,  so  that  the  dead  leaves  remain  on  the 
tree,  as  in  the  beech  and  in  various  oaks.  In  most 
herbs  there  is  no  definite  absciss  layer,  the  leaves 
remaining  attached  until  after  death. 

Deciduous  and  evergreen  trees.  — Deciduous  trees, 
as  commonly  understood,  shed  all  their  leaves  at 
once,  at  the  beginning  of  an  unfavorable  season, 
while  evergreens  shed  their  leaves  from  time  to 
time,  or,  if  all  at  once,  only  after  the  new  leaves 
have  developed  (fig.  843).  While  the  distinction 
between  evergreen  and  deciduous  trees  is  well  marked  in  cold  tem- 
perate climates,  such  is  not  the  case  in  the  tropics,  where  the  same 
species  or  even  the  same  individual  may  be  evergreen  one  year  and  de- 
ciduous the  next,  or  evergreen  in  low  grounds  and  deciduous  elsewhere; 
or  the  tree  top  may  be  deciduous  and  the  basal  limbs  evergreen.  Decidu- 
ous trees  (figs.  844,  845)  scarcely  need  subdivision  (except  that  some 
tropical  forms  may  have  two  or  more  periods  of  leaf  shedding  and  re- 
newal each  year),  but  evergreens  may  be  subdivided  into  (i)  the 
tender-leaved  evergreens  of  the  rainy  tropics,  such  as  tree  ferns  and 


FIG.  842.  — A  dia- 
grammatic vertical  sec- 
tion through  the  basal 
region  of  a  mature  leaf 
of  the  cottonwood 
(Populus  deltoides), 
showing  the  initial 
stages  of  leaf  fall;  the 
absciss  layer  develops 
along  the  plane,  a ;  the 
cortical  tissues  (c)  sepa- 
rate first,  and  the  leaf 
falls  when  the  vascu- 
lar region  (T;)  is  rup- 
tured; somewhat  mag- 
nified. 


FIG.  843.  — A  winter  landscape,  showing  the  contrast  between  deciduous  and  evergreen 
trees ;  the  evergreens  are  the  Austrian  pine  (Pinus  Laricio) ;  the  deciduous  trees  are 
willows  (Salix  alba)  and  silver  maples  (Acer  saccharinum)\  the  pines  are  excurrerit,  while 
the  other  trees  are  deliquescent ;  Chicago,  111.  —  Photograph  by  FULLER. 


FIG.  844.  —  A  sugar  maple  (Acer  sac- 
charum)  that  has  grown  in  the  open  and  thus 
is  symmetrical,the  general  shape  being  conical 
with  a  rounded  apex;  this  figure  represents 
a  deciduous  tree  in  summer  condition ;  Lan- 
caster, Ohio.  —  Photograph  by  HYDE. 


FIG.  845.  —  The  same  tree  shown  in  fig. 
844,  but  in  winter  condition;  note  that  the 
conical  shape  is  due  to  the  gradual  change  in 
branch  direction  from  horizontal  or  descend- 
ing at  the  base  to  ascending  and  finally  to 
vertical  at  the  apex.  —  Photograph  by  HYDE. 


584 


LEAVES 


58S 


bananas  (fig.  846);  (2)  evergreens  with  broad  and  stiff  leaves,  the  so- 
called  broad-leaved  sclerophylls,  as  the  live  oak  and  the  holly  (fig.  809) ; 
(3)  evergreens  with  stiff,  needle-like  leaves,  as  in  the  pine  and  the  spruce 
(fig-  955);  (4)  succulent  desert  evergreens,  as  Agave  (fig.  921);  and  (5) 
leafless  evergreens, 
such  as  the  cacti  (fig. 
1035)  and  Ephedra. 
A  transition  to  de- 
ciduous trees  is  seen 
in  the  potential  ever- 
greens, such  as  Ilex 
decidua,  which, 
though  deciduous  in 
the  northern  states 
(as  suggested  by  the 
specific  name),  is 
evergreen  farther 
south,  as  are  various 
oaks  that  are  decid- 
uous in  the  north. 
Magnolia  grandi- 
flora,  a  true  ever- 
green in  the  Gulf 
states,  retains  its 
leaves  at  its  northern 
limit,  although  they 
die  before  the  winter 
is  over,  thus  resem-  FlG  846  _  A  group  of  banana  trees  (Musa) ;  the  gigantic 

bling  the   beech    and        leaves  have  been  much  frayed  by  the  wind  (marginal  me- 

those  oaks   in   which        chanical  tissue  being  poorly  developed),  giving  the  effect  of 

.  a  pinnate  palm  leaf;   the  shrubs  beneath  the  banana  trees 

the  autumnal  absciss      are   coffee   plants   (Coffea  arabica}.  Xaiapa,  Mexico.  - 

layer    is    imperfectly       Photograph  supplied  by  LAND. 

developed. 

The  causes  of  leaf  fall.  — The  leaf  behavior  of  deciduous  trees  and 
of  tropical  evergreens  obviously  is  related  to  external  factors,  in  the 
former  being  associated  with  climatic  periodicity  (either  of  moisture,  as 
in  the  monsoon  forests  of  India,  or  of  temperature,  as  in  the  northern 
deciduous  forests),  while  in  the  latter  it  is  associated  with  uniform 
moisture  and  temperature.  That  the  deciduous  and  the  evergreen 


«< «- 

586  ECOLOGY 

habits  are  related  to  external  conditions  may  be  inferred  from  many 
trees  and  shrubs  (e.g.  poison  ivy,  Virginia  creeper,  various  oaks)  which 
shed  their  leaves  in  regions  of  cold  winters,  but  retain  them  in  warmer 
climates;  furthermore,  various  plants  (as  the  grape  and  the  peach)  be- 
come evergreen  in  uniform  tropical  climates,  and  even  those  species  that 
remain  deciduous  (as  the  persimmon  and  the  mulberry)  have  much 
longer  periods  of  leafage. 

The  exact  factors  involved  in  leaf  fall,  that  is,  in  the  development  of 
the  absciss  layer,  are  imperfectly  known.  In  the  monsoon  forest  and 
in  other  regions  of  periodic  drought,  it  is  probable  that  leaf  fall  results 
directly  from  the  desiccation  incident  to  the  increased  transpiration 
and  decreased  absorption  during  the  dry  period.  Autumnal  leaf  fall 
in  cool  climates  probably  is  due  to  desiccation  resulting  from  continued 
transpiration  at  a  time  when  absorption  is  diminished  by  reason  of  low 
temperature,  although  desiccation  due  to  dryness  in  the  soil  or  air  may 
cause  the  absciss  layer  to  develop  in  early  summer.  A  severe  frost 
in  early  autumn  may  retard  leaf  fall  through  injury  to  the  tissues  that 
develop  the  absciss  layer. 

Leaf  fall  may  result  also  from  protracted  wet  weather,  or  from  the  transference  of 
a  plant  from  a  dry  house  to  a  moist  chamber ;  possibly  the  reduction  of  transpira- 
tion if  accompanied  by  strong  turgor  pressure  may  result  here  in  the  injection  of 
air  spaces,  and  hence  in  impaired  gas  exchange  and  death.  Early  leaf  fall  some- 
times is  induced  by  diminished  light  (as  in  the  lower  leaves  of  tall  herbs  or  of  forest 
trees)  and  by  the  attacks  of  parasitic  plants  and  animals.  In  most  cases  leaf  fall 
seems  to  be  associated  with  some  impairment  of  activity,  but  why  such  impairment 
should  stimulate  the  development  of  a  separation  layer  is  not  clear.  The  cause  of 
leaf  fall  in  evergreens  is  as  yet  scarcely  to  be  conjectured,  there  being  little  or  no 
obvious  relation  to  external  factors,  except  when  the  old  leaves  are  in  a  sense 
pushed  off  by  growing  shoots.  Perhaps  leaf  activity  gradually  becomes  impaired 
through  the  continued  accumulation  of  excreta  and  the  increased  clogging  of  the 
stomata  by  dust,  or  perhaps  such  leaf  fall  is  governed  by  internal  factors. 

The  advantages  of  leaf  fall  and  of  the  evergreen  habit.  —  The  shedding 
of  leaves  at  the  inception  of  a  cool  or  dry  period  is  of  inestimable  advan- 
tage, especially  in  trees  with  delicate  leaves,  because  of  the  enormously 
reduced  transpiration  thus  resulting.  The  leafless  tree  is  one  of  the 
most  perfectly  protected  of  plant  structures,  since  impervious  bud 
scales  and  bark  cover  all  exposed  portions.  So  close  is  the  relation  be- 
tween leaf  texture  and  leaf  fall  that  in  temperate  or  in  cold  climates 
one  almost  may  determine  by  the  feel  of  a  leaf  whether  it  is  deciduous 
or  evergreen.  While  evergreens  are  more  subject  to  winter  transpiration 


LEAVES  587 

than  are  deciduous  trees,  they  have  compensatory  advantages,  such  as 
lower  summer  transpiration,  less  danger  from  frosts  in  the  growing 
season,  and  readiness  for  synthetic  activity  at  all  seasons.  The  broad- 
leaved  sclerophylls  abound  chiefly  in  regions  of  winter  rain,  their  ad- 
vantage seeming  to  consist  in  the  possibility  of  utilizing  every  day 
suitable  for  synthesis,  while  they  are  also  well  protected  from  winter 
cold  and  summer  drought.  The  advantage  of  the  delicate  evergreen 
leaf  is  obvious  where  uniform  moisture  and  temperature  prevail. 

Protective  features  in  the  cell  sap  and  in  the  protoplasm.  —  Many  plants  appear 
adequately  protected  from  transpiration  and  from  other  dangers,  although  lacking  in 
such  protective  structures  or  forms  of  behavior  as  have  been  described.  For  exam- 
ple, many  plants  transpire  less  and  some  much  less  in  hot,  dry  weather  than  at  other 
times;  in  the  Mediterranean  region,  transpiration  is  low  not  only  in  winter  (as 
would  be  expected),  but  also  in  midsummer,  and  cases  are  known  where  transpira- 
tion in  hot,  dry  air  is  reduced  to  one-sixth  of  the  amount  recorded  in  moist  air. 
Such  behavior  is  due  in  part,  of  course,  to  stomatal  closure  and  to  an  increase  in 
the  concentration  of  the  cell  sap,  but  it  is  due  in  much  larger  part  to  complex 
causes  that  are  as  yet  unknown ;  when  most  of  the  water  has  evaporated,  the  plant 
enters  a  state  of  comparative  inactivity,  and  transpiration  is  greatly  reduced.  Most 
plant  activities  take  place  between  o°  C.  and  45°  C.,  and  at  certain  temperatures 
(varying  with  the  species)  beyond  those  at  which  growth  is  checked,'  life  itself  is 
destroyed.  Death  from  freezing  generally  has  been  attributed  to  the  desiccation  of 
the  protoplasm  incident  to  the  withdrawal  from  the  cell  sap  of  water  which  contrib- 
utes to  the  formation  of  ice  crystals.1  Many  plants  show  remarkable  resistance  to 
freezing  temperatures,  and  it  is  probable  that  such  resistance  in  many  instances  is 
due  to  high  osmotic  pressure  in  the  cell  sap ;  for  example,  in  Phycomyces,  ice  does 
not  form  at  temperatures  above  —17°  C.  High  pressure  may  be  habitual,  as  in 
many  xerophytes  (p.  493),  or  there  may  be  changes  in  the  pressure,  accompanying 
the  temperature  changes.  An  instance  of  varying  pressure  is  seen  in  the  numerous 
northern  evergreens,  whose  leaf  cells  in  winter  contain  sugar  instead  of  starch,  and 
thus  have  a  more  concentrated  cell  sap  than  in  summer ;  it  is  believed  also  that 
sugar  retards  the  coagulation  of  proteins  which  otherwise  would  be  induced  when 
the  cell  salts  become  concentrated.  The  resistance  of  red  leaves  to  low  tempera- 
tures also  may  be  associated  with  their  high  sugar  content.  The  injury  occasioned 
in  early  autumn  or  in  late  spring  by  a  frost  that  in  winter  would  be  harmless  may 
be  explained  in  part  by  the  lower  sugar  content  at  such  seasons.  It  must  be  ad- 
mitted, however,  that  many  cases  of  resistance  to  freezing  or  to  desiccation  as  yet 
remain  unexplained.  Experiments  have  shown  that  while  there  is  an  undoubted 
relation  between  the  death  point  and  the  osmotic  pressure  of  the  cell  sap,  it  is  far 
from  being  an  exact  relation,  and  there  are  many  cases  in  which  such  an  explana- 
tion is  totally  inadequate.  Perhaps  the  best  illustrations  of  such  unexplained  resist- 
ance are  found  among  the  bacteria,  algae,  and  lichens.  Bacteria  in  the  so-called 

1  However,  there  is  evidence  that  in  many  cases  death  may  take  place  without  actual 
ice  formation,  also  that  ice  formation  does  not  always  result  in  death. 


588 


ECOLOGY 


resting  stages  are  able  to  endure  the  high  temperatures  of  hot  springs  or  the  low 
temperature  of  liquid  hydrogen,  and  are  able  to  withstand  the  desiccation  of  the 
desert.  Even  algae,  though  characteristic  hydrophytes,  may  occur,  apparently  un- 
protected, on  dry  rocks  or  in  the  snow  and  ice,  the  blue-green  algae  in  particular 
being  about  as  resistant  as  are  the  bacteria.  Lichens  absorb  and  transpire  water 
quickly,  enduring  long  droughts  in  a  desiccated  condition  without  injury ;  indeed, 

they  may  be  regarded  as  among  the  most  resistant 

of  plants,  in  spite  of  their  lack  of  obvious  protective 
structures,  a  fact  that  is  all  the  more  remarkable 
since  they  are  complexes  of  algae  and  fungi,  groups 
which  separately  flourish  best  in  water  and  in  moist 
woods  respectively.  Desiccation  in  these  plants 
induces  merely  a  resting  stage  or  stage  of  suspended 
animation,  in  which  the  small  amount  of  water 
needed  to  preserve  life  is  retained  with  great  tenac- 
ity. Very  probably  it  is  the  ability  of  these  plants 
to  retain  this  necessary  modicum  of  water  that 
accounts  for  their  great  resistance  to  detrimental 
factors.  The  ultimate  cause  of  resistance  here  would 
seem  to  be  some  "specific  property  of  the  proto- 
plasm," whose  nature  is  as  yet  unknown. 

Protection  by  coverings  of  snow  and  of  dead 
leaves.  —  The  mar. tie  of  fallen  leaves  which  covers 
the  ground  in  forests,  and  the  dead  leaves  remain- 
ing on  the  grasses  and  other  plants  of  meadows  and 
swamps,  are  of  great  value  in  protecting  herbaceous 
vegetation  from  the  rigors  of  winter.  On  some  trees, 
as  in  Yucca  (fig.  847)  and  in  various  palms  (fig.  951), 
the  leaves  or  leaf  bases  remain  on  the  stem  after 
death,  forming  a  thick  protective  layer.  Similarly, 
in  cold  climates  the  snow  cover  is  of  great  protective 
value,  although  winter  thaws  and  irregular  drifting 
often  leave  the  ground  bare  and  the  vegetation  un- 
protected. In  alpine  meadows  the  deep  and  long- 
enduring  mantle  of  snow  most  effectively  protects 
the  delicate  alpine  herbage  from  the  severities  of 
winter.  Layers  of  snow  or  leaves  tend  to  conserve 
the  soil  warmth,  and  thus  are  of  value  in  protecting 
the  subjacent  vegetation  from  the  deleterious  effects 
of  sudden  temperature  changes,  but  they  are  of 
much  greater  value  in  reducing  transpiration  to  a  minimum  at  a  time  when  the 
low  soil  temperature  prevents  absorption.  The  winter  killing  of  unprotected  wheat 
and  of  other  vegetation  is  in  most  instances  due  to  excessive  transpiration  rather 
than  to  freezing. 

Summary  on  transpiration  and  carbohydrate  synthesis.  —  A  review 
of  the  preceding  pages  indicates  the  existence  of  a  reciprocal  relation 


FIG.  847.  —  A  tree  yucca 
(Yucca,  arbor escens);  note  the 
rigid,  needle-like,  many-ranked 
leaves,  which  lop  back  against 
the  stem,  serving  long  after 
death  as  a  protective  covering ; 
among  the  shrubs  is  the  creo- 
sote bush  (Larrea  tridentata); 
Victor,  California.  —  Photo- 
graph by  E.  W.  COWLES. 


LEAVES  589 

between  the  structural  requirements  associated  with  transpiration  and 
carbohydrate  synthesis.  The  form  of  leaf  best  fitted  for  maximum  syn- 
thesis is  least  fitted  for  the  reduction  of  transpiration,  and  vice  versa. 
Hydrophytes  and  mesophytes  as  a  class  have  thin,  expanded  leaves  well 
fitted  for  synthetic  activity,  because  their  structure  is  such  as  to  facili- 
tate the  reception  of  light  and  the  absorption  of  carbon  dioxidr.  Xero- 
phytes,  on  the  other  hand,  including  most  plants  of  alpine  and  arctic 
regions  and  also  those  of  salt  marshes  and  peat  bogs,  have  small,  thick 
leaves,  or  leaves  otherwise  suited  for  reducing  transpiration.  Transpira- 
tion is  a  minor  danger  among  hydrophytes  and  mesophytes,  because  the 
water  supply  commonly  is  adequate,  just  as  insufficient  light  for  synthesis 
is  rarely  a  danger  in  xerophytes.  To  a  large  extent  the  features  that 
facilitate  synthesis  on  the  one  hand,  or  the  reduction  of  transpiration 
on  the  other,  are  determined  by  external  factors;  indeed,  in  many  in- 
stances transpiration  itself  occasions  the  production  of  the  very  struc- 
tures (cutin,  wax,  hairs,  etc.),  which  minimize  the  dangers  that  it  causes. 
It  would  appear,  however,  that  many  plant  structures  are  not  thus 
related  to  environment.  The  xerophytic  features  of  such  leaves 
as  those  of  the  ericads,  conifers,  and  begonias,  features  that  are 
equally  advantageous  with  those  of  other  xerophytic  leaves  in  reduc- 
ing transpiration,  appear  inflexible  when  subjected  to  varying  conditions. 
But  as  conditions  of  soil  and  climate  are  subject  to  constant  change, 
those  species  whose  structures  become  modified  accordingly  would  seem 
to  be  best  fitted  to  survive.  The  extinction  of  species  often  may  have 
resulted  from  a  lack  of  plasticity. 

6.     VARIATIONS   IN    LEAF    FORM 

The  significance  of  leaf  variation.  —  Distinctions  between  plant  spe- 
cies commonly  are  based  upon  the  forms  of  leaves  and  of  other  organs, 
hence  the  determination  of  the  causes  underlying  form  is  among  the 
most  fundamental  of  problems.  In  many  species  (e.g.  Sagittaria  hetero- 
phylla,  figs.  848-853)  there  is  a  wide  variation  in  leaf  form  which  is 
connected  definitely  with  external  causes.  When  comparable  differences 
in  leaf  form  constitute  specific  characters,  it  is  a  tenable  hypothesis 
that  the  present  species  are  the  fixed  descendants  of  once  plastic  an- 
cestors that  had  a  range  of  variation  broad  enough  to  include  differ- 
ences as  great  as  those  that  to-day  characterize  distinct  species.  For 
example,  among  the  buttercups  there  are  some  species  (as  Ranunculus 


590 


ECOLOGY 


septentrionalis}  which   always  grow    on   land,  and  whose    leaves  are 
divided  but  not  finely  dissected,  while  other  species  (as  R.  circinatus) 


848 


849 


852 


853 


850 


851 


FIGS.  848-853.  —  Leaf  variation  in  an  arrowhead  (Sagittaria,  heterophylla}:  848,  a 
representative  air  leaf;  849,  850,  air  leaves  from  plants  in  deeper  water  than  those 
bearing  such  a  leaf  as  figured  in  848;  note  the  reduction  or  absence  of  the  basal  lobes; 
851,  852,  leaves  from  plants  in  deep  water;  853,  a  submersed  bladeless  leaf  (phyllode); 
all  the  variants  here  figured  may  be  found  in  a  single  vegetative  colony  connected  (at 
least  originally)  by  underground  stems;  they  may  be  found  also  on  a  single  individual 
at  different  developmental  stages,  the  phyllode  appearing  first  and  the  broad  leaf  last. 

always  grow  in  the  water  and  have  dissected  leaves;  still  other  species 
(as  R.  multifidus  or  R.  aquatilis,  figs.  854-857)  have  leaves  of  both  sorts 
and  all  kinds  of  intergradations,  depending  upon  the  habitat.  The 


854 


857 


FIGS.  854-857. — Leaf  variation  in  the  white  water-buttercup  (Ranunculus  aquatilis): 
854,  a  water  leaf,  entirely  submersed  during  development;  855,  856,  leaves  transitional 
between  air  leaves  and  water  leaves;  857,  an  air  leaf. 

hypothesis  is  that  the  former  species,  characterized  by  slight  plasticity, 
each  have  come  from  an  ancestry  comparable  as  to  plasticity  with  the 
present  R.  aquatilis.  Another  hypothesis  is  possible,  namely,  that  some 


LEAVES 


forms  always  have  been  rigid  and  others  always  plastic.  Quite  apart 
from  evolutionary  considerations,  the  study  of  the  cause  of  leaf  form  is 
important,  because  of  its  bearing  upon  the 
fundamental  problems  of  plant  behavior, 
and  because  of  its  relation  to  the  role  of 
leaves,  including  the  advantages  and  dis- 
advantages associated  with  the  different 
leaf  forms  in  various  habitats. 

Form  variations  in  thalloid  plants.  — 
The  variations  in  body  form  exhibited  by 
algae  and  fungi  are  in  many  respects  com- 
parable to  those  of  leaves,  though  some- 
what simpler,  thus  clearly  meriting  con- 
sideration here.  In  nature  the  alga, 
Stigeoclonium,  exhibits  two  widely  con- 
trasting forms:  one,  the  palmella  form, 
once  thought  to  belong  to  the  separate 
genus,  Palmella,  is  common  on  moist  bark 
and  consists  of  relatively  thick-walled 
spherical  cells,  which  divide  in  any  plane, 
and  either  cohere  in  colonies  or  become 
isolated  (fig.  858) ;  the  other  form  is  fila- 
mentous, the  individual  cells  being  elon- 
gated and  relatively  thin-walled,  and  di- 
viding in  but  one  direction  (fig.  859).  It 
has  been  shown  that  if  the  filamentous 
form  is  grown  in  a  medium  of  relatively 
high  osmotic  pressure,  the  palmella  form 
is  produced,  the  cells  soon  bulging  out  and 
becoming  spherical,  and  later  separating; 
subsequent  divisions  occur  in  all  planes 
(fig.  860).  On  the  other  hand,  the  fila- 
mentous form  is  produced,  if  the  palmella 
cells  are  grown  in  a  medium  of  relatively 
low  osmotic  pressure.  While  only  young 
palmella  cells  can  grow  into  filaments,  adult  filament  cells  are  capable 
of  developing  directly  into  palmella  cells,  contrary  to  the  general  rule 
that  adult  forms  are  not  plastic.  The  filamentous  form  appears  to  be 
the  more  vigorous,  probably  because  the  low  concentration  of  the 


FIGS.  858-860.  —  Variation 
in  Stigeoclonium:  858,  the 
palmella  form,  consisting  of 
isolated  spherical  (a)  or  oblong 
(i)  cells;  c,  d,  vegetative  repro- 
duction by  means  of  fission,  c 
showing  an  early  stage  in  which 
a  dividing  wall  is  formed,  and  d, 
a  later  stage,  just  before  the  two 
daughter  cells  separate;  859,  the 
filamentous  form,  in  which  the 
elongated  individual  cells  cohere 
in  simple  or  branched  chains; 
860,  a  filament  that  has  been 
placed  in  a  concentrated  solu- 
tion and  is  beginning  to  break 
up  into  the  palmella  stage;  note 
the  rounding  of  the  cells  and 
their  subsequent  separation; 
highly  magnified. — After  LIV- 
INGSTON (drawn  from  a  photo- 
graphic reproduction). 


592 


ECOLOGY 


a 


medium  facilitates  absorption  and  consequently  a  dilute  cell  sap, 
which  in  turn  is  thought  to  favor  growth  and  luxuriance.  Long 
cultivation  in  a  given  medium  results  in  "  accommodation "  to  that 
medium ;  for  example,  the  cell-sap  concentration  in  a  plant  grown  for 

a  long  time  in  a  medium 
of  low  concentration  is 
less  than  that  in  one  simi- 
larly grown  in  a  medium  of 
high  concentration.  Fur- 
thermore, a  filament  grown 
for  a  long  time  in  a  dilute 
solution  reacts  more  quickly 
to  a  concentrated  solution 
than  one  that  has  just  been 
produced  from  palmella 
forms  of  long  standing. 

As  might  be  expected,  sea 
water  induces  the  palmella 
form  of  Stigeoclonium ;  also 
if  a  salt  solution  of  low  con- 
centration is  allowed  to  evap- 
orate, the  palmella  form 
gradually  develops,  owing  to 
the  slow  increase  in  concen- 
tration. The  palmella  form 
also  may  be  induced  by  ex- 
posure to  transpiration  (the 
condition  under  which  this 
form  probably  develops  on 
trees),  or  to  low  tempera- 
tures, even  if  the  solution 
is  dilute.  All  changes  in 
form  thus  far  noted  may  be 
related  to  water,  the  pal- 
mella form  being  produced  in  conditions  where  absorption  is  low  (solu- 
tions of  high  concentration  or  low  temperatures)  or  evaporation  high 
(air  cultures),  or,  in  other  words,  where  the  cell  sap  becomes  relatively 
concentrated;  the  filamentous  form,  on  the  other  hand,  is  produced 
if  absorption  is  high  and  transpiration  lacking,  that  is,  if  the  cell 


FIG.  86 1.  —  Leaf  variation  as  exhibited  or- 
dinarily by  the  mermaid  weed  (Proserpinaca 
palustris) ;  note  the  finely  dissected  water  leaves 
(w),  the  nearly  entire  air  leaves  (a),  and  the  tran- 
sitional leaves  (/)  just  above  the  water  surface. 


LEAVES 


593 


sap  becomes  relatively  dilute.  The  palmella  form  can  be  produced 
also  by  chemical  stimulation,  notably  by  toxic  salts  (e.g.  salts  of 
copper,  lead,  and  silver),  and  by  solutions  of  bog  water,  where  the  low 
concentration  excludes  osmotic  pressure  as  a  causative  factor. 

So  far  as  known,  other  algae  usually  react  to  changes  in  the  concentration  of  the 
medium  after  the  fashion  of  Stigeoclonium,  but  the  data  are  scanty.  The  fresh- 
water alga,  Mougeotia,  when  grown  in 
salt  water  for  a  time,  becomes  so 
thoroughly  "  accommodated  "  to  the 
new  environment  that  death  ensues  if 
it  is  transferred  suddenly  to  fresh  water; 
probably  such  "  accommodation  "  con- 
sists in  increased  concentration  of  the 
cell  sap.  Growth  in  media  of  low 
concentration  causes  the  marine  alga, 
Cladophora  trichotema,  to  become  more 
slender,  while  thickened  cell  walls  re- 
sult from  an  increased  concentration. 
Batrachospermum,  when  grown  in 
weak  light,  develops  only  the  embry- 
onic or  juvenile  stage,  long  known  as 
the  separate  genus  Chantransia.  The 
reactions  of  Stichococcus  appear  to  be 
the  reverse  of  those  of  Stigeoclonium, 
low  concentration  inducing  the  development  of  isolated  spherical  cells,  while  high 
concentration  induces  the  development  of  filaments  of  elongated  cells,  once  referred 
to  the  genus  Rhaphidium.  One  of  the  fungi,  Basidiobolus  ranarum,  reacts  much 
after  the  manner  of  Stigeoclonium,  increased  concentration  (and  also  chemical  stimu- 
lation) resulting  in  shorter  cells  with  thicker  walls  and  in  divisions  in  various  planes. 
Mucor  and  other  fungi  produce  yeastlike  cells  through  chemical  stimulation. 

Form  variations  in  amphibious  plants.  —  The  phenomena. — No  plants 
show  greater  variations  in  leaf  form  and  structure  than  do  amphibious 
plants,  which  may  be  subject  alternately  to  submergence  and  to  desic- 
cation. For  example,  the  mermaid  weed,  Proserpinaca  palustris,  has 
almost  entire,  lanceolate  air  leaves,  and  finely  pinnatifid  water  leaves, 
larger  in  outline,  though  of  less  weight  (figs.  861,  862-864).  The  vari- 
ations of  Radicula  aquatica,  one  of  the  cresses,  are  very  similar,  and 
even  more  striking,  since  the  air  leaves  are  quite  entire,  while  the  water 
leaves  may  be  twice  or  thrice  pinnately  dissected.  In  the  water  parsnip, 
Slum .  cicutaefolium,  the  early  radical  leaves  are  much  dissected,  while 
the  later  leaves  are  simply  pinnate.  In  various  buttercups  there  are 
similar  form  changes,  involving  palmate  rather  than  pinnate  leaves  (figs. 


862  863  864, 

FIGS.  862-864.  —  Leaves  of  the  mermaid 
weed  (Proserpinaca  palustris):  862,  an  air 
leaf;  863,  a  water  leaf;  864,  a  transitional 
leaf. 


594 


ECOLOGY 


854-857).  Another  group  of  amphibious  plants,  represented  by  Alisma, 
Castalia,  and  Potamogeton  natans,  have  narrow,  thin,  submersed  leaves 
and  broad,  thick,  aerial,  or  floating  leaves,  while  in  Sagittaria,  similar 
differences  are  supplemented  by  the  development  of  basal  lobes  (figs. 
848-853). 

Not  all  amphibious  plants,  even  in  the  above  genera,  are  equally  plastic.  For 
example,  Radicula  palustris  has  pinnately  compound  leaves  in  all  habitats,  and 
Sagittaria  graminea  and  S.  lancifolia  develop  air  leaves  without  basal  lobes.  Few 
plastic  species  are  equally  variable  at  all  times,  the  early  leaves  of  Proserpinaca  and 
Sium  being  much  divided,  regardless  of  the  habitat,  while  the  latest  leaves  react 
much  less  readily  than  do  the  plastic  intermediate  leaves.  After  Sagittaria  has 
commenced  to  produce  sagittate  air  leaves,  submergence  results  commonly  in  the 
development  of  new  air  leaves  with  long  petioles  rather  than  in  a  renewed  develop- 
ment of  water  leaves.  However,  Limnophila  seems  to  be  about  equally  plastic  at 
all  times,  and  in  Myriophyllum  heterophyllum,  even  the  emersed  flowering  branches 
develop  water  leaves  upon  submergence. 

The  causes  of  leaf  variation  in  amphibious  plants.  —  If  a  Proserpinaca 
plant  is  removed  from  the  water,  entire  leaves  at  once  begin  to  develop 
instead  of  dissected  leaves;  on  the  other  hand,  dissected  leaves  soon 
appear  if  an  air-grown  specimen  is  placed  in  water  (fig.  865).  Alter- 
nations of  the  two  leaf  forms  may  be  produced  by  alternating  the  con- 
ditions (fig.  866) .  Such  experiments  show  the  extreme  plasticity  of  the 
plant,  and  the  close  relation  existing  between  leaf  form  and  environ- 
ment. Among  the  factors  suggested  as  responsible  for  these  form  changes 
is  nutrition.  In  Sagittaria  and  Castalia  the  smallest,  narrowest,  and 
lightest  leaves  are  developed  in  the  deepest  water,  where  but  little  light 
penetrates,  and  where,  therefore,  the  manufacture  of  carbohydrates 
necessarily  is  slight.  Even  in  Sium  and  Proserpinaca,  the  water  leaves, 
though  larger  than  the  air  leaves,  contain  less  structural  material.  In 
such  plants  as  Castalia  and  Alisma  each  new  leaf  is  larger  than  the 
preceding,  as  though  the  food  supply  increases  with  the  increasing  ex- 
panse of  foliage,  involving  a  constant  increase  in  food-making  power. 
Furthermore,  each  new  leaf  usually  is  better  placed  for  light  reception 
than  is  the  preceding  leaf.  The  nutrition  theory  is  favored  further  by 
the  fact  that  the  removal  of  roots  or  leaves  is  followed  by  the  renewed 
development  of  small  water  leaves. 

The  nutrition  theory  is  inadequate,  because  it  accounts  for  differences 
in  size  rather  than  in  form,  and  because  nutrition  is  not  a  simple 
factor,  but  a  complex  of  many  factors.  Light  has  been  regarded  as 


LEAVES 


595 


a  direct  formative  stimulus,  but  this  is  made  improbable  by  the 
fact  that  leave's  just  below  the  water  surface  differ  strikingly  from 
those  just  above,  there  being  no  intergrading  series  corresponding  with 
the  gradual  decrease  in  light;  besides,  air  leaves  develop  as  readily 


866 


FIGS.  865-866.  — 865,  a  shoot  of  the  mermaid  weed  (Proserpinaca  palustris)  that  has 
been  placed  in  water  after  growing  for  some  time  in  the  air;  note  the  transitional  leaves  (/) 
between  the  air  leaves  (a)  below  and  the  water  leaves  (w)  above;  Proserpinaca  has  a 
phyllotaxy  of  high  rank,  there  being  a  number  of  orthostichies  (p.  549). — After  McCAL- 
LUM  (drawn  from  a  photographic  reproduction);  866,  a  shoot  of  the  mermaid  weed 
(Proserpinaca  palustris)  that  has  been  grown  successively  in  water,  air,  water,  and  air; 
note  the  corresponding  sets  of  leaves,  w,  a,  w',  a'.  —  After  McCALLUM  (drawn  from  a 
photographic  reproduction). 


in  shade  as  in  full  sunlight.  Variations  in  temperature,  or  in  the 
amount  of  oxygen  or  carbon  dioxid,  seem  to  have  little  formative 
significance. 

The  factor  which  appears  most  directly  related  to  leaf  form  in  am- 
phibious plants  is  transpiration.     In  Proserpinaca  the  water  leaf  can  be 


596  ECOLOGY 

produced  in  a  saturated  atmosphere,  appearing  to  show  that  transpira- 
tion results  in  the  formation  of  air  leaves,  and  freedom  from  transpiration 
in  the  formation  of  water  leaves.  This  theory  is  strengthened  by  the 
experimental  production  of  air  leaves  in  concentrated  solutions  of 
potassium  chlorid  or  of  calcium  nitrate,  the  high  osmotic  pressure  of  the 
medium  having  the  effect  of  transpiration  in  increasing  the  concentra- 
tion of  the  cell  sap,  precisely  as  in  Stigeoclonium.  Why  the  dissected 
form  should  result  if  the  cell  sap  is  dilute,  and  the  entire  form  if  it  is 
more  concentrated,  cannot  now  be  told,  the  exact  mechanics  of  form 
changes  being  but  little  understood. 

While  the  cells  in  Stigeoclonium  are  almost  always  plastic,  the  leaf  of 
Proserpinaca  loses  its  plasticity  after  attaining  a  length  of  three  or  four 
millimeters,  the  form  then  in  development  continuing  to  maturity  re- 
gardless of  habitat  changes.  Furthermore,  some  leaves  appear  to  be 
fixed  from  the  outset,  those  following  the  cotyledons  and  those  develop- 
ing on  horizontal  autumnal  shoots  being  dissected,  while  the  late  stem 
leaves  are  likely  to  be  entire  regardless  of  environment.  More  puz- 
zling still  is  Slum  in  which  the  cotyledons  are  followed  by  palmate 
leaves  of  mesophytic  aspect,  regardless  of  conditions,  these  being  fol- 
lowed by  dissected  pinnate  water  leaves,  and  later  by  simply  pinnate 
air  leaves.  The  submergence  of  an  old  plant  results  not  in  the  de- 
velopment of  dissected  pinnate  water  leaves,  but  of  palmate  leaves 
like  those  first  appearing  in  the  seedling.  In  the  water  lilies  both 
submersed  and  floating  leaves  are  developed  in  the  water,  so  that 
variations  in  the  water  relation  hardly  can  be  assumed  to  be  causative 
factors,  as  in  Proserpinaca. 

Observations  like  those  noted  in  the  preceding  paragraph  have  led  to 
the  theory  that  the  plasticity  of  amphibious  plants  is  more  apparent 
than  real,  the  phenomena  of  leaf  variation  representing  mere  stages  in 
development.  The  palmate  leaves  of  Sium  and  the  narrow  submersed 
leaves  of  Sagittaria  and  of  Castalia  from  this  viewpoint  are  regarded  as 
juvenile  leaves,  while  the  pinnate  leaves  of  Sium,  the  floating  leaves  of 
Castalia,  and  the  sagittate  air  leaves  of  Sagittaria,  are  supposed  to  be 
adult.  The  theory  has  been  carried  even  farther,  representing  that  the 
stages  in  the  life  of  the  individual  (the  ontogeny)  repeat  similar  stages  in 
the  life  of  the  race  (the  phylogeny),  illustrating  what  is  known  as  reca- 
pitulation. For  example,  a  remote  ancestor  of  Sium  might  be  imagined 
to  have  been  a  palmate-leaved  mesophyte,  and  a  more  recent  ancestor, 
a  pinnately  dissected  hydrophyte,  while  the  present  species  might  be 


LEAVES  597 

regarded  as  an  amphibious  plant  tending  once  more  toward  meso- 
phytism.  However,  there  is  no  valid  reason  for  supposing  that  juvenile 
leaves,  if  indeed  such  terms  as  juvenile  and  adult  represent  the  facts, 
can  furnish  a  trustworthy  clue  to  ancestral  adult  forms.1  Various  fac- 
tors may  induce  the  replacement  of  the  adult  by  the  juvenile  state,  a 
phenomenon  known  as  rejuvenescence.  This  is  not  regarded  as  a  reac- 
tion to  a  new  condition,  but  as  an  indication  of  a  sudden  shock,  which 
causes  the  plant  to  return  to  a  youthful  stage.  External  factors  deter- 
mine when,  but  not  what,  the  change  shall  be.  The  development  in 
Slum  of  mesophytic  juvenile  leaves  rather  than  of  dissected  leaves, 
when  an  adult  stem  is  placed  in  water,  is  cited  as  supporting  the  reju- 
venescence hypothesis. 

The  rejuvenescence  theory,  at  best,  is  a  statement  of  facts  rather 
than  an  explanation,  and  is  likely  to  obscure  the  truth.  To  say  that 
a  change  in  form  is  due  to  rejuvenescence,  is  merely  to  say  that  it  is  due 
to  unknown  causes,  the  latter  statement  being  less  misleading  than  the 
former.  But  in  such  a  plant  as  Stigeoclonium,  or  even  Proserpinaca 
at  certain  stages,  it  is  scarcely  correct  to  speak  of  rejuvenescence,  inas- 
much as  definite  external  factors  produce  definite  results,  obliterating 
any  supposedly  normal  succession  of  stages.  In  the  poison  ivy  the 
juvenile  stage  may  be  eliminated  even  in  the  seedling,  if  the  developing 
plant  is  well  nourished.  Here  and  in  Castalia  and  Sagittaria,  where 
water  as  a  formative  factor  appears  to  be  replaced  by  the  complex  of 
factors  known  as  nutrition,  the  assumption  of  such  a  complex  is  prefer- 
able to  the  assumption  of  rejuvenescence,  particularly  because  future 
researches  may  analyze  nutrition  into  its  component  factors. 

The  advantages  of  leaf  variation  in  amphibious  plants.  —  The  strik- 
ing plasticity  of  amphibious  plants  has  led  to  a  search  for  marked 
advantages  in  the  different  forms  produced,  but  no  such  marked  advan- 
tages are  known,  nor  is  there  any  satisfactory  proof  that  the  reactions  are 
adaptive.  The  structural  features  of  the  two  forms  clearly  are  beneficial, 
the  water  leaf  often  having  capacious  air  spaces  and  epidermal  chloro- 
phyll, while  the  air  leaf  has  small  air  spaces  and  abundant  stomata.  In 

1  A  more  tenable  theory  than  that  of  recapitulation  is  the  repetition  theory,  which 
represents  that  present  juvenile  stages  repeat  the  juvenile  stages  of  ancestral  forms;  even 
here  it  is  to  be  recognized  that  juvenile  as  well  as  other  stages  are  subject  to  evolu- 
tionary modification.  If  a  present  juvenile  stage  happens  to  resemble  an  ancestral  adult 
stage,  it  means  merely  that  the  ancestral  form,  as  compared  with  the  present  form, 
changed  but  little  in  passing  from  youth  to  maturity.  In  many  cases,  as  in  Proserpinaca, 
it  is  thought  that  the  juvenile  leaves  differ  greatly  from  ancestral  adult  leaves. 


598  ECOLOGY 

those  forms  with  dissected  water  leaves,  features  that  have  been  sug- 
gested as  advantageous  are  the  nitration  of  light  rays,  an  easy  yielding 
to  currents,  and  a  relative  increase  of  absorptive  area.  If  such  water 
leaves  as  those  of  Sagittaria  are  due  to  poor  nutrition,  their  form 
scarcely  would  be  imagined  to  have  advantageous  significance.  As  to 
air  leaves,  so  far  as  they  are  expanded  (as  in  Sagittaria),  they  favor 
increased  synthesis,  while  so  far  as  they  are  small  and  compact  (as  in 
Proserpinaca),  they  are  suited  for  low  transpiration.  The  dying  of  air 
leaves  in  water  or  of  water  leaves  in  air  is  due,  more  probably,  to  lack  of 
fitness  in  leaf  structure  than  in  leaf  form. 

Form  variations  in  land  plants.  —  Variation  in  leaf  size  and  propor- 
tion, —  The  most  universal  and  best  understood  of  form  variations  are 
not  those  involving  notable  changes  in  shape,  but  those  in  which  the 
changes  are  chiefly  of  size  and  proportion.  As  previously  shown, 
xerophytic  or  sun  leaves  differ  from  mesophytic  or  shade  leaves  in  that 
they  are  considerably  smaller  and  thicker.  Not  only  do  these  differences 
characterize  xerophytic  species  on  the  one  hand  and  mesophytic  species 
on  the  other,  but  there  are  also  many  plastic  species  (as  Tilia  americana, 
Rhus  Toxicodendron)  in  which  such  differences  are  induced  readily  by 
growing  the  plants  in  different  environments.  When  Tropaeolum  is 
grown  in  dry  air  and  dry  soil,  the  leaves  are  much  smaller  and  thicker 
than  in  moist  air  and  moist  soil,  those  in  the  latter  not  infrequently 
being  five  times  as  large  as  those  in  the  former.  Smaller  leaves  develop 


867 


FIGS.  867,  868.  —  Diagrammatic  cross  sections  of  harebell  leaves  (Campanula  ro- 
iundifolia):  867,  a  mesophytic  leaf  from  a  moist  shaded  habitat;  868,  a  xerophytic  leaf 
from  a  dry  sunny  habitat;  note  that  the  xerophytic  leaf  is  much  the  narrower  and 
thicker;  v,  veins;  considerably  magnified. 

when  the  soil  is  dry  and  the  air  moist  than  when  the  soil  is  moist  and 
the  air  dry,  appearing  to  indicate  that  diminished  absorption  outweighs 
increased  transpiration  as  a  size-reducing  factor.  Many  species  behave 
in  a  similar  though  less  striking  manner  (figs.  867,  868).  Even  such 
succulent  plants  as  Sedum  and  Sempervivum  may  develop  thin,  ex- 
panded leaves  in  moist  air. 


LEAVES 


599 


Not  only  may  small,  thick  leaves  be  developed  where  the  air  and  the 
soil  are  dry,  but  also  in  the  presence  of  any  factor  that  impairs  root 
absorption.  For  example,  it  has  been  shown  in  the  case  of  more  than 
two  hundred  species  that  portions  of  the  same  individual  plant  develop 
more  xerophytic  leaves  when  grown  in  alpine  regions  than  when  grown 
in  the  lowlands  (figs.  869,  870;  also  figs.  1051,  1052);  this  result  doubt- 
less is  due  in  part  to 
reduced  absorption  on 
account  of  low  tempera- 
ture and  perhaps  in  part 
to  increased  transpira- 
tion. Similar  results  have 
been  obtained  in  maritime 
cultures,  absorption  being 
impaired  by  the  presence 
of  sodium  chlorid,  and  in 
bog  cultures,  where  the 
fact  of  impaired  absorp- 
tion is  variously  explained 
(p.  537);  the  small,  thick 
leaves  of  a  bog  individual 
of  Ledum  groenlandicum 
contrast  strikingly  with 
the  large,  thin  leaves  of 
a  forest  individual;  the 
maritime  forms  of  many 
species  have  thicker  leaves 
than  do  the  inland  forms, 
chiefly  by  reason  of 
greater  palisade  develop- 
ment.1 


FIGS.  869,  870.  —  Experimental  variation  in  the 
dandelion  (Taraxacum  officinale):  869,  a  plant  grown 
in  a  lowland  garden,  having  large  and  relatively  thin 
leaves  and  long  scapes  (s);  870,  a  plant  grown  in  an 
alpine  garden  (originally  a  portion  of  the  same  in- 
dividual as  869),  having  small  and  relatively  thick 
leaves  and  short  scapes  (s') ;  note  that  the  root  and  the 
inflorescence  are  much  less  reduced  than  are  the  other 
organs;  both  figures  are  drawn  to  the  same  scale. 
—  From  BONNIER. 


The  theory  above  outlined 
may  be  called  the  water  theory 
of  leaf  size  and  proportion,  since  the  absorption  and  the  evaporation  of  water  appear  to 
be  controlling  factors.  There  is  another  prominent  theory,  however,  in  which  light  is  re- 
garded as  the  dominant  factor.  Adherents  of  the  light  theory  speak  commonly  of  sun 

1  It  may  be  worth  noting  that  submersed  leaves  are  as  thin  and  as  expanded  in  bog 
ponds  as  in  other  ponds,  in  spite  of  the  supposed  presence  of  substances  unfavorable  to 
absorption;  however,  the  absence  of  transpiration  in  such  plants  may  be  the  significant 
factor. 


6oo 


ECOLOGY 


and  shade  leaves  rather  than  of  xerophytic  and  mesophytic  leaves.  While  there  is  no 
evidence  that  light  is  a  direct  factor,  it  is  of  undoubted  indirect  importance  through 
its  influence  both  upon  carbohydrate  synthesis  and  upon  transpiration.  Increased 
synthesis  implies  increased  available  food  for  leaf  construction,  and  hence,  probably, 
results  in  increased  size,  while  increased  transpiration,  as  has  been  seen,  results  in 
decreased  size.  The  simplest  situ- 
ation is  in  the  water,  where  syn- 
thesis is  unhindered  by  transpira- 
tion, and  here  (as  has  been  noted 
for  Sagittaria  and  Castalid)  the 
smallest  leaves  develop  where  the 
light  intensity  is  least.  In  air 
leaves  synthesis,  and  therefore  the 
food  available  for  leaf  construc- 
tion, increases  as  the  light  in- 
creases to  a  certain  optimum,  be- 
yond which  there  is  a  decrease. 
Thus,  intense  direct  light  in  con- 
trast with  diffuse  light  opposes  the 
development  of  expanded  leaves, 
partly,  perhaps,  because  of  less 


871 


872 


FIGS.  871,  872.  —  871,  a  young  mesophytic  individual  of  the  harebell  (Campanula 
rotundifolia\  showing  leaf  variation ;  note  the  broad  basal  leaves  and  the  narrower  stem 
leaves;  872,  the  upper  portion  of  a  xerophytic  individual  of  the  harebell  (Campanula 
rotundifolm),  showing  the  characteristic  linear  stem  leaves. 


available  food,  but  much  more,  probably,  because  of  increased  transpiration. 
Maximum  leaf  size  appears  to  be  found  in  the  moist  tropics,  where  transpi- 
ration is  low,  and  yet  where  there  is  sufficient  light  for  the  optimum  of  synthesis. 
It  is  believed  usually  that  a  moderate  increase  of  carbon  dioxid  favors  an  in- 
crease in  leaf  size;  indeed,  the  luxuriance  of  Carboniferous  vegetation  often  has 
been  ascribed  to  a  supposed  abundance  of  this  gas.  However,  a  large  increase 
in  the  percentage  of  carbon  dioxid  has  been  shown  to  result  in  decreased  leaf 


LEAVES 


601 


It  would  seem  that  the  chief  determining  factors  of  leaf  size  and  pro- 
portion are  those  that  control  the  water  supply.  High  transpiration, 
whether  caused  by  dry  air,  wind,  light,  or  high  temperature,  and  low 

absorption,  whether  caused  by 
dry  soil,  soil  salts,  soil  toxins, 
low  oxygen  pressure,  or  low  tem- 
perature, are  undoubtedly  the 
dominating  influences  in  deter- 
mining the  small  size  and  the 
great  thickness  of  the  xerophytic 
leaf.  Furthermore,  a  combina- 
tion of  factors  is  more  effective 
than  any  single  factor,  both  as 
to  the  rapidity  of  development 
and  as  to  the  degree  of  xero- 
phytism  attained;  an  "alpine" 
leaf,  for  example,  is  produced 
most  quickly  by  exposing  a  plant 
to  intense  sunlight,  high  tem- 
perature, and  dry  air  by  day, 


FIG.  873.  —  Leaf  variation  in  the  hare- 
bell (Campanula  rotundifolia) ;  the  apex  of 
the  shoot  has  been  cut  off ,  and  lateral  buds 
have  developed  at  b  and  b';  note  that  the 
first  leaves  of  the  axillary  rosettes  are  short 
and  roundish,  as  in  seedlings  and  basal 
rosettes;  such  an  occurrence  sometimes  is 
called  a  reversion  to  a  juvenile  stage.  —  From 
FAMILLER. 


875 


FIGS.  874,  875.  —  Leaf  variation  in  the 
arbor  vitae  (Thuja  occidentalis);  874,  a 
seedling,  showing  the  cotyledons  (c)  and 
the  awl-shaped  "juvenile"  leaves  (j)\ 
875,  an  older  plant,  showing  "  juvenile  " 
leaves  (j)  and  the  first  lateral  branches 
bearing  scale-shaped  "adult"  leaves  (a). 


and  to  low  temperature  by  night.  The  large  size  and -the  thinness 
of  the  mesophytic  leaf,  on  the  other  hand,  are  due  to  the  less 
extreme  influence  of  the  factors  (transpiration,  heat,  light,  etc.)  pro- 
ducing the  xerophytic  leaf,  supplemented  by  the  optimum  amount  of 


6O2 


ECOLOGY 


light  and  carbon  dioxid  for  synthesis.     In  the  xerophytic  leaf  there 
is  a  relatively  large  amount  of  cell   division  parallel  to  the  surface, 


FIGS.  876-879.  —  Leaf  variation  in  Geum  virginianum;  876,  a  basal  leaf  from  a  winter 
rosette;  877,  a  higher  leaf  from  a  winter  rosette;  878,  a  leaf  from  the  lower  part  of  a  stem; 
879,  a  leaf  from  the  upper  part  of  a  stem. 

resulting  in  dorsiventral  enlargement,  while  in  the  mesophytic  leaf,  sur- 
face enlargement  results  from  the  extensive  lateral  development  of  the 

individual  cells  sup- 
plemented some- 
ti  mes  by  an  increased 
number  of  cell  divi- 
sions perpendicular 
to  the  surface. 

Variation  in  the 
leaf  contour  of  land 
plants. — The  hare- 
bell (Campanula 
rotundifolid)  has  a 
basal  rosette  of  peti- 
oled,  round,  or  ovate 
leaves,  while  the  up- 

FIGS.  880,  881.  —  Leaf  variation  in  the  black  oak  (Quercus  per  stem  leaves  are 

velutina);  880,  an  upper  leaf,  exposed  to  strong  sunlight;  881,  narrowly  lanceolate 
a  shade  leaf  from  a  lower  branch  of  the  same  tree;  leaves  re-  ,.  (~ 

sembling  881  especially  characterize  basal  shoots  (suckers).  °r  even  nnear  (ngs- 

871,    872).      Early 

experiments  related  these  leaf  variations  to  light,  the  large  basal  leaves 
being  thought  to  result  from  shade  or  diffuse  light,  and  the  narrow 
upper  leaves  from  intense  light.  However,  the  basal  leaves  are 


LEAVES 


603 


round  on  exposed  rocks,  and  the  stem  leaves  are  nearly  as  narrow 
in  the  shade  as  in  the  sun.  Both  in  sun  plants  and  in  shade  plants 
there  is  a  gradation  from  broad  basal  leaves  to  narrow  stem  leaves,  the 
chief  difference  being  that  at  any  given  node  the  leaf  of  the  sun  plant 
is  narrower  and  thicker.  If  the  terminal  shoot  is  removed,  lateral 
shoots  develop,  their  first  leaves  being  round,  regardless  of  conditions. 

Furthermore,   any   sudden    change,  even 
fy^^  that   from  shade  to  light,  results  in  the 

arrest  of  the  terminal  shoot  and  in  the 
development  of  lateral  shoots  with  round 
basal  leaves  (fig.  873).  Thus,  Campanula 
appears  to  resemble  Sium  in  that  there  is 


882  '  V  883  «  884 

FIGS.  882-884.  —  Shoots  of  a  mulberry  (Morns'),  showing  leaf  variation ;  882,  a  vig- 
orous vertical  shoot,  showing  relatively  entire  leaves  spirally  arranged  on  the  stem;  883, 
a  horizontal  shoot  with  leaves  somewhat  lobed  and  all  in  one  plane  facing  the  light,  as 
a  result  of  stem  and  petiole  twisting;  884,  a  shoot  similar  to  883  but  with  smaller  and  more 
lobate  leaves. 

a  more  or  less  fixed  succession  of  leaf  forms,  any  shock  to  the  plant 
causing  "rejuvenescence." 

Variations  comparable  to  those  of  Campanula  are  exhibited  by  Satureja  glabra 
(figs.  985-988)  and  by  various  species  of  Arabis  and  Lechea.  In  Nicotiana  (fig, 
786)  and  in  many  similar  plants  there  is  a  very  gradual  change  from  the  base  to 
the  apex  of  the  stem,  the  leaves  becoming  progressively  smaller,  shorter,  and  thicker, 
as  well  as  more  erect;  the  changes  in  transpiration  from  the  base  to  the  apex  prob- 


604 


ECOLOGY 


ably  suffice  in  large  part  to  explain  such  cases.1  In  Thuja  (fig.  874)  the  first  leaves 
are  needle-shaped,  as  in  the  juniper,  but  after  a  year  or  two  there  appear  flattened 
lateral  branches  with  appressed  awl-shaped  leaves  (fig.  875).  Thenceforth  awl- 
shaped  leaves  continue  to  develop  through  life,  but  needle  leaves  again  develop 
if  the  plant  is  placed  in  a  moist  chamber.  Whether  this  represents  a  direct  re- 
action to  the  new  conditions  or  merely  "  rejuvenescence  "  is  not  known.  Very  strik- 
ing changes  in  form  are  exhibited  by  Eucalyptus  globulus,  which  has  a  thinnish, 
horizontal,  ovate  shade  leaf  and  a  thick,  vertical,  lanceolate-falcate  sun  leaf,  the 
differences  appearing  to  be  due  largely  to  differences  in  transpiration. 

In  Geum  virginianum  and  Ranunculus  abortivus,  and  in  many  similar 
plants,  the  leaf  changes  are  more  complicated  than  in  Campanula, 

the  basal  rosettes  of 
roundish  winter  leaves 
being  succeeded  by 
variously  divided  stem 
leaves;  in  Geum  the 
round  leaves  are  fol- 
lowed by  pinnate  and 
later  by  three-parted 
leaves  (figs.  876-879). 
In  Silphium  lacinia- 
tum,  narrow,  undi- 
vided early  leaves  are 
followed  by  broader, 
much-lobed  adult 
leaves.  In  various 
oaks  not  only  are  the 

FIGS.  885,  886.  —  Leaf  variation  in  the  barberry  (Ber- 
beris  vulgaris) ;  885,  a  young  shoot,  whose  lower  bud  (b) 
is  subtended  by  an  ordinary  foliage  leaf  (/),  while  the 
upper  (&')  is  subtended  by  a  spine  (s) ;  the  leaf  and  spine 
are  homologous  (i.e.  similar  in  position)  and  develop  from 
similar  primordia);  886,  a  bud  similar  to  V  in  885  but 
somewhat  older. 


early  leaves  relatively 
entire  and  the  later 
leaves  lobed,  as  in 
Geum,  but  the  upper 
leaves  are  more  lobed 


than  the  lower,  what- 
ever their  time  of  appearance,  while  leaves  on  vigorous  young  shoots 
(such  as  stump  suckers)  are  scarcely  lobed  at  all  (figs.  880,  88 1). 
Perhaps  the  lobation  of  the  upper  leaves,  as  well  as  their  smaller 
size  and  greater  thickness,  is  due  to  the  increased  transpiration  to 
which  they  are  subject.  The  large  leaves  of  vigorous  suckers  also 

1  In  some  plants,  as  the  cottonwood,  the  upper  leaves  are  larger  than  the  lower,  ap- 
pearing to  be  better  nourished. 


LEAVES 


605 


appear  better  nourished  than  do  the  other  leaves.  These  observations 
would  suggest  that  the  lobed  leaf  represents  a  sort  of  skeleton, 
which,  under  favorable  vegetative  conditions  (e.g.  freedom  from  ex- 
cessive transpiration  or  good  food  supply),  may  become  filled  out  into 
an  entire  leaf. 

This  view,  perhaps,  is  supported  by  the  fact  that  various  leaves  (as  in  Ricinus) 
are  more  divided  in  youth  than  at  maturity,  the  regions  between  the  principal  veins 
developing  last.  If  such  leaves  while  young  were  exposed  to  higher  transpiration 
or  to  decreased  food  supply,  they  probably  would  remain  in  the  more  lobate  condi- 
tion. In  the  mulberries  the  more  lobate  leaves  usually  are  small  and  on  slender 


FIGS.  887,  888.  —  Leaf  variation  in  the  Japan  ivy  (Psedera  triciispidata) ;  887,  a  three- 
lobed  simple  leaf  from  the  upper  portion  of  a  vine;  888,  a  compound  leaf  with  three  leaf- 
lets from  the  lower  portion  of  a  vine. 

branches  (as  if  poorly  nourished),  while  the  simple  leaves  commonly  are  larger 
and  on  more  luxuriant  shoots  (figs.  882-884).  Early  in  the  year  the  leaves  of  the 
staminate  tree  may  be  larger  and  more  entire  than  are  those  of  the  pistillate  tree, 
perhaps  because  in  the  latter  the  food  otherwise  available  for  leaf  construction  is 
utilized  in  fruit  development.  In  the  barberry  there  are  gradations  between 
ordinary  leaves  and  leaf  skeletons  which  are  reduced  essentially  to  spines  (figs. 
885,  886),  and  it  is  possible  that  the  latter  are  due  to  poor  nutrition  or  perhaps  to 
excessive  transpiration  (see  discussion  of  stem  spines,  p.  741). 

While  the  compound  air  leaves  thus  far  considered  differ  from  divided 
water  leaves  in  appearing  where  transpiration  is  high,  rather  than  where 
it  is  low,  in  each  case  their  development  seems  to  be  associated  with  poor 
nutrition.  The  lack  of  rigorous  experimental  data  forbids  further  analy- 
sis, and  makes  dubious  even  the  suggestions  here  given. 

While  the  Campanula  and  Geitm  categories  of  leaf  variation  are  poorly 
enough  understood,  chiefly,  perhaps,  because  of  inadequate  experi- 
mentation, there  are  many  cases  of  changing  form  that  are  not  under- 
stood at  all.  In  a  number  of  plants  (as  Lepidium,  Lactuca)  the  large, 


6o6 


ECOLOGY 


thin, rosette  leaves  are  more  divided  than  are  the  small,  thick,  stem  leaves. 
In  Cytisus  scoparius  the  three-lobed  lower  leaves  are  followed  by  simple 
(or  more  correctly,  one-lobed)  leaves,  greatly  reduced  in  size,  and  in 
the  Japan  ivy  the  lower  leaves  are  three-parted,  while  the  upper  leaves 
are  simple  (though  with  similar  palmate  venation)  and  often  as  large 
as  or  even  larger  than  those  below  (figs.  887,  888) .  In  Leonurus  Cardiaca 
the  intermediate  stem  leaves  are  more  divided  than  are  those  below  or 


FlGS.  889-894.  —  Leaf  variation  in  the  motherwort  (Leonurus  Cardiaca) ;  889,  a 
basal  rosette  leaf;  890,  one  of  the  more  apical  rosette  leaves;  891,  a  leaf  from  the  lower 
part  of  a  stem;  892,  893,  leaves  from  the  upper  part  of  a  stem;  894,  a  leaf  from  the  floral 
region;  note  that  lobation  and  leaf  size  increase  for  a  time,  culminating  in  891,  and  reach- 
ing a  minimum  in  894. 

above  (figs.  889-894),  and  in  Sassafras  almost  any  leaf  may  be  entire  or 
variously  lobed,  apparently  without  relation  to  transpiration  or  nutrition, 
or  even  to  the  phenomena  of  rejuvenescence.  One  of  the  most  extraor- 
dinary cases  of  leaf  dimorphism  is  exhibited  by  the  staghorn  fern 
(Platycerium) ,  in  which  there  are  erect  leaves  that  bear  the  sporangia, 
and  sterile  leaves  that  adhere  closely  to  the  ground  and  keep  the  substra- 
tum constantly  moist.  The  sterile  leaves  appear  to  react  to  contact  and 
gravity  rather  than  to  light,  ultimately  forming  an  overlapping  series, 
the  lowermost  of  which  gradually  become  transformed  into  humus. 

The  advantages  of  leaf  variation  in  land  plants.  —  In  so  far  as  the 
basal  or  lower  stem  leaves  of  any  plant  are  large  and  thin,  they  favor 


LEAVES 


607 


the  optimum  of  synthesis,  while  in  so  far  as  the  upper  leaves  are  small  and 

thick,  they  are  protected  from  excessive  transpiration.     In  essentially 

all  of  the  contour  categories  noted,  that  of  Lepidium  with  compound 

basal  leaves,  as  well  as  that  of  Campanula  or  Geum  with  simple  basal 

leaves,  these  differences  in  size  and  thickness  obtain,  and  the  advan- 

tages follow  as  cited.     Advantages  from  differences  in  contour,  however, 

are   not  so  obvious. 

It  often  is  assumed 

that    each    of    these 

differences   is    bene- 

ficial,  and   that   the 

very  fact  of  change, 

whether   from   with- 

out or  from  within,  is 

prima  facie  evidence 

of    usefulness.     But 

attempts  to  discover 

advantages  have  met 

with   failure.      It  is 

true  that  compound 

leaves    have    been 

thought  to  be  useful 

in  the  sifting  of  light 
/  v      i     , 

* 
plants  (as  Lepidium) 

have  their  compound 
I  r    i  j   • 

low,  and  in 
no  case  is  there  evi- 
dence that  the  capacity  of  leaves  to  sift  light  has  had  much  effect  upon 
the  survival  of  species.  Probably  contour  variability  in  leaves  is  of  no 
special  import  in  determining  the  success  or  failure  of  plants. 

Asymmetry  and  anisophylly.  —  In  certain  plants  (as  Celtis  and  Begonia)  the 
leaves  exhibit  asymmetry,  the  basal  region  bulging  more  on  one  side  than  on  the 
other,  giving  a  general  oblique  effect  (fig.  895).  It  has  been  shown  in  a  number 
of  instances  that  leaf  asymmetry  is  due  to  unequal  illumination,  the  bulged  portion 
having  received  moreTight  in  its  development,  because  of  its~more  favored  posi- 
tion. The  smaller  portion  commonly  develops  close  to  the  stem  and  often  is 
shaded  by  the  next  leaf.  Probably  the  light  influence  is  indirect  (i.e.  affecting 
synthetic  activity)  rather  than  direct.  By  twisting  a  petiole  or  by  making  incisions 
so  as  to  check  the  water  supply,  it  is  possible  to  produce  in  Begonia  a  symmetrical 


FIG.  895.  —  A  horizontal  shoot  of  the  hackberry  (Celtis 
occidentalis),  showing  leaf  asymmetry;  note  that  the  leaves 
form  a  "mosaic,"  the  expanded  portion  of  the  base  of  one 
blade  coming  into  juxtaposition  with  the  contracted  portion 
of  the  base  of  the  adjoining  blade  ;  all  the  leaves  are  in  one 

plane  by  reason  of  stem  twisting  or  petiole  growth. 


6o8 


ECOLOGY 


leaf  or  even  a  leaf  in  which  the  basal  asymmetry  is  reversed,  while  leaves  commonly 
symmetrical  in  like  manner  may  be  induced  to  become  asymmetrical.     Anisophylly 

is  well  illustrated  by  such  a  plant 
as  Selaginella  (fig.  896),  which 
has  two  sorts  of  leaves,  large 
and  small,  more  or  less  regularly 
placed  in  relation  to  each  other, 
and  also  by  the  leafy  liverworts, 
which  have  two  dorsal  rows  of 
green  leaves  and  a  ventral  row 
of  small,  colorless  leaves  (am- 
phigastria).  Anisophylly,  like 
asymmetry,  is  believed  to  be  due 
to  the  unequal  action  of  various 
factors,  among  which  light  and 
gravity  appear  the  more  impor- 
tant. In  many  plants  anisophylly 
characterizes  the  leaves  of  hori- 
zontal shoots,  where  the  influence 
of  such  factors  is  unequal,  while 
the  leaves  of  erect  shoots  are 
equal.  In  some  cases  at  least, 


FlG.  896.  —  A  plant  of  Selaginella  apus,  illus- 
trating anisophylly  (i.e.  inequality  in  leaf  size); 
note  that  large  and  small  leaves  adjoin  one  another 
in  regular  fashion ;  the  structures  depending  from 
the  horizontal  stems  are  rootlike  organs  known  as 
rhizophores  (r) ;  note  the  dichotomous  branching. 


the  primordia  of  anisophyllous 
leaves  are  equal,  and  it  has  been 
shown  that  upon  emergence  from 

the  bud,  one  leaf  of  a  pair  has  a  more  favored  position  and  hence  manufactures 
more  food  than  does  the  other,  making  possible  its  greater  development.  In  some 
plants  anisophylly  seems  unrelated  to  external  factors.  Both  asymmetry  and  ani- 
sophylly have  been  considered  advantageous  from  the  standpoint  of  light-reception, 
since  they  favor  the  development  of  leaf  mosaics  and  the  maximum  utilization  of  space. 


7.     THE  ABSORPTION    OF  WATER    AND   OF    NON-GASEOUS 
SOLUTES    BY    LEAVES 

General  remarks  on  leaf  absorption.  —  While  roots  are  the  chief 
regions  of  entry  for  water  and  salts,  there  are  many  plants  in  which 
these  substances  enter  directly  into  the  chlorophyll-bearing  organs.  In 
such  cases  the  division  of  labor  between  absorptive  and  synthetic  organs 
is  not  pronounced,  and  there  may  be  little  or  no  transportation  of  water 
and  salts  through  a  specialized  conductive  system.  As  a  class,  however, 
leaves  are  disadvantageously  placed  for  water  absorption,  since  they  are 
in  much  less  frequent  contact  with  water  than  are  roots,  and  generally 
are  subject  to  transpiration  rather  than  to  absorption;  the  cutinized  epi- 
dermal walls  of  most  leaves,  which  are  advantageous  in  checking  tran- 


LEAVES 


609 


spiration,  are  equally  effective  in  checking  absorption.  Where  the  outer 
walls  are  of  cellulose,  absorption  and  also  transpiration  may  take  place, 
so  that  leaf  absorption  is  advantageous  chiefly  where  transpiration  is  low 
or  wanting. 

Absorption  by  chlorophyll-bearing  organs  in  water  plants.  —  Algae 
and  bryophytes.  —  In  nearly  all  water  plants  the  outer  walls  are  of  cel- 
lulose, and,  since  there  is  constant  water  contact  and  freedom  from  tran- 
spiration, no  conditions  are  more  suitable  for  water  absorption  by  chlo- 
rophyll-bearing organs.  In  many  algae  there  is  a  homogeneous  green 
body  which  absorbs  water  through  its  entire  surface.  In  other  forms, 
as  in  Chara  and  Bryopsis  (figs.  1074, 1075)  and  in  large  marine  algae  (fig. 
751),  there  are  rhizoids,  which  are 
regarded  as  anchorage  organs. 
Probably  the  rhizoids  are  permeable, 
but  the  much  more  extensive  chlo- 
rophyll-bearing portion  with  its 
permeable  walls  is  vastly  more 
important  from  the  standpoint  of 
absorption.  The  aquatic  liverwort, 
Riccia  fluitans,  has  a  homogeneous 
green  body  without  rhizoids,  its 
mode  of  absorption  being  compara- 
ble to  that  of  algae  with  similar 
aspect.  In  aquatic  mosses  (as  in 
Fontinalis}  water  probably  enters 
chiefly  through  the  leaf  surfaces. 

Vascular  plants.  —  There  are 
some  rootless  aquatic  ferns  and  seed 
plants  in  which  all  water  and  salts 
must  enter  through  the  leaf  or  stem 
surface,  as  in  Utricularia  vulgaris 
(fig.  909)  and  in  Ceratophyllum.  In 
Salvinia  there  are  synthetic  floating 
leaves  and  absorptive  water  leaves, 
the  latter  being  finely  dissected  and 
quite  unlike  ordinary  leaves  (fig. 
897).  In  Wolffia  (fig.  997)  there  is  a  thalloid  body,  of  which  the 
submersed  lower  part  is  a  region  of  absorption,  while  the  emersed  upper 
part  is  a  region  of  synthesis  and  gas  exchange.  Most  submersed  seed 


FIG.  897.  —  A  plant  of  Salvinia 
natans,  showing  the  broad  floating  leaves, 
whose  upper  surfaces  have  aerial  rela- 
tions, and  the  dissected,  descending  sub- 
mersed leaves,  which  absorb  water  and 
salts  from  the  medium  and  which  bear 
reproductive  organs ;  note  the  abundant 
leaf  hairs.  —  From  COULTER  (Part  I). 


610  ECOLOGY 

plants,  however,  have  soil  roots,  so  that  experimentation  is  necessary 
to  determine  the  absorptive  region.  The  presence  of  cellulose  epider- 
mal walls  and  of  a  slightly  developed  conductive  system  has  led  to  a 
general  belief  in  the  dominance  of  leaf  absorption. 

That  in  attached  submersed  plants  root  absorption  is  not  inconsequential  has 
been  shown  in  some  instances,  lithium  salts  having  been  supplied  to  the  medium 
around  the  roots  and  later  detected  in  the  plant,  and  the  ascent  of  water  in  the 
conductive  tract  has  been  observed;  the  abundant  development  of  root  hairs  in 
many  instances  and  the  greater  luxuriance  of  plants  in  loam  than  in  gravel  also  ap- 
pear to  argue  for  root  absorption.  However,  there  is  scarcely  any  doubt  that  in 
water  plants  leaf  absorption  generally  is  much  greater  in  amount  than  is  root  ab- 
sorption, though  it  is  likely  that  plants  differ  in  respect  to  the  relative  impor- 
tance of  the  two;  for  example,  Potamogeton  may  depend  more  upon  root  absorp- 
tion than  does  Elodea,  since  it  grows  less  readily  in  an  aquarium  without  a  soil 
substratum.  Leaf  absorption  would  seem  the  more  advantageous,  since  it  does  not 
involve  the  development  of  a  conductive  tract  and  the  consequent  translocation  of 
water. 

Absorption  by  chlorophyll-bearing  organs  in  land  plants. — Algae  and 
lichens.  —  Some  green  algae  (as  Vaucheria)  and  various  blue-green 
algae  form  felts  on  moist  soil,  the  entire  surface  being  absorptive,  as  in 
water  forms;  here  the  wet  soil  may  provide  constant  water  contact, 
though  transpiration  at  times  may  be  excessive.  No  group  illustrates 
absorption  by  the  chlorophyll-bearing  organs  better  than  do  the  lichens. 
For  example,  the  reindeer  lichen  (Cladonia  rangiferina,  fig.  898),  which 
is  stiff,  brittle,  and  gray  when  dry,  becomes  soft,  flexible,  highly  elastic, 
and  green  when  wet,  by  reason  of  the  large  amount  of  water  it  is  able  to 
absorb.  Upon  entering  the  lichen,  the  water  passes  rapidly  by  capillar- 
ity between  the  hyphae  and  more  slowly  through  the  cell  membranes 
and  lumina.  Lichens  as  a  class  are  exposed  to  strong  transpiration,  and 
soon  lose  most  of  the  water  that  is  so  quickly  absorbed. 

Bryophytes.  —  Probably  all  mosses  and  liverworts  are  able  to  absorb 
water  through  the  chlorophyll-bearing  organs,  whenever  they  are 
moistened  by  rain  or  dew.  In  some  leafy  liverworts  water  catchment 
and  absorption  are  facilitated  by  a  cuplike  arrangement  of  the  leaves, 
but  most  liverworts  and  many  trailing  mosses  (as  species  of  Hypnum) 
grow  so  near  the  substratum  that  water  absorption  generally  is  possi- 
ble without  special  structural  arrangements.  The  situation  is  different 
in  erect  mosses  with  more  or  less  solitary  stems  (as  Polytrichum,  Catha- 
rinea,  Climacium),  in  which  rhizoid  absorption  probably  exceeds  leaf  ab- 
sorption in  amount,  the  latter  being  confined  chiefly  to  periods  when 


LEAVES 


611 


the  leaves  are  moist  from  rain  or  dew;  in  addition  to  lessened  opportu- 
nity for  leaf  absorption,  such  mosses  also  are  exposed  to  greater  tran- 
spiration than  are  mosses  in  general.  In  other  mosses  and  liverworts 
rhizoid  absorption  probably  supplements  leaf  absorption,  but  it  is 
probable  that  in  most  cases  the  latter  exceeds  the  former  (p.  517). 
Remarkable  cases  of  leaf  absorption  are  afforded  by  various  mosses 


FIG.  898.  —  Cushion  plants,  which  absorb  water  readily  through  the  aerial  organs; 
the  light-colored  cushions  are  reindeer  lichens  (Cladonia  rangiferina),  and  the  dark-colored 
cushion  is  a  moss,  Bartramia  pomiformis ;  Grand  Marais,  Minn.  —  From  MAcMiLLAN. 

that  grow  in  dense  cushions  (as  Dicranum,  Leucobryum,  Sphagnum, 
fig.  898).  Such  cushions  may  be  regarded  as  systems  of  capillaries  con- 
nected with  the  substratum ;  thus  a  constant  supply  of  water  is  available 
for  the  leaves,  except  in  very  dry  weather.  Since  each  year  the  individual 
stems  of  a  cushion  grow  at  the  apex  and  die  at  the  base,  it  is  unlikely 
that  the  soil  rhizoids  are  of  any  significance  in  absorption,  at  least  after 
the  cushion  is  a  few  years  old;  the  aerial  rhizoids  that  develop  in  some 
species  probably  facilitate  capillary  activity.  The  most  noteworthy  of 


6l2 


ECOLOGY 


the  cushion  mosses  is  the  peat  moss,  Sphagnum,  in  which  capillary 
phenomena  are  facilitated  further  by  drooping  lateral  branches  (fig. 
977),  and  especially  by  the  presence  of  dead  air-containing  cells  with 
porous,  fiber-thickened  walls  intercalated  regularly  among  the  syn- 
thetic cells  (fig.  899) .  Not  only  do  these  dead  cells  fill  with  great  rapidity, 
but  the  presence  of  porous  cell  walls  in  the  stem  gives  the  plant  an  in- 
ternal capillary  system  which  sup- 
plements the  external  capillary  system 
possessed  in  common  with  other 
cushion  mosses. 

Leucobryum  much  resembles  Sphagnum, 
except  that  porous  dead  cells  form  external 
absorptive  layers  comparable  to  an  absorp- 
tive epidermis  (as  in  orchid  roots),  the 
chlorophyll-bearing  cells  occupying  the 


Jl 


FIG.  899.  — A  surface  view  of  a  part 
of  a  leaf  of  Sphagnum,  showing  the  nar- 
row, elongated  living  cells  (c)  containing 
chloroplasts,  and  the  larger,  colorless 
dead  cells  (h)  with  their  spiral  thicken- 
ings and  pores  (p~) ;  highly  magnified. 
—  From  COULTER  (Part  I). 


FIG.  900.  —  A  cross  section  of  a  leaf  of 
Leucobryum,  showing  the  central  chlorophyll 
layer  (c)  with  its  chloroplasts,  and  the  peripheral 
layers  (e)  whose  cells  communicate  with  one 
another  by  means  of  the  pores  (h)  that  per- 
forate the  walls ;  highly  magnified. 


median  region  of  the  leaf  (fig.  900).  The  dry  dead  cells  cause  the  characteristic 
white  aspect  of  the  moss  (whence  the  name,  Leucobryum,  i.e.  white  moss).  Inter- 
esting capillary  phenomena  of  quite  another  sort  are  exhibited  by  Polytrichum, 
a  moss  whose  leaves  become  closely  appressed  to  the  stem  during  dry  weather; 
if  the  base  of  such  a  stem  is  placed  in  water,  the  leaves  open  out  almost  instantly  by 
reason  of  the  capillary  ascent  of  water  along  the  dry  sheathing  leaf  bases  and  between 
the  chlorenchyma  plates  of  the  leaves  (figs.  901,  902).  Doubtless  much  water  enters 
in  this  manner,  thus  supplementing  ordinary  leaf  and  rhizoid  absorption. 

As  in  the  air  roots  of  orchids,  the  absorptive  process  in  lichens  and  in 
cushion  mosses  has  two  phases,  the  first  being  a  capillary  phenomenon, 


LEAVES 


613 


which  consists  of  a  rapid  ascent  of  water  between  the  stems  or  of  a  rapid 
filling  of  air  spaces  (or  of  both  in  Sphagnum  and  Leucobryum) ;  the  second 
phase  is  represented  by  a  slow  osmotic  movement,  as  in  root  hairs. 
The  water  which  is  absorbed  so  rapidly  by  lichens  and  by  cushion  mosses, 
is  lost  more  slowly  by  transpiration,  Sphagnum  being  able  to  absorb  as 
much  in  a  minute  as  is  lost  by  ordinary  transpiration  in  a  week.  While 
some  cushion  mosses  are  mesophytic  (as  in  Bar- 
tramia  and  some  species  of  Dicranum),  others 
tend  toward  xerophytism  (as  in  Leucobryum),  and 
even  Sphagnum  may  be  called  a  bog  xerophyte. 
Lichens  usually  are  pronounced  xerophytes,  and 
(as  with  some  mosses)  absorption  must  be  a  com- 
paratively infrequent  phenomenon  (particularly 
since  only  liquid  water  can  be  absorbed  in  quan- 
tity), while  exposure  to  transpiration  is  frequent.1 
For  aerial  absorption  to  be  advantageous  in  such 
plants,  it  must  be  accompanied  by  an  ability  to 
endure  prolonged  desiccation,  an  ability  possessed 
by  lichens  and  by  some  mosses  in  superlative 
degree. 


FIGS.  901,  902.  — 
Aspects  of  Polytrichum 
commune,  a  xerophytic 
moss:  901,  a  leafy 
shoot,  as  seen  when 
the  water  supply  is 
adequate;  902,  a  simi- 
lar shoot  that  has  been 
exposed  to  desiccation, 
the  loss  of  water  hav- 
ing caused  the  leaves 
to  become  appressed 
to  the  stem ;  when  the 
base  of  such  a  shoot  is 
placed  in  water,  the 
leaves  soon  assume  the 
position  seen  in  Fig. 
901. 


Vascular  plants.  —  The  aerial  leaves  of  ferns  and  seed 
plants  are  cutinized,  and  water  absorption  commonly  is  so 
slight  as  to  be  without  significance.2  Wilted  leaves,  when 
placed  in  contact  with  water,  absorb  enough  to  enable 
them  to  recover  their  usual  turgescence,  but  this  phenom- 
enon probably  is  of  little  significance  in  nature.  Water 
absorption  has  been  predicated  as  a  function  of  many 
living  leaf  hairs,  especially  in  xerophytes,  largely,  perhaps, 
because  their  water  supply  is  scant,  and  because  no  other 
use  is  known  for  these  hairs.  The  felted  hairs  of  Centaurea 
have  been  thought  to  be  useful  in  this  way,  since  the  leaf  can  take  up  thirteen  percent 
of  its  weight,  when  placed  in  water  for  a  day.  Salt-secreting  hairs,  as  in  Reaumuria, 
have  been  shown  to  absorb  water.  Recent  experiments  show  that  xerophytic  leaves  as 
a  class  possess  less  capacity  for  absorption  than  do  leaves  in  general,  any  water  taken 
up  being  lost  with  rapidity,  since  it  does  not  penetrate  to  the  living  cells.  Nor 
should  leaf  absorption  be  expected  a  priori,  above  all  in  xerophytes,  since  a  cuti- 

1  Some  recent  experiments  appear  to  show  that  certain  mosses  in  a  few  seconds  can  ab- 
sorb sufficient  water  vapor  to  increase  their  weight  25  to  75  per  cent,  and  that  fruticose 
lichens  may  absorb  enough  water  vapor  to  be  of  appreciable  advantage. 

2  In  the  filmy  ferns,  which  are  largely  delicate  plants  of  tropical  forests,  cutinization  is 
slight  and  absorption  by  leaves  often  surpasses  in  amount  absorption  by  subterranean 
organs. 


614 


ECOLOGY 


nized  epidermis  is  as  effective  in  keeping  water  out  as  in  keeping  it  in.  Recently 
it  has  been  shown  that  such  salt-containing  plants  as  Salicornia  are  able  to  absorb 
water  somewhat  readily  through  the  aerial  organs.  Though  adequate  experiment 
is  lacking,  there  is  some  reason  for  believing  that  the  leaf  sheaths  of  many  grasses 
and  umbellifers  and  the  cup-shaped  leaf  bases  of  Silphium  perfoliatum  have  some 
absorptive  efficiency. 

Water  absorption  in  epiphytes.  —  Thallophytes  and  bryophytes.  —  In 
regions  with  cold  winters  true  epiphytes  are  confined  essentially  to  the 


FIG.  903.  —  A  live  oak  (Querctis  virginiana)  festooned  with  the  long  moss  (Tillandsia 
usneoides},  an  epiphytic  member  of  the  pineapple  family,  which  absorbs  liquid  water  by 
means  of  specialized  scale  hairs ;  Tampa,  Fla.  —  Photograph  by  E.  W.  COWLES. 

lower  plants,  and  in  these  groups  absorption  is  dominantly  a  function 
of  chlorophyll-bearing  organs  rather  than  of  rhizoids.  The  widespread 
distribution  of  these  plants  in  exposed  situations  is  due  in  large  part  to 
their  ability  to  endure  prolonged  desiccation,  since  they  quickly  lose  most 
of  the  absorbed  water  by  transpiration  through  the  surface  by  which  it  en- 
tered. Among  such  xerophytic  epiphytes  are  some  algae  (as  Pleurococcus) 
and  liverworts  (as  Frullania)  and  many  mosses  (notably  Orthotrichum), 
but  the  most  representative  epiphytic  group  is  that  of  the  lichens. 


LEAVES 


Probably  the  most  xerophytic  of  all  plants  are  the  crustose  lichens 
(as  Buellia),  which  are  either  epiphytic  or  epilithic  (i.e.  growing  on  rocks), 
appearing  embedded  within  the  substratum.  Most  crustose  lichens  ab- 
sorb water  chiefly  through  the  upper  surface,  though  some  species  have 
upper  surfaces  which  are  not  readily  wetted  or  which  are  covered  with 
an  impermeable  crust.  On  dry  rocks,  at  least,  such  plants  absorb  water 
chiefly  during  or  immediately 
following  precipitation,  soon 
drying  out  again,  and  enter- 
ing a  period  of  inactivity; 
the  active  period  may  be 
longer  in  the  case  of  bark 
lichens,  owing  to  the  greater 
retentiveness  of  the  substra- 
tum. Foliose  lichens  (for  ex- 
ample, Parmelia;  fig.  mi) 
are  more  leaflike  and  are  at- 
tached to  rock  or  tree  surfaces 
by  evident  rhizoids ;  perhaps 
also  they  are  less  xerophytic, 
though  capable  of  withstand- 
ing prolonged  desiccation. 
Foliose  lichens  commonly  ab- 
sorb through  both  surfaces, 
but  mainly  through  the  lower 


surface,  except  in  a  few 
instances  (as  in  species  of 
Parmelia  with  black  under 


FIGS.  904,  905. — Absorptive  scale  hairs  of 
Tillandsia:  904,  a  scale  hair,  as  seen  in  surface 
view;  905,  a  scale  hair,  as  seen  in  cross  section; 
the  outer  cells  (c)  are  dead,  and  absorb  water 
readily  when  moistened,  thus  elevating  the  scale 
and  opening  the  channel,  aa,  along  which  water 
passes  by  capillarity;  subsequently  the  water 
_c  \  rm  i  ,•  enters  the  living  stalk  cells  (5);  e,  epidermis; 

surfaces).      The     gelatinous     highly  magnifiedg 
lichens   (as  Collemd)  absorb 

great  quantities  of  water  and  remain  for  some  time  as  mucilaginous 
masses.  Possibly  the  rhizoids  supplement  the  chlorophyll-bearing 
organs  in  absorption,  but  to  what  extent  if  any  is  unknown.  Some 
mosses  (as  Andreaea  and  Grimmia)  are  epilithic  xerophytes  and  with- 
stand prolonged  desiccation  without  harm,  reviving  rapidly  during  rain 
as  a  result  of  leaf  absorption. 

Bromeliaceae.  —  The  most  remarkable  instance  among  seed  plants  of  absorption 
by  aerial  leaves  is  in  the  Bromeliaceae,  a  family  of  tropical  epiphytes,  repre- 
sented in  the  southern  United  States  by  Tillandsia,  especially  T.  usneoides,  the 


616  ECOLOGY 

"  long  moss,"  a  rootless  plant  with  minute  leaves  (fig.  903).  In  these  epiphytes  the 
chief  organs  of  absorption  are  specialized  hairs,  with  which  the  long  moss,  for  ex- 
ample, is  completely  covered;  that  these  plants  do  not  depend  upon  root  absorp- 
tion is  shown  by  their  occasional  development  on  telegraph  wires,  and  by  the  fact 
that  they  blow  from  place  to  place,  continuing  their  activity  wherever  they  lodge. 
Probably  the  long  moss  represents  an  extreme  epiphytic  form  that  has  been  de- 
rived from  an  ancestry  similar  to  its  relative,  the  pineapple;  even  now  there  ex- 
ists a  series  of  intergrading  forms,  those  at  the  pineapple  end  of  the  series  having 
ordinary  absorptive  soil  roots  and  scattered  or  localized  absorptive  scales,  while 
those  at  the  Tillandsia  end  have  only  anchorage  roots  or  no  roots  at  all  and  have 
an  abundant  development  of  absorptive  scales  (fig.  969). 

The  absorptive  scales  or  scale  hairs  of  the  Bromeliaceae  are  epidermal  struc- 
tures consisting  of  a  sunken,  multicellular,  thin-walled,  living  stalk,  capped  by 
a  protruding,  multicellular,  shieldlike  organ,  whose  cells  are  dead  and  have  thick 
cellulose  walls  (figs.  904,  905).  Liquid  water  is  absorbed  by  the  dry  scale  hairs 
with  remarkable  rapidity,  but  the  rest  of  the  epidermis  is  highly  cutinized  and  im- 
permeable. When  the  surface  is  moistened,  the  dead  cells  fill  with  water,  causing 
an  expansion  of  the  structure  which  lifts  it  from  the  rest  of  the  epidermal  surface, 
thus  making  possible  the  movement  of  water  along  the  capillary  passages  between 
the  scale  and  the  subjacent  epidermis,  as  well  as  through  the  scale.  The  water  then 
enters  the  living  stalk  cells  osmotically,  as  in  root  hairs,  the  presence  of  sugar  in  the 
cells  facilitating  the  process.  When  evaporation  begins,  the  dead  cells  lose  their 
water  and  the  scale  collapses,  thus  closing  the  region  where  capillary  water  enters, 
and  reducing  the  amount  of  water  lost.  It  has  been  shown  that  salts  as  well  as 
water  may  enter  these  plants  through  the  scale  hairs.  The  Bromeliaceae  furnish 
the  only  conspicuous  well-attested  example  among  the  higher  plants  of  the  absorp- 
tion of  water  by  aerial  leaves,  and  the  only  well-attested  structure  in  any  group,  which 
both  facilitates  absorption  and  retards  transpiration.  Nothing  is  known  concern- 
ing the  steps  in  the  evolution  of  these  hairs,  and  their  rigidity  gives  little  hope  of 
obtaining  through  experiment  a  clue  to  their  origin. 

Food  absorption  by  the  leaves  of  carnivorous  plants.  —  The  sundews. 
— A  few  plants  possess  the  power  of  absorbing  and  digesting  animal  food. 
The  best  understood  of  these  is  the  sundew,  Drosera,  a  bog  plant  whose 
leaves  have  prominent  glandular  hairs,  which  usually  are  wine-red  in 
color  and  tipped  with  viscid  secreted  drops  that  glisten  in  the  sunlight 
like  dewdrops  (fig.  906).  Insects,  which  make  chance  visits  or  which 
are  attracted  by  the  leaf  brilliancy  or  color,  are  held  by  the  viscid 
drops,  and  their  efforts  to  escape  result  in  contact  with  other  drops,  so 
that  they  are  held  still  more  securely.  The  presence  of  the  insect  stim- 
ulates the  glandular  hairs  to  secrete  more  actively  and  also  differently, 
the  more  important  substances  secreted  being  formic  and  other  acids 
and  proteolytic  (i.e.  protein-digesting)  enzyms,  which  transform  into 
solutes  the  digestible  portions  of  the  insect  body.  The  presence  of  the 


LEAVES 


617 


insect  further  incites  an  incurving  of  the  leaf  margins  and  of  the  glan- 
dular hairs  not  originally  touched,  so  that  most  of  or  even  all  the  hairs  may 
take  part  in  secretion  and  digestion.  The  latter  phenomenon  is  facili- 
tated, especially  if  the  insect  alights  at  the  leaf  center,  by  the  fact  that  the 
hairs  are  progressively  longer  from  the  center  outward.  The  hair  struc- 
ture is  somewhat  complicated,  there  being  two  peripheral  layers  of 


906 


FIGS.  906,  907.  —  The  absorptive  and  digestive  glandular  hairs  of  a  sundew  (Drosera 
rotundifolia) :  906,  a  leaf,  showing  the  conspicuous  glandular  hairs  (g)  covering  the  upper 
surface;  the  hairs  at  the  right  are  inflected  toward  an  entangled  insect;  note  that  the 
hairs  are  tipped  by  a  drop  of  secreted  liquid  (d),  which  attracts  insects  to  the  leaf  and 
also  entangles  them;  907,  the  terminal  capitate  portion  of  a  glandular  hair,  as  seen  in  a 
median  longitudinal  section;  the  conductive  bundle  (v)  entering  the  hair  from  the  leaf 
is  much  enlarged  at  its  terminal  portion,  where  it  is  composed  of  tracheids  (J);  the  ter- 
minal tracheids  are  inclosed  by  the  protective  sheath  or  endodermis  (e),  outside  of  which 
are  two  epidermal  layers  of  secretory  cells  (s  s') ;  note  the  palisade-like  nature  of  the  cells 
in  the  outer  secretory  layer;  w,  a  drop  secreted  by  these  layers;  906,  somewhat  magnified ; 
907,  highly  magnified.  —  906  after  KERNER,  907  after  DEBARY. 

secretory  cells  containing  the  wine-red  pigment,  underneath  which  is  the 
endodermis  and  a  branch  of  the  leaf  conductive  system ;  the  latter  con- 
sists chiefly  of  a  row  of  tracheids,  which  diverge  in  the  terminal  part  of 
the  hair,  thus  presenting  an  .enlarged  surface  (fig.  907).  After  the 
digestible  portions  have  been  absorbed,  the  secretions  cease,  and  the 
glandular  hairs  assume  their  original  position.  It  is  said  that  inorganic 
bodies,  such  as  dirt  particles,  excite  no  enzym  secretions  or  leaf  move- 
ments. 


6i8 


ECOLOGY 


Besides  the  various  species  of  Drosera,  other  members  of  the  same  family,  as 
Drosophyllum,  have  irritable  glandular  hairs.  Two  of  the  most  remarkable  genera 
are  Aldrovanda  and  Dionaea,  which  have  sensitive  leaf  blades  that  close  suddenly 
when  irritated,  and  prevent  the  escape  of  alighting  insects  (figs.  657-659).  Im- 
pact upon  the  stiff  outer  part  of  certain  hairs  is  perceived  by  delicate  cells  beneath 
and  transmitted  to  the  region  where  movement  is  effected.  The  secretion  of 
digestive  fluids,  and  subsequent  digestion  and  absorption 
take  place  as  in  Drosera. 

Pitcher  plants.  —  The  pitcher  plants  (Sarracenia,  Nepen- 
thes, etc.),  like  Drosera,  commonly  are  bog  plants.  The 
pitcher-like  leaf  blades  of  Sarracenia  (fig.  908)  are  partly 
filled  with  rain  water,  into  which  insects,  by  chance  or 
attracted  by  the  bright  colors,  frequently  wander  and  are 
drowned.  For  crawling  insects,  entrance  is  easy  and  exit 
difficult  by  reason  of  stiff  downward-pointing  hairs  at  the 
edge  of  the  pitcher.  In  Nepenthes  (fig.  656)  nectar  is 
secreted  at  the  pitcher  edge,  doubtless  forming  an  addi- 
tional attraction.  Proteolytic  enzyms  have  been  discovered 
in  the  pitchers  of  Nepenthes,  the  glands  occurring  at  the 
base  of  cavities  and  consisting  of  spherical  multicellular 
structures,  below  which  are  the  terminal  tracheids  of  a 
conductive  bundle,  as  in  Drosera.  Enzym  secretion  prob- 
ably does  not  occur  in  Sarracenia,  though  it  is  possible 
that  the  products  of  insect  decay  may  enter  the  plant, 
much  as  in  saprophytes.  In  Dischidia,  an  epiphytic 
pitcher  plant,  there  are  double  pitchers,  one  inside  the 
other;  the  outermost  pitcher  is  a  sort  of  living  flower  pot 
in  which  earth  and  water  collect  and  into  which  adventi- 
tious roots  penetrate  from  other  parts  of  the  plant. 

Butterivorts  and  bladder  worts.  —  A  third  family  of  carniv- 
orous plants  is  represented  by  the  butterworts  (Pinguicula) 
and  the  bladderworts  (Utricularia},  which  commonly  inhabit 
swamps  or  ponds.  On  the  leaf  blade  of  Pinguicula,  as  in 
Drosera,  there  are  glandular  hairs  that  secrete  viscous 
substances,  and  the  leaf  margins,  but  not  the  hairs,  also 
incurve  when  alighting  insects  irritate  the  leaf.  The  hairs 
are  of  two  sorts ;  some  with  relatively  long  stalks  hold  fast 
to  the  visiting  insects,  while  shorter  hairs,  consisting  of  an 
eight-celled  disk  with  a  hidden  stalk,  are  thought  to  be  more 
The  cells  secrete  enzyms  only  when  the  hairs  are  irritated. 
The  bladders  of  Utricularia  have  an  opening  at  one  end,  within  and  about  which  are 
a  number  of  hairs  and  other  structures  which  are  so  arranged  as  to  prevent  egress, 
though  permitting  easy  entrance,  much  on  the  principle  of  an  eel-trap  (figs.  909, 
910).  Minute  water  animals  often  crawl  or  swim  into  the  bladders,  where  they 
are  detained.  The  presence  of  the  imprisoned  animals  has  led  to  a  general  belief 
in  their  utilization  by  the  plant,  the  forked  hairs  of  the  inner  surface  of  the  bladder 
being  supposed  to  play  a  part  in  the  process,  and  there  is  some  evidence  of  the 


FIG.  908.  —  A  leaf 
of  the  pitcher  plant 
(Sarracenia  purpurea) ; 
usually  such  pitcher 
leaves  are  partly  filled 
with  water  into  which 
insects  often  crawl  or 
fall. 

efficient  in  absorption. 


LEAVES 


619 


secretion  of  enzyms  and  of  the  digestion  of  animal  food.  Bladders  may  assist  in 
the  flotation  of  the  plant,  though  in  such  a  role  the  eel-trap  structures  can  have  no 
significance ;  it  may  be  noted  that  bladders  occur,  though  less  abundantly,  in  land 
species.  Possibly  the  bladders  have  no  role  of  importance.  Insects  are  held  by 
viscid  secretions  on  the  stems  of  Silene  antirrhina  (hence  called  catchfly),  and 
are  drowned  in  the  water-containing  leaf  cups  of  Silphium  perfoliatum  and  Dip- 

sacus  sylvestris,  but  in  none  of  these 
cases  is  there  evidence  of  enzym  se- 
cretion or  of  food  digestion. 


FIG.  909.  —  A  leaf  of  a  bladderwort 
(Utricularia  vulgaris),  showing  numerous 
capillary  divisions,  many  of  which  bear 
bladders  (£>),  especially  near  the  place  of 
attachment  to  the  main  leaf  axis  (a);  note 
the  apertures  (p]  of  the  bladders  into  which 
small  aquatic  animals  may  crawl  or  swim. 


FIG.  910. — A  longitudinal  section 
through  the  bladder  of  a  bladderwort 
(Utricularia  neglecta),  showing  long  ex- 
ternal hairs  (h)  about  the  entrance,  an 
elastic  valve  (-y)  which  entering  animals 
readily  push  back,  a  cushion  (c~)  on  which 
the  valve  rests,  and  the  interior  cavity  of 
the  bladder  (i)  in  which  the  animals  re- 
main imprisoned ;  the  cavity  is  lined  with 
small  branched  hairs  (s),  the  so-called 
absorptive  organs  of  the  bladder;  con- 
siderably magnified.  —  From  KERNER. 


The  significance  of  the  carnivorous  habit.  —  The  general  restric- 
tion of  carnivorous  plants  to  bogs  has  led  to  the  view  that  the  car- 
nivorous habit  is  advantageous  in  supplementing  the  nitrogen  supply, 
which  has  been  thought  inadequate  in  such  habitats.  However, 
the  vast  majority  of  bog  plants  have  no  such  unusual  method 
of  getting  nitrogenous  food,  and  yet  they  thrive  as  well  as  or 
better  than  do  the  carnivores.  Even  in  the  sundew  the  advan- 
tage of  animal  food,  so  far  as  known,  is  slight,  and  in  other  plants 
the  proof  of  such  advantage  is  wanting.  In  all  carnivorous  plants 
animal  food  probably  is  a  comparatively  incidental  feature  of  nutri- 
tion. In  view  of  the  foregoing,  the  question  of  the  evolution  of 
the  carnivorous  habit  arouses  much  interest.  It  has  been  thought 


620 


ECOLOGY 


that  the  glandular  hairs  of  Drosera  have  been  derived  from  water- 
secreting  glands  through  gradual  specialization,  but  there  is  no  good 
evidence  for  this  view;  even  the  experimental  method  of  attack  is 
likely  to  prove  unavailing  here.  The  theory  of  origin  through  natural 
selection  seems  particularly  inadequate,  partly  because  in  most  cases 
only  highly  developed  organs  can  be  of  use,  and  partly  because  in  all 
cases  the  use  appears  too  slight  to  account  for  preservation  through  the 
operation  of  natural  selection. 


8.     LEAVES    AS  ORGANS  OF  SECRETION  AND 
EXCRETION 

General  remarks  on  secretion  and  excretion.  —  Secretion  usually  in- 
volves the  elaboration  of  new  materials  by  specialized  glands  or  glandu- 
lar regions,  whereas  excretion  involves 
the  elimination  of  waste  by  any  organ. 
While  the  products  of  secretion  often 
play  an  explicit  role  in  subsequent  ac- 
tivities, there  are  many  cases  in  which 
no  such  role  is  known,  so  that  it  is  im- 
possible to  regard  all  secreta  as  useful 
and  all  excreta  as  useless  substances. 
Plants  as  a  whole  have  less  waste  than 
animals,  probably  because  they  utilize 
simpler  raw  materials,  taking  in  relatively 
little  useless  matter.  Plants  also  differ 
from  animals  in  that  their  excreta  usually 
accumulate  in  reservoirs  or  in  dead  or 
inactive  tissues  instead  of  passing  off, 
although  excreta  are  lost  in  large  amount 
through  leaf  fall  and  to  some  extent 
through  special  organs.  In  any  event, 
the  accumulation  of  waste  products  in 
active  cells  is  distinctly  disadvantageous, 
not  only  because  some  waste  products 
are  toxic,  but  also  because  any  such  accumulations  interfere  with  cell 
activity. 

Water  exudation.  —  Hydathodes.  —  When  certain  plants  (as  Tropae- 
olum)  are  placed  for  some  time  in  a  moist  chamber,  liquid  water  is  ex- 


FiG.  911.  —  A  surface  view  of 
a  nasturtium  leaf  (Tropaeolum), 
showing  large  water  stomata  (w) 
just  over  the  terminal  portion  of 
the  vein  (whose  course  is  indicated 
by  broken  lines),  and  numerous, 
smaller  air  stomata  (a)  over  the 
mesophyll  region  of  the  leaf;  note 
the  irregularity  of  stomatal  orien- 
tation, generally  characteristic  of 
dicotyl  leaves;  considerably  mag- 
nified. 


LEAVES 


621 


uded  from  the  ends  of  the  principal  veins,  collecting  as  drops.  The  water 
passes  to  the  exterior  through  structures,  known  as  water  stomata,  which 
differ  from  ordinary  air  stomata  in  their  position  at  the  ends  of  veins, 
in  always  remaining  open,  in  the  lack  of  the  peculiar  cutinization  char- 
acteristic of  air  stomata,  and  often  by  their  large  size  (as  in  Tropaeolum, 
fig.  911).  Underneath  the  aperture  is  an  air  cavity,  below  which  is 
a  loose  tissue,  the  epithem,  made  up  of  small 
delicate  cells  without  chlorophyll ;  under- 
neath this  are  diverging  terminal  tracheids, 
representing  the  end  of  a  conductive  bundle 
(fig.  912).  The  entire  structure,  consisting 
of  tracheids,  epithem,  and  water  stoma  is 
called  a  hydathode  (i.e.  water  way).  In 
many  plants,  as  in  Primula  and  Fuchsia,  the 
hydathodes  occur  at  the 
tips  of  bluntish  leaf  teeth 

(fig- 9i3)- 

The    distribution    and 

significance   of    hyda- 
thodes. —  Not    much   is 

known    concerning    the 

general    distribution    of 

hydathodes   by   habitats 

or    regions;     commonly 

they  are  regarded  as  most 

characteristic    of   the 

humid  tropics,  although 

present   in    many  herbs 

of  low  grounds  and  humid 
woods.  The  amount  of  excreted  water  may  be  very  large ;  the  tips  of 
Colocasia  leaves  have  been  known  to  drip  water  at  the  rate  of  190  drops 
per  minute,  and  the  leaves  of  Conocephalus  are  said  to  lose  in  one  night 
an  amount  equal  to  a  fourth  of  their  weight.  The  substance  excreted  is 
nearly  pure  water,  o.i  per  cent  or  less  representing  the  proportion  of 
salts ;  closely  related  to  hydathodes  are  the  chalk  glands  of  Saxifraga 
and  the  salt  glands  of  Tamarix,  which  excrete,  respectively,  large  quan- 
tities of  calcium  carbonate  and  sodium  chlorid.  The  prevalent  theory 
as  to  the  advantage  of  hydathodes  is  that  they  are  a  means  of  escape  for 
a  surplus  of  water  in  plants  with  high  turgor  pressure  ("root  pressure"), 


t       me 

FIG.  912. — A  radial 
longitudinal  section  through 
a  leaf  tooth  of  the  Chinese 
primrose  (Primula  sinensis\ 
showing  a  hydathode  with 
divergent  tracheids  (t)  termi- 
nating the  conductive  bundle, 
above  which  is  the  colorless 
epithem  (d),  the  stomatal 
cavity  (c),  and  one  of  the 
guard  cells  (x)  of  the  water 
stoma  at  the  tip  of  the  tooth; 
i,  intercellular  air  spaces  in 
the  epithem ;  e,  epidermis ;  m, 
chlorenchyma ;  highly  magni- 
fied. —  From  HABERLANDT. 


FIG.  913.  — A  por- 
tion of  the  leaf  margin 
of  Fuchsia,  showing 
blunt  teeth  (/),  each 
of  which  represents  a 
hydathode  at  a  vein 
terminus. 


622  ECOLOGY 

at  times  when  the  atmospheric  humidity  is  too  great  for  transpiration. 
Water  exudation  thereby  prevents  the  injection  of  air  spaces  with  water 
and  the  consequent  impairment  of  respiration  and  carbohydrate  syn- 
thesis. There  are  those  who  regard  the  injection  of  air  spaces  as  an 
imaginary  danger  and  hydathodes  as  structures  of  no  evident  value;  the 
weight  of  opinion,  however,  is  for  the  view  first  stated. 

The  mechanics  of  exudation.  —  The  mechanics  of  exudation  appear  relatively 
simple,  water  being  forced  through  the  influence  of  turgor  pressure  along  the  path 
of  least  resistance,  namely,  through  the  epithem  and  water  stomata.  The  phenome- 
non has  much  in  common  with  Heeding,  which  may  be  denned  as  the  exudation  of 
sap  from  a  wounded  surface.  As  with  exudation,  the  amount  of  bleeding  is  deter- 
mined largely  by  turgor  pressure,  and  it  is  especially  evident  when  sap  is  flowing 
abundantly  in  spring.  Dew  and  excreted  water  are  likely  to  be  mistaken  for  one 
another,  particularly  as  the  same  atmospheric  conditions,  facilitate  both,  and  as 
both  are  likely  to  appear  on  leaf  teeth. 

Various  water-excreting  organs.  —  In  addition  to  undoubted  hydathodes, 
organs  are  found  which  have  been  supposed  to  excrete  water.  Among  such  are 
structures  somewhat  like  those  above  described,  but  on  submersed  leaves  (as  in 
Proserpinaca  and  Ranunculus};  while  transpiration  is  excluded  in  such  plants, 
there  may  be  an  excretion  of  liquid  water,  if  there  is  a  conductive  stream,  as  some- 
times is  supposed  (p.  610).  Water  excretion  has  been  thought  to  be  the  role  of  epi- 
dermal glands  which  occur  close  to  the  terminal  tracheids  in  grooves  near  the  leaf 
margins  of  certain  ferns.  Many  hairs  (as  in  Phaseolus  and  Lathraea),  known  as 
trichome  hydathodes,  have  been  thought  to  excrete  water  through  glandular  activity. 
While  the  exudation  of  water  by  these  hairs  has  been  doubted,  it  is  easy  to  believe 
in  such  a  role  because  of  their  close  resemblance  to  nectar  glands.  Indeed,  in  the 
so-called  water  calyxes  of  certain  tropical  plants  there  are  capitate  trichome  hyda- 
thodes that  excrete  large  quantities  of  water  and  small  amounts  of  sugar,  thus  grad- 
ing into  true  nectaries.  In  fungi,  water  exudation  may  occur  at  any  point  on  the 
plant  surface,  the  entire  body  frequently  being  covered  with  drops  of  water,  especially 
in  caves,  mines,  and  moist  chambers. 

The  influence  of  external  factors  upon  hydathode  development.  —  Little  is  known 
concerning  the  evolution  of  hydathodes  or  of  the  influence  of  external  factors  upon 
their  development.  When  the  hydathodes  of  Conocephalus  are  poisoned  by  cor- 
rosive sublimate,  the  air  spaces  become  injected  with  water,  and  in  a  few  days  hyper- 
trophied  tissues  are  said  to  protrude  from  the  leaf  and  excrete  water.  While  these 
have  been  called  substitute  hydathodes,  and  regarded  as  a  sort  of  emergency  adap- 
tation, they  are  probably  nothing  more  than  a  case  of  edema  (p.  633),  appearing,  as 
usual,  where  an  excess  of  water  is  present. 

The  secretion  of  oils,  resins,  and  mucilage. — Glandular  hairs. — 
Glandular  hairs  are  distributed  widely  in  plants,  and  their  secretions 
are  often  odoriferous  (as  in  Pelargonium  and  in  the  mints),  and  usually 
viscid.  Most  glandular  hairs  are  multicellular  epidermal  outgrowths 


LEAVES 


623 


FIG.  915. — A  capi- 
tate, multicellular 
glandular  hair  from  a 
geranium  leaf  (Pelar- 
gonium}, showing  the 
accumulation  of  an  oil 
drop  (o)  just  beneath 
the  cuticle  (c) ;  highly 
magnified. 


with  a  head  and  with  a  more  or  less  evident  stalk;  the  cells,  both  in 
the  head  and  in  the  stalk,  vary  in  number  from  one  to  several  and  are 
rich  in  cytoplasm  (figs.  914,  915;  also  fig.  632).  In  the  mints  the 
glandular  hairs  occur  in  leaf  depressions  and  are  relatively  stalkless. 
In  some  plants  (as  in  Silene)  there  is  a  region  of  palisade-like  secretory 
cells  instead  of  glandular 
hairs,  while  in  many  plants 
ordinary  epidermal  cells  ex- 
crete wax,  varnish,  etc.,  as 
previously  noted  (p.  570).  In 
oil  glands  the  secretions  gather 
within  the  walls  of  the  head 
cells,  where  they  press  the 
cuticle  away  from  the  other 
layers  of  the  wall,  ultimately 
bursting  it  and  discharging  to 
the  exterior.  The  cuticle  may  or  may  not  regen- 
erate, but  in  any  event  old  glands  lose  the  power 
of  excretion,  the  oil  ac- 
cumulating in  the  cell 
lumen.  Many  water 
plants  (as  Brasenia  and 
Nymphaea,  fig.  916;  also 
%•  805)  possess  slime 
glands,  which  secrete 
copiously.  In  the  gold- 
back  and  silverback  ferns 
(Gymno gramme)  there  is 
a  glandular  waxy  secre- 
tion copious  enough  to 
give  the  leaves  their  char- 
acteristic color. 

Internal  glands.  —  Many  plants  possess  in- 
ternal glands,  which  often  appear  as  trans- 
lucent dots,  as  in  the  leaves  of  Citrus  and 
Eucalyptus.  In  most  cases  the  glands  are 
spherical,  there  being  a  peripheral  layer  of 


FIG.  914.  —  Hairs 
from  a  vervain  leaf 
(Verbena  stricta);  con- 
trast the  pointed,  thick- 
walled,  unicellular 
"protective"  hair  (/>) 
with  the  capitate,  thin- 
walled,  multicellular 
glandular  hair  (&),  the 
latter  being  much  the 
richer  in  protoplasm 
both  in  the  stalk  and 
in  the  head  (h);  n, 
nucleus;  highly  mag- 
nified. 


FIG.  9 1 6. — Multicellular 
slime  glands  of  the  water 
shield  (Brasenia  Schreberi) ; 
note  the  stalk  cells  (/),  the 
slime-secreting  cells  (c~),  and 
the  superficial  slime  layer, 
whose  outer  limit  is  indi- 
cated by  the  dotted  line 
(s);  highly  magnified. 


glandular  cells  which  secrete  into  a  common  central  reservoir  (fig.  917). 
Often  this  structure  is  surrounded  by  a  relatively  impermeable  pro- 


624 


ECOLOGY 


tective  layer.  Usually  the  reservoir  does  not  discharge  to  the  ex- 
terior, but  in  Eucalyptus  and  in  various  Rutaceae  there  are  cover  cells, 
which  after  a  time  rupture  at  a  definite  spot  or  along  the  walls,  allowing 
the  secretions  to  pass  off.  In  some  cases,  as  in  Peperomia  and  in 
various  liverworts,  there  are  single  cells  that  secrete  mucilage  (figs.  745, 
928).  The  substances  secreted  by  internal  glands  resemble  those  se- 
creted by  glandular  hairs,  and  likewise  are  often  odoriferous. 

The  role  of  oils,  resins,  and  mucilage.  —  But  little  is  known  concern- 
ing the  role  of  oils,  resins,  and  mucilage,  though  speculation  along  this 
line  has  been  abundant.  Wax  coats  previously  have  been  seen  to  rep- 
resent waste  products  that  are 
of  incidental  value  in  protect- 
ing against  excessive  transpira- 
tion (p.  570).  A  similar  theory 
has  been  proposed  concerning 
excreted  volatile  oils,  on  the 
ground  that  they  absorb  heat 
in  large  amount,  but  it  is  most 
unlikely  that  these  oils  are 
present  in  sufficient  abundance 
to  check  transpiration.  Simi- 
larly improbable  is  the  common 
theory  that  sticky  glandular 
hairs,  which  are  especially  abun- 


FIG.  917.  —  An  internal  oil  gland  (a)  in  the 
orange  (Citrus  A urantium);  0,  oil  drops;  highly 
magnified.  —  From  TSCHIRCH. 


dant  on  floral  stems,  are  of  value  in  keeping  crawling  insects  away 
from  the  flowers,  or  the  theory  that  various  secretions  which  are- 
unpalatable  or  even  poisonous  (as  in  Primula)  may  lead  to  the  better 
preservation  of  plant  species  from  grazing  animals.  Somewhat  more 
probable  but  scarcely  authenticated  are  the  roles  commonly  ascribed  to 
slime  in  water  plants,  such  as  protection  from  snails  and  other  water 
herbivores  and  from  water  currents.  In  gelatinous  lichens  and  in  am- 
phibious algae,  mucilaginous  secretions  may  protect  from  desiccation. 
In  the  economy  of  aquatic  life  as  a  whole,  slime  plays  an  important 
part,  since  it  is  a  perfect  culture  medium  for  many  algae,  bacteria,  and 
small  animals.  Probably  there  is  no  adequate  reason  for  believing  that 
such  secretions  as  oils,  resins,  and  mucilage  are  of  any  particular  value  in 
the  economy  of  plants.  Doubtless,  for  the  most  part,  they  represent  waste 
products,  whose  removal  is  of  greater  value  than  their  retention.  Any 
incidental  gain  that  these  secretions  may  have  probably  is  small. 


LEAVES 


625 


The  influence  of  external  factors  upon  gland  development  and  secretion.  —  Almost 
nothing  is  known  concerning  the  influence  of  external  factors  upon  gland  develop- 
ment or  secretion.  In  some  cases  glandular  and  "  protective  "  hairs  arise  from 
similar  primordia  (as  in  Verbena,  fig.  914),  and  it  has  been  claimed  that  the  condi- 
tions to  which  such  primordia  are  subjected  determine  the  kind  of  hair  that  develops ; 
for  example,  in  Spiraea,  primordia  that  are  exposed  to  severe  conditions  (especially 
to  cold)  develop  into  long,  thick-walled  protective  hairs,  while  other  primordia 
develop  into  glands.  Similarly,  resins  and  oils  are  thought  to  be  secreted  more 
abundantly,  even  in  the  same  species  (as  in  Rumex  Acetosella  and  in  Primula  obco- 
nica),  in  xerophytic  than  in  other  situations.  Slime  glands  are  better  developed  in 
the  water  leaves  than  in  the  air  leaves  of  Myriophyllum  proserpinacoides. 

Crystals  and  cystoliths.  —  Crystals.  —  Calcium  oxalate  is  a  very  com- 
mon substance  in  plants,  occurring  in  the  form  of  needlje-shaped  crystals 
(raphides,  fig.  918),  or  crystal  aggregates  (fig.  919), 
or,  much  more  rarely,  in  the  form  of  isolated 
crystals  with  more  or  less  equal  axes.  Crystals 
commonly  are  confined  to 
parenchymatous  cells,  which 
often  are  arranged  in  longi- 
tudinal rows  close  to  the  con- 
ductive bundles.  Raphides 
usually  are  grouped  in  bundles, 
the  individual  crystals  being 
oriented  in  a  common  direc- 
tion; they  are  particularly 
abundant  in  monocotyls  with 
slimy  cell  sap,  while  crystal 
aggregates  are  perhaps  more 
common  in  the  stems  of  di- 
cotyls.  Crystals  sometimes 


FIG.  918.  —  A  cor- 
tical cell  from  the  stem 
of  the  wandering  Jew 
(Zebrina  pendula), 

showing  a  group  of  occur  in  veryminute  form,  con- 
stituting the  so-called  crystal 
sand. 

Calcium  oxalate  crystals  are 
undoubted  excreta,  represent- 


needle-like  crystals  (ra- 
phides) of  calcium  ox- 
alate; note  the  paral- 
lelism of  the  crystals 
to  one  another  and  to 

ZSLAAS     '«*  by-products   of    metabo- 
magnified.  lism.     Oxalic  acid  in  the  free 

state,  existing  as  a  solute  in 
the  cell  sap,  is  believed  to  be  poisonous,  especially  if  present  in  large 
amount,  though  in  the  various  sorrels  it  is  abundant  enough  to  give 


FIG.  9 19. — A  longi- 
tudinal section  of  the 
cortex  of  a  petiole  of 
the  hop  tree  (Ptelea  tri- 
foliata),  showing  longi- 
tudinal rows  of  short 
cells,  each  with  a  com- 
pound crystal  of  cal 
cium  oxalate;  highly 
magnified. 


626 


ECOLOGY 


them  their  characteristic  taste.  Even  if  not  poisonous,  free  oxalic  acid 
certainly  is  deleterious,  since  its  formation  interferes  with  further  cell 
activity,  as  does  sugar  or  any  other  product  of  metabolism,  unless  trans- 
formed into  an  insoluble  substance  or  removed  to  other  cells.  Thus  the 
chief  advantage  of  crystals  is  in  removing  oxalic  acid  from  solution. 
Sometimes  it  is  held  that  crystals  are  beneficial  in  removing  calcium 
from  solution,  especially  in  calcareous  soils,  though  this  view  has  not  met 
with  general  acceptance.  It  has  been  held  also  that  raphides  by  their 
sharp  points  protect  plants  from  injury  by  snails,  but  the  evidence  for 
this  theory  is  inadequate.  Still  more  untenable  are  the  theories  that 
crystals  give  mechanical  support  to  plants  and  that 
they  represent  accumulations  of  calcium  that  later 
may  be  utilized.  In  most  cases  it  is  not  necessary 
or  even  desirable  to  seek  a  subsidiary  function  for 
the  excreted  products  of  plants ;  if  in  certain  instances 
they  have  such  a  function,  it  must  be  regarded  as 
wholly  incidental. 


FIG.  920.  —  A 

spindle-shaped  cys- 
tolith  with  warty 
protuberances  from 
the  leaf  of  Pelli- 
onia ;  highly  mag- 
nified. 


Cystoliths.  —  In  various  Urticalcs  there  occur  aggregates  of 
calcium  carbonate,  known  as  cystoliths,  whose  rounded  rather 
than  angular  projections  readily  distinguish  them  from  crystals. 
In  Pilea  and  Pellionia  they  are  spindle-shaped  and  lie  free 
in  the  cell  (fig.  920),  while  in  Ficus  they  are  stalked  and 
mulberry-shaped.  When  treated  with  acid,  the  rounded  knobs 
effervesce,  leaving  an  insoluble,  stratified  cellulose  skeleton. 
While  the  role  of  cystoliths  is  unknown,  it  is  probable  that  as 
waste  products  they  may  serve  to  remove  from  solution  an  excess  of  calcium  salts, 
though  some  regard  them  as  calcium  accumulations  that  subsequently  may  be  used. 
Some  plants  contain  warty  siliceous  bodies  somewhat  resembling  cystoliths. 

Various  products  of  secretion  and  excretion.  —  Various  products,  mainly  excre- 
tions, accumulate  more  in  stems  than  in  leaves,  and  will  be  considered  elsewhere 
p.  718).  The  sharp  taste  of  mustards,  capers,  and  nasturtiums  is  due  to  an  oil, 
formed  by  the  action  of  a  ferment,  myrosin,  upon  calcium  myronate.  Many  plants 
contain  alkaloids,  most  of  which  are  violent  poisons,  as  strychnin,  atropin,  and  cocain. 
Bitter  principles  are  illustrated  by  the  absinthin  of  wormwood  and  the  aloin  of  aloes, 
and  poisonous  glucosids  are  represented  by  digitalin  and  solanin.  Such  substances 
as  a  class  probably  are  by-products  of  metabolism,  and  if,  as  suggested,  they  pro- 
tect against  animals,  either  because  poisonous  or  unpalatable,  such  protection  is 
purely  incidental.  Even  this  incidental  use  is  likely  to  be  overestimated,  since 
plants  that  are  poisonous  or  disagreeable  to  man  are  not  necessarily  so  to  all  herbiv- 
orous animals.  Furthermore,  the  organs  most  likely  to  be  eaten,  the  leaves, 
usually  contain  poisons  and  unpalatable  substances  in  less  amount  than  do  the 
other  organs. 


LEAVES  627 

9.     LEAVES  AS  ORGANS  OF  ACCUMULATION  OF  WATER 

AND  FOOD 

Food  accumulation  in  leaves.  —  Generally  it  would  be  disadvanta- 
geous for  leaves  to  serve  as  organs  of  food  accumulation  or  "  storage," 
since  this  would  impair  their  synthetic  efficiency,  partly  because  of  the 
space  that  such  foods  would  occupy,  and  partly  (in  the  case  of  soluble 
foods,  like  the  sugars)  because  increasing  concentration  retards  further 
manufacture.  Leaves  serve  as  organs  of  temporary  accumulation,  be- 
cause in  the  daytime,  foods  usually  are  manufactured  faster  than  they  are 
removed  to  other  organs;  during  the  night,  however,  removal  continues, 
so  that  the  leaf  is  relatively  free  from  accumulated  foods  by  morning.  In 
various  desert  xerophytes  in  which  the  leaves  remain  for  years,  large 
quantities  of  food  and  water  may  accumulate,  as  in  Agave,  whose 
developing  flower-stalk  so  drains  the  leaves  of  their  contents  that  they 
fall  back  limp  and  wrinkled  (figs.  921,  922). 

General  features  of  water-accumulating  leaves.  —  Water  retention.  — 
In  leaves  the  accumulation  and  retention  of  water  in  conspicuous 
amount  is  much  more  common  than  that  of  foods,  and  is  especially 
characteristic  of  succulent  xerophytes,  such  as  the  Crassulaceae  and 
Chenopodiaceae.  The  water  retentiveness  of  such  plants  is  well  shown 
when  attempts  are  made  to  dry  them;  Crassulaceae  and  Cactaceae 
frequently  grow  for  days  or  even  for  weeks  while  being  dried  under 
pressure  for  herbarium  specimens.  This  retentiveness  is  due  in  some 
instances  to  high  cutinization  (Cactaceae),  or  to  waxy  coats  (Crassula- 
ceae) ,  or  to  both  combined  (Agave) ;  most  succulent  plants,  however, 
exhibit  weak  cutinization.  Another  character  favoring  retentiveness  in 
succulent  leaves  is  a  small  evaporating  surface  in  proportion  to  the 
volume,  but  many  orchid  leaves  which  evaporate  slowly  are  thin  and 
slightly  cutinized  (as  in  species  of  Habenaria) .  Again,  in  many  succulents 
which  have  a  cell  sap  of  high  osmotic  pressure,  evaporation  doubtless  is 
relatively  slow.  The  best  attested  cases  of  such  concentrated  cell  sap  are 
in  the  succulent  plants  of  salt  marshes,  which  may  have  an  osmotic 
pressure  equal  to  twenty  atmospheres,  and  in  various  desert  plants, 
largely  shrubs,  some  of  which  may  have  a  pressure  as  high  as  one  hun- 
dred atmospheres,  as  compared  with  a  pressure  of  five  to  ten  atmospheres 
in  plants  of  ordinary  habitats.  The  osmotic  pressure  in  cacti,  on  the 
other  hand,  has  been  shown  to  approximate  that  in  ordinary  plants. 
Probably  upon  this  basis  succulent  plants  may  be  divided  into  two  classes : 


628 


ECOLOGY 


FIG.  921.  FIG.  922. 

FIG.  921.  — The  century  plant  (A gave  americana)  at  the  time  of  the  rapid  develop- 
ment of  its  gigantic  inflorescence;  note  the  growing  flower-stalk  with  its  large  bracts  and 
the  rosette  of  stiff  foliage  leaves ;  Washington  Park,  Chicago,  111.  —  Photograph  supplied 
by  FULLER. 

FIG.  922.  —  The  century  plant  (Agave  americana),  showing  a  fully  developed  inflores- 
cence; note  that  the  leaves,  deprived  of  their  accumulated  water  and  food  by  the  growing 
inflorescence,  have  fallen  back  limp;  the  plant  dies  after  the  ripening  of  the  fruit;  Wash- 
ington Park,  Chicago,  111.  (the  same  plant  as  in  Fig.  921,  two  months  later).  —  Photo- 
graph supplied  by  FULLER. 


LEAVES 


629 


(i)  salt  marsh  and  other  succulents  (which  include  the  most  representa- 
tive forms)  having  sap  of  high  osmotic  pressure,  and  (2)  succulents 
whose  water  retentiveness  is  due  to  structural  or  other  characters, 
notably  cutinization,  as  in  the  cacti.  It  is  probable,  however,  that  no 
explanation  thus  far  given  accounts  for  all  cases 
of  extreme  water  retention  in  the  presence  of 
conditions  favoring  transpiration. 

Structural  features.  — 
While  most  leaves  are 
dorsiventral  (i.e.  with 
the  upper  and  under 
portions  different  in 
structure),  many  succu- 
lent leaves  are  cylindrical 
and  almost  radially  sym- 
metrical, like  stems  and 
roots,  instead  of  having 
only  one  plane  of  sym- 
metry, as  do  most  leaves 
(fig.  923).  *  However, 
while  the  epidermis, 
chlorenchyma,  and 
colorless  parenchyma,  as  seen  in  cross  sec- 
tion, are  essentially  uniform  in  aspect  in 
the  entire  leaf  cylinder,  the  conductive  tract 
is  dorsiventral,  as  in  ordinary  leaves,  the 
xylem  being  above  and  the  phloem  beneath 
(fig.  926).  In  succulent  leaves  the  veins 
commonly  are  buried  so  deeply  as  to  be  in- 
conspicuous from  without. 

Water  tissue.  —  All  the  living  plant  tissues 
are  composed  chiefly  of  water,  but  the  term 
water  tissue  is  employed  especially  in  succulent 

plants  to  designate  regions  of  turgescent  parenchyma  cells  with  delicate 
cellulose  walls,  thin  peripheral  layers  of  cytoplasm,  and  few  or  no  chloro- 
plasts.  In  many  succulents  the  water  tissue  is  not  sharply  delimited 
from  ordinary  chlorenchyma,  and  may  be  made  up  entirely  of  turgescent 


Fro.    923.  — A 

branch  of  Senecio  sp., 
a  desert  xerophyte,  il- 
lustrating extreme  leaf 
succulence,  the  very 
fleshy  leaves  present- 
ing a  small  surface  in 
proportion  to  their 
volume. 


I 


FIGS.  924,  925. — Cross  sec- 
tions through  the  leaf  of  Coty- 
ledon, a  succulent  xerophyte: 
924,  a  diagrammatic  section, 
showing  that  the  leaf  is  rela- 
tively thick  in  proportion  to  its 
width ;  92  5,  a  cross  section,  con- 
siderably magnified,  showing 
relatively  uniform  chloren- 
chyma cells,  except  that  the 
outermost  cells  (0)  are  rounded 
and  the  innermost  cells  (i)  an- 
gular; note  that  stomata  (s) 
occur  on  both  surfaces  and  that 
chlorophyll  and  air  spaces  are 
more  abundant  in  the  outer- 
most than  in  the  innermost 
cells. 


1  Most  dorsiventral  leaves  present  two  distinct  surfaces  and  hence  may  be  called  bi- 
facial, contrasting  with  cylindrical  equilateral  leaves. 


630 


ECOLOGY 


green  cells  (as  in  various  thin-leaved  succulents),  quite  as  in  most  leaves 
except  for  the  evident  fleshiness  of  the  organ;  or  the  leaf  may  be  thick 
with  the  chlorophyll  gradually  decreasing  toward  the  center,  the  cells 
otherwise  being  essentially  similar  in  aspect,  as  in  most  Crassulaceae 
(figs.  924,  925);  or  the  leaf  may  be  thick  with  chlorophyll  decreasing 
toward  the  center,  but  with  the  outermost  chlorenchyma  cells  elongated, 

representing  true 
palisade  cells,  while 
the  cells  toward  the 
center  become  more 
and  more  isodia- 
metric  and  also 
poorer  in  chloro- 
phyll (as  in  the 
century  plant  and 
in  various  cacti). 
Another  kind  of 
water  tissue  char- 
acterizes more  ex- 
treme succulents  (as 
Salsola  and  other 
forms  with  cylin- 
drical leaves)  and 
may  be  regarded  as 
more  representa- 
tive ;  in  these  plants 
the  water  tissue, 
which  is  composed 
of  large  isodiamet- 
ric  cells,  is  centrally 


FIG.  926.  — A  cross  section  through  a  succulent  xerophytic 
leaf,  that  of  the  Russian  thistle  (Salsola  Kali  tenuifolia),  illus- 
trating peripheral  palisade  chlorenchyma  (/>)  and  central 
water  tissue  (w);  note  the  relatively  thin  cuticle  (c\  and 
the  sharp  delimitation  between  the  chlorenchyma  and  the 
water  tissue,  the  latter  being  characterized  by  the  large  size 
of  the  cells  and  by  the  absence  of  conspicuous  air  spaces; 
such  a  leaf  is  equilateral  and  approximately  radially  sym- 
metrical, thus  having  a  small  surface  exposure  in  proportion 
to  the  leaf  volume;  dorsiventrality  is  exhibited  alone  by  the 
vascular  bundle,  the  hadrome  or  xylem  (h)  lying  above  the 
leptome  or  phloem  (/) ;  highly  magnified. 


placed  and  is  more 
or  less  sharply  delimited  from  the  peripheral  chlorenchyma  cylinder 
whose  cells  usually  are  relatively  small  and  of  palisade  shape  (figs. 
926,  927). 

A  third  kind  of  water  tissue  differs  from  all  the  rest  in  its  peripheral 
position,  the  cells  belonging  to  the  epidermis  rather  than  to  the  meso- 
phyll.  All  gradations  occur  between  an  ordinary  epidermis  with  a  single 
layer  of  colorless  water-containing  cells  and  a  many-layered  epidermis 
of  similar  but  more  turgescent  cells,  as  in  Begonia  and  Peperomia,  where 


LEAVES 


631 


the  centrally  placed  chlorenchyma  occupies  less  space  than  does  the  epi- 
dermal water  mantle  (figs.  928,  766,  767).  Perhaps  the  commonest  sort 
of  water  mantle  is  represented  in  Tradescantia,  where  there  is  a  single 
or  double  layer  of  large  turgescent  colorless  cells.  The  water  tissue  com- 
monly  is  best  developed  on  the  upper  side  of  the  leaf,  sometimes  being 
confined  to  that  side.  In  most  cases  the  cell  sap  is  highly  add. 


In  the  "  ice  plant "  (M ' esembryanthemum  crystallinum)  a  few  of  the  epidermal  cells 
are'  much  distended  and  project  considerably  beyond  the  epidermal  level,  causing 
the  leaves  to  glisten  in  the  sun- 
light. Not  infrequently  plants 
possess  isolated  colorless  tur- 
gescent  cells  in  the  midst  of  a  tis- 
sue made  up  of  cells  of  wholly 
different  character;  when  such 
cells  have  walls  with  tracheid- 
like  thickenings,  they  have 
been  called  storage  tracheids 
(fig.  772).  In  many  succu- 
lent monocotyls  (as  Aloe)  the 
cell  sap  is  very  mucilaginous, 

and  in  some  plants  of  similar         \  / 

character  mucilaginous   mate-  \  ' 

rial  is  deposited    in    the  form 
of  wall  thickenings. 


FIG.  927.  — A  sector  from  a  cross  section  of  a 
succulent  equilateral  xerophytic  leaf  (Senecio  sp.), 
illustrating  peripheral  palisade  chlorenchyma  and 
central  water  tissue,  but  showing  a  gradual  transition 
from  the  former  to  the  latter,  both  in  cell  size  and  in 
chlorophyll  abundance;  lettering  as  in  Fig.  926;  con- 
siderably magnified. 


The  causes  of  water 
accumulation  in  succu- 
lent plants.  —  Experi- 
mental data.  —  Some  suc- 
culent plants  (e.g.  S&n- 
pervivum  assimile,  figs. 
1043-1045)  when  placed  for  a  few  weeks  in  a  moist  chamber  develop 
slender  elongated  shoots  with  thin  expanded  leaves,  having  little  or 
no  suggestion  of  succulence,  while  subsequent  removal  to  dry  air 
results  once  more  in  the  development  of  short  and  stout  shoots  that 
bear  thick  succulent  leaves  of  small  size.  Such  reactions  are  quite 
like  those  previously  noted  as  characterizing  habitats  that  differ  in 
atmospheric  humidity  and  hence  in  transpiration  (see  p.  598).  But 
while  the  xerophytic  leaf  is  in  all  cases  small  and  thick  and  the  meso- 
phytic  leaf  large  and  thin,  the  thickness  in  the  succulent  xerophyte  is  due 
to  an  increased  dorsiventral  development  of  watery  tissue,  while  that  in 


632 


ECOLOGY 


other  xerophytes  (as  Ledum  or  Campanula,  figs.  867,  868)  is  due  to  in- 
creased cutinization  and  to  palisade  development.  Apparently  these 
different  xerophytic  reactions  are  due  to  a  common  cause,  namely,  expo- 
sure to  increased  transpiration. 

Not  only  does  the  reference  of  succulence  to  transpiration  bring  it  into 
line  with  other  xerophytic  characters,  such  as  cutinization  and  palisade 

development,  but  like  these,  succulence 
develops  in  analogous  situations,  as  in 
maritime  habitats.  Years  ago  it  was 
shown  that  Salicornia,  one  of  the  most 
succulent  of  halophytes,  loses  much  of 
its  succulence  when  grown  where  the 
soil  is  poor  in  sodium  chlorid,  while  its 
succulence  increases  with  the  addition 
of  this  salt  to  the  soil.  The  same  is 
true  of  Glaux  maritima,  Rhizophora, 
and  various  other  salt  plants.  The 
dorsiventral  leaves  of  the  beach  pea  and 
the  wallflower  tend  toward  equilaterality 
in  salty  soils,  precisely  as  they  do  when 
exposed  to  strong  transpiration.  Simi- 
larly, plants  of  alkali  deserts  (as  Hali- 
modendron)  show  a  reduced  development 
of  water  tissue  when  grown  in  ordinary 
garden  soil.  In  so  far  as  succulence 
involves  increased  cell  turgidity  and 
sphericity,  the  influence  of  transpiration 
or  of  salt  solutions  of  high  osmotic  pres- 
sure is  essentially  the  same  as  in  Stigeo- 
donium,  where  these  characteristics  ac- 
company high  concentration  of  the  cell 
sap,  however  produced  (p.  591).  Usually  a  desert  plant  is  exposed  to  high 
transpiration,  and  often  absorption  is  slight;  such  conditions  result  in  cell- 
sap  concentration,  which  favors  succulence.  Similarly,  a  Salicornia  plant 
in  a  salt  marsh  is  exposed  to  transpiration,  while  at  the  same  time  the  soil 
salts  make  absorption  difficult,  also  resulting  in  cell-sap  concentration. 

The  problem  of  water  accumulation  seems  to  present  greater  difficulties  than 
does  that  of  cell  shape  or  turgidity,  since  it  is  difficult  to  see  how  increased  transpi- 
ration can  stimulate  water  accumulation.  However,  the  problem  may  have  to  do 


FIG.  928.  —  A  cross  section 
through  a  Peperomia  leaf,  illustrat- 
ing peripheral  or  epidermal  water 
tissue  (w),  the  epidermis  being  three 
or  four  layers  thick  and  containing 
mucilage  glands  (g) ;  note  that  the 
uppermost  chlorenchyma  layers  (c) 
consist  of  small  closely  packed  cells 
with  abundant  chlorophyll,  while 
the  lowermost  chlorenchyma  layers 
consist  of  larger  and  more  loosely 
placed  cells  with  less  chlorophyll; 
highly  magnified. 


LEAVES 


633 


not  so  much  with  increased  water  accumulation  as  with  the  distribution  of  the  water- 
containing  cells.  Probably  a  leaf  appears  succulent,  less  because  of  the  large 
amount  of  water  it  contains  than  because  its  thickening  at  the  expense  of  ex- 
pansion concentrates  the  water  within  a  compact  region;  and  leaf  compactness  in 
contrast  to  expansion  already  has  been  seen  to  be  connected  largely  with  high 
relative  transpiration  (p.  598),  and  thus  with  cell-sap  concentration. 

Edema.  —  An  explanation  of  leaf  succulence  may  be  suggested  by  a  consideration 
of  edema,  a  phenomenon  occasionally  witnessed  when  turgor  pressure  is  high  and 
transpiration  low,  and  evidenced  externally  by  the  appearance  over  the  leaf  sur- 
face of  whitish  emergences,  known  as  intumescences  (figs.  929,  930).  Intumescences 
develop  on  the  leaves  of  Hibiscus  vitifolius,  Solanum  tuberosum,  and  various  other 


929 


FIGS.  929,  930.  —  Intumescences  produced  on  cauliflower  leaves  (Brassica  oleracea)  by 
chemical  stimulation,  the  leaves  having  been  sprayed  with  copper  ammonium  carbonate; 

929,  a  small  portion  of  the  lower  leaf  surface,  five  days  after  spraying ;  i,  intumescences : 

930,  a  cross  section  through  an  intumescence,  highly  magnified ;    note  the  greatly  hyper- 
trophied  mesophyli  cells  (/*),  which  have  broken  through  the  lower  epidermis  (e).  —  From 
VON  SCHRENK  (929  drawn  from  a  photographic  reproduction). 


plants  in  moist  chambers,  and  also  on  isolated  leaves  of  Populus  and  Eucalyptus 
and  on  the  inner  surfaces  of  pea  pods  that  are  placed  in  water;  in  the  tomato  they 
have  been  induced  by  forcing  water  into  cut  stems  and  by  heating  the  soil  in  which 
they  grow.  In  all  these  cases  a  surplus  of  water  in  the  plant  causes  the  hypertrophy 
of  the  leaf  tissues,  which,  for  lack  of  space  within,  are  forced  to  break  through  the 
epidermis  as  intumescences.  Probably  "  water  lenticels"  (p.  663),  the  "  substitute 
hydathodes  "  of  Conocephalus  (p.  622),  and  the  "  breathing  roots  "  of  Jussiaea 
(p.  508)  are  essentially  identical  with  intumescences,  not  only  having  a  similar  aspect, 
but  also  developing  under  similar  conditions.  In  some  cases  intumescences  are 
developed  also  by  chemical  stimulation,  as  in  the  leaves  of  the  cauliflower  (figs. 

929»  93°). 

Succulence  and  intumescence.  —  While  succulent  and  intumescent  leaves  seem  to 
have  certain  superficial  resemblances,  each  being  very  juicy  and  having  cells  filled 
almost  to  bursting  with  cell  sap,  they  differ  in  most  essential  respects.  Intumescence 


634  ECOLOGY 

clearly  is  associated  with  a  relative  surplus  of  water,  while  succulence  commonly 
is  associated  with  a  relative  surplus  of  salts.  The  best  conditions  for  active  growth 
border  closely  on  those  which  induce  intumescence,  water  being  present  in  sufficient 
amount  to  make  the  cell  sap  dilute  and  thus  permit  unrestricted  cell  activity.  In- 
deed, the  phenomena  of  edema  mean  that  growth  within  the  leaf  is  so  rapid  that 
expansion  fails  to  keep  pace  with  it.  Succulence,  on  the  other  hand,  results  from 
sluggish  growth,  the  water,  because  of  its  large  percentage  of  solutes,  being  utilized 
less  freely.  When  exposed  to  transpiration,  the  intumescent  plant  withers  quickly, 
while  the  succulent  plant  long  retains  its  moisture,  owing  to  the  large  amount  of 
solutes.  However,  not  all  cases  of  leaf  succulence  are  to  be  explained  thus.  In 
some  cases  the  cell  sap  is  known  to  be  dilute,  and  in  many  instances  its  condition  is 
yet  to  be  investigated.  Still  less  is  known  as  to  the  factors  involved  in  the  origin  of 
concentrated  cell  sap,  except,  perhaps,  in  halophytes.  It  has  been  suggested  that 
the  respiration  of  succulent  plants  is  imperfect,  thus  leading  to  the  accumulation  of 
osmotically  active  substances  which  facilitate  further  succulence ;  it  is  difficult, 
however,  to  understand  the  inception  of  such  a  process.  Still  more  inexplicable  is 
succulence  in  slich  plants  as  Begonia  and  Peperomia,  which  have  fleshy  leaves  in 
the  humid  atmosphere  of  the  rain  forest. 

The  advantages  of  succulence.  —  General  advantages.  —  Even  though 
a  succulent  plant  is  unwatered  for  a  long  time,  the  growing  parts  show 
no  cessation  of  activity,  new  shoots  continuing  to  develop  by  utilizing 
the  water  in  the  older  organs.  The  old  leaves  shrink  and  become 
wrinkled,  the  cells  collapsing  to  a  greater  or  less  degree.  The  func- 
tional significance  of  leaf  succulence  is  to  be  found  chiefly  in  the  pro- 
tection of  the  synthetic  tissue.  Leaves  are  in  general  ill-fitted  for  water 
retention,  since  they  are  much  more  subject  to  excessive  transpiration 
than  are  stems  and  roots.  Furthermore,  leaf  succulence  must  impair 
/  synthetic  activity,  partly  because  of  the  weakening  of  incident  light  in 
traversing  the  leaf,  and  partly  because  the  accumulation  of  solutes  im- 
•  I  pairs  sugar  formation  and  other  cell  activities.  However,  in  extreme 
xerophytes  it  is  not  a  question  between  ample  and  slight  synthetic 
activity,  but  between  slight  activity  and  none  at  all ;  the  very  presence  of 
abundant  water  and  solutes,  though  tending  generally  to  reduce  syn- 
thesis, may  here  be  the  means  of  permitting  synthesis  through  the 
protection  thus  afforded  to  the  chlorenchyma.  Such  protection  may 
vary  with  the  relative  arrangement  of  the  water  tissue  and  the  chloren- 
chyma. Whether  its  position  is  peripheral  or  central,  water  passes  from 
the  water  tissue  to  the  chlorenchyma,  whenever  the  supply  in  the  latter 
becomes  scanty,  thus  permitting  the  continuance  of  synthetic  activity. 
I  Peripheral  water  tissues  also  may  tone  down  the  intensity  of  the  inci- 
dent light,  thus  permitting  synthesis  to  continue  even  in  bright  sunlight. 


LEAVES  635 

It  has  been  shown  experimentally  that  a  leaf  with  a  surface  layer  of 
water  becomes  less  heated  on  exposure  to  light  than  does  the  same  leaf 
without  such  a  layer. 

Peripheral  water  tissues.  —  Peripheral  water  tissues  commonly  are  associated  with 
expanded  dorsiventral  leaves  and  are  especially  characteristic  of  the  tropical  rain 
forest,  where  they  are  found  particularly  in  epiphytes  (such  as  the  orchids  and  bro- 
melias),  whose  water  supply  is  more  limited  than  that  of  the  trees  on  which  they 
grow,  because  they  can  absorb  water  only  when  it  is  in  contact  in  liquid  form  with  ' 
their  aerial  organs;  similar  tissues  occur  also  in  the  leaves  of  various  trees,  as  in 
Ficus,  and  even  in  those  of  such  mesophytes  as  the  banana.  Except  in  the  epiphytes, 
the  advantage  of  water  tissue  in  the  rain  forest  is  not  obvious,  though  it  has  been 
suggested  that  the  water  mantle  is  of  value,  even  there,  as  a  means  of  reducing  the 
intensity  of  the  light  and  of  lessening  the  occasionally  high  transpiration.  Even  these 
somewhat  doubtful  advantages  must  be  lacking  in  the  case  of  Begonia  and  Pepe- 
romia,  plants  with  pronounced  leaf  succulence  that  often  live  in  the  dense  shade  of 
the  forest  bottom.  Such  plants  may  have  immigrated  .to  the  forest  from  a  former 
xerophytic  habitat,  retaining  their  xerophytic  structures ;  or  their  xerophytism  may  be 
inherent  and  quite  unrelated  to  external  conditions.  That  in  Begonia  some  species, 
at  least,  are  essentially,  rather  than  apparently,  xerophytic  is  shown  by  their 
frequent  cultivation  in  houses  in  relatively  xerophytic  conditions,  while  most  of  their 
associates  of  the  rain  forest  (such  as  the  filmy  ferns)  require  the  most  humid  of 
conditions  for  successful  cultivation.  It  is  difficult  to  imagine  any  advantage  in  suc- 
culent leaves  in  the  depths  of  the  rain  forest. 

The  significance  of  succulence  in  extreme  habitats.  —  In  the  most  ex- 
treme desert  xerophytes  and  in  the  plants  of  salt  marshes  and 
alkali  regions,  equilateral  leaves  with  centrally  placed  water  tissue  are 
very  characteristic.  There  is  little  doubt  that  succulence  represents  j 
the  culmination  of  xerophytic  characters  and  that  the  equilateral  leaf  1 
with  central  water  tissue  represents  the  most  xerophytic  of  leaf  forms.  ) 
Salt  deserts  are  the  most  unfavorable  of  habitats,  since  with  the  most 
excessive  transpiration  there  are  combined  the  poorest  conditions  for 
absorption,  by  reason  of  the  high  concentration  of  the  soil  solutions. 
Among  the  higher  plants  the  forms  which  penetrate  the  farthest  into 
these  barren  regions  are  the  succulents.  In  conditions  where  existence 
is  impossible  for  the  sagebrush  with  its  hairy  leaves,  or  for  the  creosote 
Dush  with  its  varnished  leaves,  or  even  for  the  leafless  cacti  with  their  great 
quantities  of  accumulated  water  protected  by  cutin,  there  flourish  various 
Chenopodiaceae  (such  as  Salsola,  Sarcobatus,  or  Suaeda)  with  their 
fleshy  equilateral  leaves.  Three  features  appear  to  be  responsible  for 
the  remarkable  endurance  of  such  leaves :  the  small  evaporating  surface, 
which  is  the  least  of  any  leaves ;  the  large  amount  of  water,  which  serves 


636 


ECOLOGY 


to  tide  over  long  unfavorable  periods;  and,  finally,  the  high  concentra- 
tion of  the  cell  sap,  which  makes  water  retention  possible  in  spite  of 
prolonged  exposure  to  conditions  that  cause  desiccation  in  plants  with 
dilute  cell  sap. 


10.     MISCELLANEOUS    LEAF    STRUCTURES    AND 
RELATIONS 

Leaves  as  organs  cf  run-off.  —  As  previously  noted,  hairy  coats  and  surface  ex- 
cretions often  are  of  value,  especially  in  water  plants,  in  preventing  the  wetting 
of  leaf  surfaces,  thus  facilitating  an  undisturbed   ex- 
change of  gases  through   the  stomata.     In  the  rainy 
tropics,  where  storms  occur  daily  and  where  the  air  is 


d 


FIG.  931.  — A  leaf 
of  Ficus  religiosa,  a 
tree  of  the  tropical  rain 
forest,  showing  a  so- 
called  dripping  point 
(</).  —  After  STAHL. 


FIG.  932. — A  portion  of  a  leaf  of  a  tropical  Asplenium, 
illustrating  reproduction  by  leaves;  on  the  under  sides  of 
the  ultimate  leaf  divisions  (pinnules)  are  fruit  dots  or  sori 
(s),  in  which  are  sporangia  with  their  spores;  on  the  upper 
sides  of  the  pinnules  are  bulbils  (b),  which  here  have  germi- 
nated while  connected  with  the  parent  plant,  giving  rise  to 
bulblings  whose  leaves  (/)  already  are  conspicuous. 


almost  constantly  humid,  many  leaves  have  long  attenuated  tips,  sometimes  known 
as  dripping  points  or  gutter  points,  which  are  supposed  to  facilitate  the  run-off 
of  precipitated  water  that  might  otherwise  impede  stomatal  activity  (fig.  931). 
Grooved  and  channeled  petioles  may  act  similarly.  It  has  been  suggested  that 
there  is  a  sort  of  correlation  between  run-off  and  root  direction,  horizontal  roots 
characterizing  plants  with  leaf  drip,  and  vertical  roots  those  with  petiole  drip  ;  while 
this  is  unlikely,  the  direction  of  root  growth  might  be  thus  determined  if  run-off 
were  the  sole  source  of  water,  since  roots  are  prohydrotropic. 

Leaves  as  reproductive  organs.  —  Ferns.  —  In  the  ferns  the  foliage  leaves  com- 
monly bear  sporangia,  which  are  grouped  in  brownish  fruit  dots  (sori)  on  the 
back  of  the  leaf  (as  in  Aspidium,  figs.  1128,  1129),  or  are  borne  under  the 
recurved  leaf  margins  (as  in  Pteris).  In  some  ferns  (as  Osmunda)  the  sporangia  are 
borne  on  special  reproductive  branches,  or  even  on  special  leaves  which  contrast 
strongly  with  the  foliage  leaves.  Fern  leaves  may  take  part  also  in  vegetative 
reproduction.  Cystopteris  bulbifera  and  species  of  Asplenium  develop  leaf  bulbils, 


LEAVES 


637 


which  in  Cystopteris  drop  off  and  germinate  in  the  ground.  In  Asplenium  they 
may  germinate  and  grow  to  considerable  size  while  attached  to  the  leaf  (fig.  932),  later 
falling  to  the  ground  where  development  is  continued.  When  the  leaf  tip  of  the 
walking  fern  (Camptosorus'}  comes  in  contact  with  the  ground,  a  bud  forms  from 
the  terminal  cells,  later  developing  into  a  plant.  In  Adiantum  Edgeworthii  the 
apical  cell  of  a  leaf  may  develop  directly  into  a  stem.  In  Camptosorus  some  ex- 
ternal factor  (perhaps  moisture)  stimulates  vegetative  reproduction,  but  the  stimu- 


FIG.  933.— The 
upper  portion  of  a 
plant  of  Bryophyl- 
lum    calycimim, 
showing  new  plants 
(p}  developing  from 
buds  that  originate  in  the 
leaf  sinuses;  note  also  the  adventi- 
tious roots  (r)  which  develop  at  the 
stem  nodes,  appearing  from  all  sides  where 
the  stem  is  vertical  or  ascending,  but  from  the 
under  side  only  where  the  stem  is  horizontal. 


FIG.  934.  —  Vegetative  re- 
production in  Samevieria  arti- 
ficially induced  through  the  use 
of  a  leaf  cutting;  a  bud  and  a 
copious  growth  of  roots  origi- 
nate at  the  basal  part  of  the  cut- 
ting when  placed  in  moist  soil. 


lating  factor  in  Asplenium  and  Cystopteris  is  unknown.     The  leaves  as  well  as 
various  other  organs  of  mosses  exhibit  vegetative  reproduction  (see  p.  807). 

Seed  plants.  —  In  the  seed  plants  natural  leaf  reproduction  is  a  rare  phenomenon. 
In  Tolmiea  and  in  Cardamine  pratensis  new  plants  may  appear  on  the  leaf  blade 
near  the  petiole,  and  in  Bryophyllum  at  the  sinuses  along  the  leaf  margin  (fig.  933). 
Moisture  appears  to  facilitate  development  in  Bryophyllum,  but  the  more  vigorous 
development  of  new  plants  on  a  severed  leaf,  even  in  dry  air,  seems  to  suggest  a 
release  from  some  inhibitory  factor  residing  in  the  plant  (see  p.  749).  In  many 
plants  (as  Peperomia  and  Begonia)  a  bud  soon  forms  on  a  severed  leaf  placed  in  the 
soil,  later  growing  into  a  plant ;  in  Sansevieria,  leaves  may  be  cut  into  a  number  of 
pieces,  each  of  which  will  produce  a  bud  if  placed  in  the  soil  (fig.  934).  A  number 
of  species  are  propagated  in  this  manner  by  florists,  and  it  has  been  shown  that 


638 


ECOLOGY 


as  a  class  dicotyls  have  a  much  greater  capacity  for  propagation  by  leaves  than  have 
monocotyls.  In  a  great  many  species  whose  leaves  appear  unable  to  give  rise  to 
new  plants,  roots  originate  somewhat  readily  from  the  leaves.  These  plants  rarely 
exhibit  leaf  propagation  in  nature,  chiefly  because  living  leaves  rarely  fall  to  the 
ground.  Hence  in  seed  plants  the  capacity  for  leaf  propagation  is  not  especially 
advantageous. 

Conductive  tissues  in  leaves.  —  General  features.  —  Veins  are  the  mechanical 
framework  of  leaves  and  also  the  paths  of  conduction,  and  differences  in  venation 
form  most  conspicuous  leaf  features.  Most  dicotyl  leaves  have  a  prominent  mid- 
rib, whose  branches  fork  and  anastomose  repeatedly,  thus  forming  a  reticulated  net- 


FIG.  935.  — The  skeletonized  edge  of  a  leaf  of  a  Ficus,  showing  the  anastomosing  of 
the  finer  veins.  —  From  LAND. 


work  of  small  veins  (fig.  935).  The  midrib  and  the  larger  veins  are  the  main  trunk 
lines  along  which  water  and  foods  pass,  and  the  smaller  veins  connect  these 
with  the  chlorenchyma.  In  some  dicotyls  (as  in  the  maples,  fig.  779)  there 
are  several  primary  veins,  while  most  monocotyls  have  several  to  many  equal  and 
more  or  less  parallel  primary  veins,  connected  by  rather  obscure  transverse  veins. 
The  anastomosing  of  veins  is  highly  advantageous,  since  materials  may  pass  to 
or  from  any  point  by  more  than  one  route ;  in  case  of  injury  to  a  large  vein,  this 
insures  continued  activity  in  all  parts  of  the  leaf.  In  most  conifers  and  in  many 
narrow-leaved  angiosperms  there  is  a  midrib  with  few  or  no  branches.  Veins 
often  are  inconspicuous  in  succulent  xerophytes  because  they  are  deeply  buried,  and 
in  submersed  hydrophytes,  because  the  are  poorly  developed. 

Structural  features.  —  Leaf  veins,    like   conductive    tracts    generally    (p.    682), 
are  composed  of  water-conducting  elements  (hadrome),  food-conducting  elements 


LEAVES 


639 


(leptome),  mechanical  elements  (stereome),  and  undifferentiated  parenchyma.  The 
position  of  the  hadrome  cells  in  the  upper  portion  of  the  vein  just  above  the  leptome 
(fig.  760)  and  near  the  palisade  cells  may  be  advantageous,  since  the  amount  of 

water  transported  greatly  exceeds  the  combined 
amount  of  other  substances.  Near  the  ends  of 
the  veinlets  there  are  no  tracheae,  but  chiefly 
tracheids  which  frequently  diverge  in  such  a 
way  as  to  increase  the  area  of  the  diffusing  sur- 
face (fig.  936);  com- 
monly they  are  sur- 
rounded by  a  sheath 
of  mesophyll  cells. 

Mechanical  tis- 
sues in  leaves.  — 
Mechanical  leaf  tis- 
sues, while  mostly 
lacking  in  hydro- 
phytes and  not  espe- 
cially well  developed 
in  mesophytes,  are 


FIG.  936. — A  longitudinal  radial 
section  of  a  leaf  of  the  hop  tree 
(Ptelea  trifoliata)  near  the  end  of 
a  vein,  showing  a  bundle  terminus 
with  its  tracheids  (/),  surrounded 
by  a  bundle  sheath  (b) ;  note  that 
the  upper  chlorenchyma  consists  of 
palisade  cells  (/>),  which  at  the  leaf 
margin  grade  into  the  cells  char- 
acterizing the  lower  chlorenchyma 
(s);  highly  magnified. 


FIG.   937.  —  A  cross  sec- 
tion through   a  leaf  of  the 
fragrant    olive    (Osmanthus 
developed  promi-     fragrans^    showing    a    T_ 

nently  in  many  xero-  shaped  sclereid  (s),  which  to- 
phytes,  particularly  in  those  with  stiff  evergreen  leaves,  gether  with  the  thick  cuticle 
the  so-called  sclerophylls.  As  noted  elsewhere,  the 

cutinized  outer  epidermal 

wall   is   an   important 


(c)  accounts  for  the  stiffness 
of  the  leaf;  note  the  three 
palisade  layers  (/>),  indicat- 


means  of  stiffening  in  .inS  relative  ^rophytism; 
these  and  other  leaves,  highl?  magnified- 
supplemented  in  some  cases  by  a  thickening  of  the 
lateral  epidermal  walls  (as  in  Ficus,  fig.  80 1)  or  even 
of  the  hypodermal  walls  (as  in  Pinus,  fig.  1039).  In 
the  leaves  of  xerophytic  grasses  and  sedges,  patches  of 
thick-walled  bast  fibers  and  other  mechanical  cells  may 
occur  just  beneath  the  epidermis  (fig.  835),  and  festoons 
of  such  cells  often  surround  the  conductive  bundles, 
giving  a  great  amount  of  strength  to  the  leaves.  Some- 
times (as  in  Osmanthus,  fig.  937)  evergreen  leaves  con- 
tain isolated  mechanical  cells  (sdereids)  extending  from 
the  lower  to  the  upper  epidermis,  apparently  acting  as 
(Ribes  aureum),  showing  ar-  supports  or  braces;  occasionally  these  stiff  cells  are 
cuate  veins  (v)  just  below  Y-  or  T-shaped.  The  outer  cortical  layers  of  most 
petioles  have  the  collenchymatic  thickening  charac- 
teristic of  young  stems  (p.  697).  The  tearing  of  leaves 
at  their  margins  is  prevented  largely  by  the  presence 
there  of  an  extra  amount  of  cutinization,  while  similar  protection  is  afforded  in 
many  leaves  by  marginal  veins.  Lobed  leaves  would  seem  especially  subject  to 
tearing  at  their  sinuses ;  in  some  cases  (as  in  Ribes,  fig.  938)  strong  arcuate  veins 


FIG.  938.  —  A  portion  of 
,  a  leaf  of  the  golden  currant 


the  leaf  sinuses,  where  the 
danger  of  tearing  otherwise 
would  be  considerable. 


640 


ECOLOGY 


just  beneath  the  sinus  prevent  such  tearing.  Sometimes  such  marginal  protecting 
structures  are  wanting,  as  in  the  banana  leaf,  which  consequently  is  shredded  by 
the  winds  (fig.  846). 

Leaf  tendrils.  —  Climbing  organs  in  general  will  be  considered  under  stems 
(p.  651),  but  some  plants  (as  vetches  and  peas,  figs.  939,  943)  climb  by  means  of  leaf 
organs,  the  upper  leaflets  consisting  of  slender  tendrils  instead  of  blades;  in  Cobaea 
the  tendril  ends  are  recurved  somewhat  after  the  fashion  of  grappling  hooks  (figs, 
959,  960).  Tendrils  are  irritable  organs,  which  react  by  growth  curvatures  when 

they  come  in  contact  with  a 
solid  object,  and  thus  are  en- 
abled to  coil  about  a  support. 
Some  petioles  (as  in  Tropaeo- 
lum)  are  similarly  responsive. 
Leaf  tendrils  sometimes  have 
been  called  modified  or  meta- 
morphosed leaves  or  leaflets, 


FIG.  939.  —  A  growing  shoot  of  the  sweet 
pea  (Lathyrus  odoratus),  showing  leaves  with 
a  pair  of  leaflets  (/),  a  terminal  tendril  (t\  and 
a  pair  of  stipules  (s)  at  the  base  of  the  petiole 


940 


FIGS.  940,  941.  —  Bud  protec- 
tion in  the  sycamore  (Platanus 
occidental-Is):  940,  a  portion  of 
a  twig,  showing  the  swollen  base 
of  a  petiole;  941,  a  twig,  as  in 
940,  with  enough  cut  away  to 
show  the  bud  for  the  following 
year  (d)  covered  by  the  swollen 
base  (6)  of  the  petiole  (/>). 


a  statement  that  is  unwarranted,  since  there  is  no  evidence  that  ancestrally  they 
were  ever  anything  else  than  tendrils. 

Petioles.  —  Attention  has  been  called  elsewhere  to  the  chief  advantage  of  leaf- 
stalks or  petioles,  namely,  the  facilitation  of  leaf  display  to  light  through  elongation 
and  change  of  orientation.  Petioles  are  poorly  developed  in  most  conifers  and 
monocotyls,  reaching  their  culmination  in  dicotyls,  where  usually  they  are  slender, 
elongated  organs,  contrasting  sharply  with  the  blades  (fig.  779).  Short  or  broad 
petioles  are  of  less  significance  in  facilitating  leaf  display.  While  many  petioles 
are  cylindrical,  others  are  grooved  and  still  others  (as  in  the  poplars)  are  flattened 
laterally.  Some  leaves  (known  as  phyttodes)  consist  only  of  petioles  (fig.  853). 


LEAVES 


641 


In  some  instances  petioles  are  of  value  in  protecting  developing  buds  from  trans- 
piration and  other  detrimental  factors,  as  in  Acer,  Platanus,  and  Rhus,  where 
the  buds  for  the  next  season  are  hidden  more  or  less  completely  under  the  base  of 
the  petiole  until  leaf  fall,  by  which  time  the  bud  scales  are  fully  formed  (figs.  940, 
941).  In  various  developing  umbellifer  shoots  (as  in  Angelica,  fig:  942)  the  petioles 
have  large  sheathing  bases  which  enclose  all  the  younger  parts.  In  some  aquatic 
plants  the  petioles  are  greatly  inflated,  air-containing  organs  that  help  to  float  the 
inflorescence  (as  in  Utricularia  inflate)  or  the  entire  plant  (as  in  Eichhornia). 
Variations  in  the  length  of  petioles,  due  to  external  factors,  will  be  considered 
under  stems  (p.  728). 

Stipules.  —  Many  plants,  especially  dicotyls,  possess  leaf  appendages  known  as 
stipules,  which  usually  occur  in  pairs,  one  at  each  side  of  the  petiole  near  its  base. 


944 


FIGS.  942-944.  —  942,  a  growing  shoot  of  Angelica  atropurpurea,  showing  the  large  inflated 
petiole  (p)  of  a  full-grown  leaf,  from  which  a  young  leaf  (/)  is  just  emerging;  the  petiole 
(/>')  of  this  young  leaf  is  cut  away  enough  to  show  within  it  a  still  younger  leaf  (/');  the 
petiole  (p")  of  this  younger  leaf  contains  within  it  a  still  smaller  and  younger  leaf  ;  943,  a 
part  of  a  young  shoot  of  a  wild  pea  (Lathyrus  ochroleucus),  showing  the  prominent  stipules 
(s,  s'),  and  a  young  leaf  (/)  terminated  by  a  tendril  (J);  the  stipules  early  develop  to 
their  full  size,  for  a  time  being  erect  and  enclosing  an  undeveloped  shoot,  as  at  s' ;  944,  a 
growing  shoot  of  a  loosestrife  (Lysimachia),  showing  a  gradual  transition  from  the  early 
scale  leaves  (s)  to  the  foliage  leaves  (/)  that  appear  later;  note  also  the  decussate 
phyllotaxy. 

Their  presence  or  absence  apparently  is  unrelated  to  external  factors  and  often  charac- 
terizes entire  genera  or  families ;  the  Rosaceae  and  the  Leguminosae,  for  example,  com- 
monly have  stipules,  while  the  Cruciferae  and  the  Ranunculaceae  commonly  have 
none.  Stipules  assume  different  forms,  appearing  as  spines  in  Robinia,  and  as  mem- 


642 


ECOLOGY 


branous  sheaths  surrounding  the  stem  in  the  Polygonaceae  (fig.  822),  while  they 
are  coherent  with  the  petiole  in  the  roses  (fig.  1094).  Stipules  may  persist 
through  the  life  of  the  leaf  or  they  may  be  caducous  (i.e.  falling  as  the  buds  open, 
fig.  948);  usually  in  both  cases  they  are  precocious  in  their  development,  thus 
affording  some  protection  to  the  rest  of  the  develop- 
ing leaf  or  shoot  from  transpiration  and  other  detri- 
mental factors.  Probably  this  is  the  only  role  of  most 
caducous  stipules,  and  it  is  well  illustrated  by  the  large 
stipule  of  Ficus,  which  encloses  the  developing  blade, 
falling  as  the  latter  expands  (fig.  714).  Many  cadu- 
cous stipules  are  small  and 
have  no  apparent  role. 
Precocious  persistent  stip- 
ules also  may  afford  pro- 
tection, as  in  the  peas 
(figs.  939,  943),  where  they 
stand  close  together  verti- 
cally, with  an  undeveloped 
leaf  or  shoot  enclosed  be- 
tween them;  in  Lathyrus 
ochroleucus,  frosts  may  kill 
young  parts  protruding 
from  the  stipules,  while  not 
injuring  the  enclosed  parts. 
The  chief  role  of  persistent 
stipules,  which,  in  contrast 
with  caducous  stipules,  are 
almost  always  green,  is  the 
manufacture  of  foods;  in 
plants  with  large  stipules, 
as  in  the  peas,  this  role 
assumes  quantitative  im- 
portance, and  in  Lathyrus 

Aphaca,  the  stipules  much  surpass  the  blades  in  size 
and  in  synthetic  capacity.  If  the  developing  blades  of 
Prunus  serotina  are  cut  away,  the  stipules  grow  to  a 
considerable  size  and  live  for  a  longer  time  than  usual ; 
doubtless  they  manufacture  more  food  than  under 
ordinary  conditions. 


946 

FIGS.  945,  946.  —  Cross 
sections  of  leaves  of  a  loose- 
strife (Lysimachia):  945,  a 
section  through  a  scale  leaf, 
showing  a  relatively  undif- 
ferentiated  mesophyll  (m), 
resembling  sponge  tissue,  and 
the  prominently  cutinized 
lower  (outer)  epidermis  (e); 
946,  a  section  through  a  foli- 
age leaf,  showing  a  row  of 
conspicuous  palisade  cells 
(/>),  a  broken  row  of  shorter 
palisade  cells  (pf),  and  the 
sponge  tissue  (s);  figs.  945 
and  946  highly  and  equally 
magnified. 


FIG.  947.  —  An  inflor- 
escence bud  of  the  arti- 
choke (Cynara  Scolymus\ 
showing  the  overlapping 
scale  leaves  (forming  an 
involucre),  which  protect 
the  delicate  flower  buds 
within;  the  leaves  are  ar- 
ranged in  many  ortho- 
stichies,  illustrating  a  phyl- 
lotactic  system  of  high 
rank. 


Scale  leaves.  —  General  features.  —  Scale  leaves  contain  little  or  no  chlorophyll, 
and  hence  are  not  foliage  organs;  usually  they  are  small  yellowish  or  brownish  struc- 
tures attached  to  the  stem  by  a  broad  base,  and  without  prominent  veins  or  leaf 
teeth;  the  mesophyll  commonly  remains  undifferentiated  through  life. 

Subterranean  scale  leaves.  —  Underground  stems,  at  least  when  young,  are  clothed 
more  or  less  thickly  with  scale  leaves.  In  bulbs  the  closely  imbricated  scale  leaves  are 
much  thickened,  making  up  the  main  body  of  the  organ;  obviously  the  role  of  such 
scale  leaves  is  the  accumulation  of  surplus  food  and  water,  which  commonly  are 


LEAVES 


643 


s'- 


utilized  during  subsequent  development  (fig.  991).  The  scale  leaves  of  rhizomes 
are  much  smaller  and  thinner  than  bulb  scales,  and  often  are  more  ephemeral,  ap- 
pearing to  have  no  role  of  importance,  except  where  they  protect  the  growing  stem 
apex  as  it  pushes  through  the  soil  (as  in  Spartina,  figs.  979,  980). 

In  many  plants  with  both  aerial  and  subterranean  stems  (as  Lysimachia,  fig.  944) 
there  are  all  gradations  between  scale  leaves  and  foliage  leaves,  the  former 
having  undifferentiated  colorless  mesophyll,  and  a 
strongly  cutinized  lower  epidermis,  and  the  latter  having 
green  palisade  and  sponge  cells  (figs.  945,  946).  Both 
kinds  of  leaves  have  similar  positions  and  arise  from 
similar  primordia.  Furthermore,  in  many  cases,  foliage 
leaves  develop  from  scale  primordia  when  exposed  from 
the  outset  to  light  and  air,  and  scale  leaves  may  develop 
in  the  soil  from  the  primordia  of  foliage  leaves.  Thus 
the  distinction  between  such  leaves  is  not  inherent,  but 
a  matter  of  relation  to  external  conditions,  though  the 
precise  factors  involved  are  imperfectly  known. 

Bud  scales.  —  In  most  trees  and  shrubs  of  cold  and 
arid  climates,  buds  are  formed  in  the  growing  season 
previous  to  their  full  development ;  after  reaching  a 
certain  size,  they  remain  for  some  months  in  compara- 
tive quiescence.  In  most  cases  the  outermost  leaf 
primordia  attain  their  full  development  the  first  season, 
becoming  hard  and  thick  scales  (figs.  952,  953,  1057- 
1059);  on  the  other  hand,  the  innermost  primordia 
beneath  the  closely  imbricated  outer  scales  are  incom- 
pletely or  not  at  all  developed  until  the  following  season, 
when  they  grow  into  foliage  leaves.  Bud  scales  protect 
the  embryonic  shoot  by  reducing  transpiration  and  by 
minimizing  the  effect  of  sudden  temperature  changes,1 
their  thick  cutin  or  cork  layer  often  being  supple- 
mented by  an  external  resin  coat  (as  in  the  cottonwood) 
or  by  internal  hairs  or  by  both  combined  (as  in  the  horse 
chestnut) ;  the  scales  also  are  beneficial  in  protecting 
the  delicate  inner  portion  of  the  bud  from  mechanical 
injuries.  The  protective  efficiency  of  bud  scales  is 
shown  by  the  injury  done  to  germinating  buds  by  a  spring  frost  that  would  have 
been  harmless  if  occurring  before  the  shoot  had  emerged  from  the  scales,  though  part 
of  the  harm  is  due  to  the  fact  that  germinating  buds  contain  much  more  water  than 
do  the  resting  buds,  and  hence  are  more  subject  than  the  latter  to  injury  through 
frost.  The  winter  buds  of  Viburnum  lantanoides  and  Cornus  sanguinea  are  without 
scales,  the  buds  of  the  latter  being  protected  by  a  dense  growth  of  hairs. 

Bud  scales,  like  subterranean  scales,  grade  into  foliage  leaves  (fig.  948).     In  the 
horse  chestnut  and  in  various  maples  the  scales  represent  the  basal  portion  of  the  leaf, 

1  The  importance  of  bud  scales  in  protecting  from  low  temperatures  often  is  overesti- 
mated; in  the  winter,  ice  forms  abundantly  in  the  bud  tissues. 


FIG.  948. — A  develop- 
ing shoot  of  the  choke 
cherry(Prunus  virginiana), 
showing  a  gradual  tran- 
sition from  the  outer  scale 
leaves  (s)  of  the  winter 
buds  which  are  shed  early, 
through  the  inner  scale 
leaves  (s1)  which  elongate 
as  the  bud  opens,  to  the 
ordinary  foliage  leaves  (I) ; 
note  also  the  slender  sti- 
pules (st). 


644 


ECOLOGY 


as  is  shown  by  the  fact  that  some  of  the  inner  scales  are  tipped  by  a  minute  leaf 
blade  (fig.  1160).  When  such  a  bud  germinates,  the  outer  scales  drop  off,  while  the 
inner  scales  progressively  assume  more  and  more  the  characters  of  foliage  leaves,  in 
color  and  persistence,  as  well  as  in  shape  and  size.  In  Viburnum  Lentago  the  bud 
is  protected  by  two  large  scales  with  long  attenuated  tips, 
which  in  spring  enlarge  at  the  end  into  small  green  blades 
(figs.  949,  950).  Not  only  do  such  facts  show  clearly  the 
essential  morphological  equivalence  of  foliage  leaves  and 
scale  leaves,  but  the  identical  possibilities  of  their  pri- 
mordia  are  capable  of  experimental  demonstration.  The 
removal  of  the  leaves  from  a  pine  or  lilac  shoot  while  the 
buds  are  forming  is  followed  by  the  development  into 
foliage  leaves  of  primordia  that  otherwise  would  become 
scale  leaves,  and  in  most  plants  the  removal  of  the  termi- 
nal bud  during  development  is  followed  by  the  develop- 
ment into  shoots  of  lateral  buds  which  otherwise  would 
have  'remained  as  primordia.  Probably,  therefore,  the 
stimuli  which  determine  whether  primordia  develop  into 
bud  scales  or  into  foliage  leaves  are  external,  but  the  pre- 
cise factors  involved  are  unknown.  Among  the  features 
of  buds  most  in  need  of  explanation  are  these:  the 
arrest  of  the  shoot  primordia  at  a  certain  definite  stage 
in  development,  apparently  without  external  inhibitory 
influence;  the  failure  of  the  external  leaf  primordia 
to  develop  into  foliage  leaves;  and  the  development  in 
unusual  thickness  of  cutin  or  cork  layers  on  the  ex- 
posed under  surfaces  of  the  scale  leaves.  Experiment 
has  thrown  some  light  on  the  cause  of  arrested  shoot 
development,  as  will  appear  elsewhere  (p.  735),  but  why 
leaf  primordia  that  apparently  are  exposed  to  favorable 
conditions  fail  to  reach  their  developmental  possibilities 
remains  to  be  explained.  Nor  is  it  understood  why  scales 
develop  cutin  in  such  great  amount,  when  foliage  leaves 
growing  under  apparently  similar  conditions  exhibit  rel- 
atively slight  cutinization ;  sometimes  (as  in  Tilia}  the 
bud  scales  have  cork  as  well  as  cutin,  while  the  foliage  leaves  have  cutin  only.  It  is 
possible  that  the  relatively  high  transpiration  of  late  summer  is  here  a  factor  of 
importance.  Aerial  scale  leaves,  other  than  those  of  winter  buds,  occur  in  many 
inflorescences  (fig.  947),  in  various  plants  without  chlorophyll  (as  Monotropa,  fig. 
1104),  and  even  in  some  green  plants  (as  Asparagus  and  Equisetum,  figs.  1054, 
1055)  in  which  the  stems  are  the  chief  synthetic  organs. 


FIGS.  949,  950. — 
Shoots  of  the  sweet  vi- 
burnum (Viburnum  Len- 
tago) :  949,  a  shoot  as  seen 
in  late  winter,  showing 
the  two  prominent  scale 
leaves  (s),  which  enclose 
the  bud;  950,  the  same 
shoot  in  early  spring, 
showing  the  leaf  blades 
(/)  which  have  developed 
through  the  renewal  of 
growth  in  the  bud  scales 
(s  of  fig.  949)  and  also 
the  leaves  (/')  which  have 
developed  from  the  bud 
within  the  scale  leaves; 
note  the  leaf  scars  of  the 
previous  season  (r). 


CHAPTER  III —  STEMS 
i.     STEMS   AS    ORGANS   OF   DISPLAY 

General  remarks  on  stems.  —  Stems  are  concerned  chiefly  with 
reproduction  and  with  the  display  of  other  organs,  horizontal  stems 
taking  the  chief  part  in  the  former  and  erect  stems  in  the  latter. 
Though  making  up  the  greater  part  of  the  stem,  the  conductive  and 


FIG.  951.  —  Palmetto  trees  (Sabal  Palmetto),  illustrating  the  tree  rosette  habit  and 
the  absence  of  branching;  note  the  persistent  leaf  bases  which  form  a  protective  stem 
covering;  Ocala,  Fla.  —  Photograph  by  E.  W.  COWLES. 

mechanical  tissues  may  be  regarded  as  structures  incident  to  the  display 
of  leaves  and  reproductive  organs,  since  they  give  these  organs  water  and 
mechanical  support.  Usually  the  character  of  the  stem  determines  the 
form  of  the  plant  body,  as  is  indicated  by  the  old  and  obvious  classifica- 
tion of  plants  into  trees,  shrubs,  and  herbs.  A  tree  possesses  a  tall, 

645 


646 


ECOLOGY 


woody,  perennial  stem,  usually  with  a  single  primary  trunk;  a  shrub 
possesses  a  similar  but  smaller  stem,  usually  with  a  number  of  approxi- 
mately equal  shoots  arising  at  or  near  the  base;  an  herb  is  not  con- 
spicuously woody  and  usually  has  an  annual  aerial  stem  and  often  also 

a  perennial  underground  stem.  These  dis- 
tinctions, however,  are  arbitrary,  gradations 
between  trees  and  shrubs  being  common; 
herbs  and  shrubs  also  intergrade,  especially 
in  the  tropics,  where  all  degrees  of  woodi- 
ness  exist  and  where  herbaceous  stems  often 
endure  for  two  or  more  years.  Herbs  may 
be  subdivided  further  into  forms  whose  stems 
are  wholly  subterranean  (as  in  many  ferns), 
forms  whose  stems  are  wholly  aerial  (as  in 
annuals),  forms  which  have  both  aerial  and 
subterranean  stems  (as  in  most  peren- 
nials), and  forms  which  apparently  are 
stemless  (as  in  some  rosette 
plants). 

The  display  of  foliage  leaves 
by  stems.  —  Elongation  and 
branching.  —  From  the  stand- 
point of  the  maximum  display 
of  foliage  to  light  and  air,  the 
most  significant  stem  features 
are  elongation, 
branching,  and 
erect  ness.  Elon- 
gation is  a  char- 
acteristic feature  of 
most  aerial  stems, 
though  it  is  lacking 
in  various  rosette 
plants;  growth  in 
height  proceeds 
with  extreme  slowness  in  certain  other  cases,  particularly  in  the 
cycads,  some  of  which  are  said  to  require  centuries  to  reach  a  height 
of  two  or  three  meters.  Branching  is  relatively  slight  in  the  pterido- 
phytes,  cycads,  and  monocotyls  (fig.  951),  reaching  its  culmination  in 


FIGS.  952,  953. — Twigs  of  a  maple  (Acer): — 952,  a  termi- 
nal twig  in  its  natural  position;  note  the  erect  terminal  bud  (6) 
of  the  main  shoot,  all  other  buds  being  ascending  rather  than 
erect,  whether  they  are  the  terminal  buds  (b')  of  lateral  shoots 
or  the  lateral  buds  (//')  of  the  main  shoot;  s,  bud  scales;  note 
that  the  terminal  buds  are  larger  than  the  lateral  buds;  953, 
a  horizontal  twig  in  its  natural  position ;  note  the  ascending 
curvatures  of  the  main  (m)  and  lateral  (a)  shoots. 


STEMS  647 

the  conifers  and  dicotyls.  Thus  it  appears  to  be  associated  with  those 
groups  that  exhibit  considerable  growth  in  diameter  from  year  to  year, 
and  it  can  be  recognized  readily  that  the  enormous  spread  of  a  dicotyl 
tree  would  be  quite  impossible  but  for  such  diametral  enlargement  of 
the  trunk.  However,  the  connection  between  diametral  enlargement 
and  branching  is  not  absolute,  as  is  indicated  by  the  cycads,  which 
rarely  branch,  though  increasing  in  stem  diameter.  Some  monocotyls 
and  some  extinct  pteridophytes  exhibit  branching  or  increase  in  stem 
diameter  or  sometimes  both  combined. 

Stem  erectness.  —  Most  aerial  stems  tend  to  grow  erect,  being  pro- 
priototropic  and  apogeotropic.  In  a  dark  chamber,  erectness  is  due 
solely  to  apogeotropism,  but  in  ordina_ryi]ial)itats  light  and  gravity  co- 
operate in  determining  erectness,  the  light  influence  being  the  stronger, 
as  is  well  shown  when 
plants  are  exposed  to  one- 
sided illumination  (figs. 
952>  953)-  When  an 
apogeotropic  stem  is 

placed     horizontally,     the  FIG.  954.  —  A  plant  of  Euphorbia  maculata,  illus- 

growing  tip  SOOn  becomes       trating  the  prostrate  habit;   note  also  that,  although 

,,  |,     ,,        ,,  the  phyllotaxy  is  decussate,  the  leaves  are  in  one  plane 

erect,  but  usually  the  older      owi£/to  the  ^wisting  of  the  horizontal  stems 
parts  of  the  stem  remain 

horizontal;  however,  in  certain  -grasses  (as  in  the  cereals)  the  whole 
stem  once  more  becomes  erect  through  differential  growth  in  the 
lower  nodes.  Most  subterranean  stems  and  some  aerial  stems,  particu- 
larly those  that  are  prostrate  or  running,  show  little  tendency  toward 
erectness  (fig.  954).  Many  of  the  latter  are  erect  when  young  and 
have  erect  tips  through  life,  suggesting  that  horizontality  in  such  cases 
may  be  due,  in  part  at  least,  to  the  lack  of  sufficient  mechanical  tissue 
to  permit  of  erectness.  Some  fruiting  stems  grow  downward  (as  in  the 
peanut  and  in  many  water  plants),  while  in  other  water  plants  (as  in 
Potamogeton)  the  reproductive  stems  are  more  rigidly  erect  than  are  the 
vegetative  stems. 

Lateral  branches.  —  The  most  important  exception  to  erectness  in 
aerial  stems  is  in  the  lateral  branches,  which  grow  in  various  directions, 
thus  resembling  the  diverse  directions  of  lateral  roots.  The  resulting 
plant  contour  usually  is  symmetrical,  especially  in  those  conifers  whose 
trunks  are  excurrent  (i.e.  extending  to  the  summit),  and  whose  outline 
is  approximately  an  elongated  cone  (fig.  955).  The  cause  of  directional 


648 


ECOLOGY 


diversity  in  branching  is  complicated  and  but  vaguely  understood.  If 
the  terminal  bud  of  the  erect  main  shoot  is  removed,  certain  strong 
lateral  branches  hitherto  ascending  obliquely  soon  begin  to  grow  erect 
(as  in  Picea),  or  buds  hitherto  inactive  develop  into  erect  shoots  (as  in 
Araucaria),  As  in  roots,  the  main  axis  seems  to  inhibit  verticality  in 


FIG.  955.  —  Alpine  spruces  (Picea)  and  firs  (Abies),  illustrating  the  spirelike  contour 
and  excurrent  habit  that  is  characteristic  of  various  conifers  of  high  altitudes;  Rocky 
Mountains,  Mont.  —  Photograph  supplied  by  ELROD. 

the  lateral  branches.  However,  aerial  stems  differ  strikingly  from  roots 
in  that  light  is  an  important  factor  in  determining  the  orientation  of 
lateral  branches;  lateral  stems,  even  those  that  descend,  usually  grow 
in  the  direction  of  maximum  incident  light. 

The  advantages  of  conical  shape,  elongation,  and  stem  twisting.  — 
Other  things  being  equal,  the  greater  the  power  of  stems  to  elongate  and 


STEMS 


649 


to  branch  in  diverse  directions,  the  greater  is  the  possibility  of  maxi- 
mum foliage  display.  A  broad-based  cone,  consisting  of  an  excurrent 
trunk  with  branches  diverging  therefrom  from  base  to  apex  at  a  con- 
stantly decreasing  angle,  would  seem  to  be  the  best  of  all  contours  for 
lighting,  in  proportion  to  the  amount  of  structural  material  involved. 


FIG.  956.  —  A  bur  oak  tree  (Quercus  macrocarpa)  in  winter,  showing  representative 
deliquescence ;  note  the  tortuous  branching  characteristic  of  this  species  of  oak ;  Chicago, 
111.  —  Photograph  by  LAND. 

Habits  of  this  sort  reach  their  culmination  in  various  arboreal  conifers, 
but  are  found  in  scarcely  less  perfection  in  some  dicotylous  trees,  as  the 
oaks  and  maples,  though  in  the  latter  the  cones  commonly  are  shorter 
and  broader  and  often  truncated  (figs.  844,  845) .  In  many  alpine  coni- 
fers (fig.  955)  and  in  various  trees  of  warm  climates  (as  the  cypress  and 


650  ECOLOGY 

Lombardy  poplar),  the  stem  contour  is  a  narrow,  elongated  cone;  it  has 
been  suggested  that  such  shapes  are  well-fitted  for  protection  from  in- 
tense light.  The  display  of  foliage  also  is  facilitated  in  high  degree 
by  the  variability  of  the  internodes  in  respect  to  length,  a  variability  com- 
parable to  that  of  petioles  and  attended  with  similar  advantages  (see 
p.  725  for  discussion  of  causes).  Internodal  elongation  makes  possible 
not  only  the  prevention  of  shading  in  large-leaved  plants,  but  also,  in 
many  cases,  the  elevation  of  plants  above  their  surroundings.  In  hori- 
zontal stems,  leaf  display  often  is  facilitated  by  stem  twisting,  through 
which  the  leaves  are  brought  into  a  common  plane  transverse  to  inci- 
dent light  (fig.  781). 

Stem  contours  other  than  conical.  —  Though  the  prevailing  tree  contour  is  conical, 
there  are  many  exceptions,  particularly  among  deliquescent  trees  (i.e.  trees  whose 
main  stem  is  replaced  by  the  diverging  upper  lateral  branches),  such  as  the  bur 
oak  (fig.  956),  the  silver  maple,  and  especially  the  American  elm,  whose  shape  is  that 
of  a  flaring  vase.  A  prevalence  of  horizontal  branching  characterizes  the  cedar  of 
Lebanon,  while  some  hawthorns  and  acacias  have  an  almost  umbrella-like  contour, 
owing  to  the  numerous  descending  branches.  The  extreme  drooping  of  the  branches 
in  the  weeping  willow  (due,  perhaps,  to  a  slight  development  of  mechanical  tissue) 
gives  a  characteristic  rounded  contour  to  the  crown.  Many  trees,  especially 
willows,  are  asymmetric,  the  main  trunks  curving  to  one  side  as  they  develop. 
So  far  as  known,  these  diverse  tree  shapes  are  not  inherently  advantageous.  The 
causes  are  quite  as  little  known,  though  much  more  likely  to  repay  investigation. 
In  some  cases,  as  in  Araucaria  and  Pinus  Strobus,  the  branches  appear  to  be  in 
whorls,  the  tree  thus  being  divided  into  stories ;  this  habit  results  from  the  develop- 
ment at  intervals  of  numbers  of  lateral  buds.  An  interesting  habit  is  that  of  the 
rosette-bearing  trees  (as  in  the  ferns,  cycads,  and  palms,  fig.  951),  characteristic 
of  tropical  forests.  While  the  lack  of  branches  in  such  plants  appears  disadvan- 
tageous, the  elevation  of  the  crown  of  leaves  makes  possible  a  relatively  favorable 
display  of  foliage.  The  palmetto  often  has  a  short  trunk  or  none  at  all  in  dry 
open  habitats,  while  it  has  a  tall  slender  trunk  in  deep  woods. 

Foliage  display  in  shrubs  and  herbs.  —  Most  shrubs  and  herbs  that  have  aerial 
stems  exhibit  essentially  the  same  methods  of  display  as  do  trees,  their  branches 
ascending  or  diverging  variously  and  exhibiting  conical  and  other  contours.  Herbs 
with  stems  wholly  underground  are  poorly  situated  for  leaf  display,  though  some 
forms  (as  Pteris)  have  greatly  elongated  petioles,  which  raise  the  leaf  blades  well 
into  the  light.  Rosette  plants  also  are  situated  somewhat  poorly  for  light  reception, 
though  the  leaves  originate  above  the  soil  level  instead  of  below  it. 

The  significance  of  trees  and  grasses  in  foliage  display.  —  Trees 
are  the  culminating  forms  of  the  plant  world  from  the  standpoint  of 
stem  development  and  the  display  of  foliage  to  light,  their  great  height 
and  size  making  possible  the  development  and  adequate  display  of  an 


STEMS 


651 


enormous  number  of  leaves.  Their  height  and  size  in  turn  are  made 
possible  by  the  perennial  nature  of  their  aerial  stems,  which  permits  each 
season  the  resumption  of  growth  where  it  ceased  the  year  before,  and 
also  by  their  diametral  increase,  which  permits  the  development  of  a 
supporting  trunk  sufficient  to  bear  the  weight  of  the  constantly  increas- 
ing branches.  Trees  dominate  the  vegetation  in  most  regions  where  they 
grow,  other  plants  being  eliminated  except  such  as 
can  endure  the  shade;  among  trees  those  dominate 
ultimately  whose  seedlings  can  germinate  in  the 
shade.  In  prairies  and  in  treeless  swamps,  how- 
ever, there  is  a  predominance  of  sun-requiring  herbs, 
among  which  rhizomatous  monocotyls,  such  as 
many  grasses,  sedges,  and  rushes,  take  a  prominent 
place.  Perhaps  from  the  standpoint  of  world  dom- 
inance plants  with  grasslike  foliage  may  be  placed 
second  only  to  the  trees.  From  the  foregoing  the 
conclusion  must  not  be  drawn  that  plants  other 
than  trees  and  grasses  are  without  comparative 
advantage.  Of  all  vegetative  organs  the  leaves 
of  trees  are  the  farthest  removed  from  the  water 
supply  and  the  most  exposed  to  the  dangers  of 
transpiration.  Thus  the  tree  habit  is  possible  only 
through  the  development  of  extensive  conductive, 
mechanical,  and  protective  tissues. 

Lianas  in  relation  to  leaf  display.  —  Definition 
of  lianas.  —  Lianas  are  plants  that  ascend  by  climb- 


FIG.  957.  —  Apart 


,       i  of  the  climbing  stem 

mg  or  by  leaning  upon  other  plants  or  upon  any     of  a  scarlet  runner 
adequate   support,  their    mechanical    tissue    being     bean  (Phaseolus  mui- 

insufficient  to  permit  them  to  stand  erect,  though     tiflarus),  a  representa- 
,  1*1  •  tive  sinistrorse  twiner. 

they  are  prophototropic  and  apogeotropic. 

Twiners.  —  Possibly  the  most  specialized  climbers  are  those  that  twine, 
for  in  them  the  growing  tip  of  the  main  stem  executes  movements, 
known  as  revolving  nutations,  whereby  a  widening  circumference  comes 
within  the  sweep  of  the  elongating  stem.  After  coming  into  contact  with 
an  erect  stem,  the  continuation  of  the  nutatory  movements  results  in 
twining. 

If  the  lower  part  of  a  coil  appears  from  behind  the  support  at  the  observer's 
left  and  the  upper  part  disappears  at  his  right,  the  twiner  is  called  sinistrorse  (as 
in  the  bean,  fig.  957,  and  in  the  dodder,  fig.  1081).  If,  as  is  more  rarely  the  case, 


652 


ECOLOGY 


FIGS.  958-960.  —  958,  tendrils  of  a 
wild  cucumber  (Echinocystis  lobata); 
note  the  reverse  directions  of  the  spirals, 
those  at  t  being  dextrorse  and  those  at  t' 
sinistrorse;  note  also  the  coiling  of  the  tendril  ends  about  the  support;  959,  960,  tendrils 
of  Cobaea  scandens:  959,  a  compound  leaf  with  three  pairs  of  leaflets  and  a  terminal 
dichotomously  branched  tendril;  960,  the  tip  of  one  of  the  tendril  branches  considerably 
magnified,  showing  the  recurved  hooks  which  hold  the  plant  to  a  support. 


the  lower  part  of  a  coil  appears  at  the  right  and  the  upper  part  disappears  at  the 
left,  the  twiner  is  called  dextrorse  (as  in  the  hop).  Common  native  twiners  are  the 
bittersweet  (Celastrus)  and  the  dodder  (Cuscuta),  the  latter  being  a  parasite  in  which 

contact  stimulation  incites  the  develop- 
ment of  sucking  organs  (haustoria)  that 
serve  also  to  hold  the  climber  firmly  to 
'  the  supporting  plant.     In  twiners  and 

in  other  lianas  there  is  a  striking 
development  of  conductive  tissues 
(p.  689). 


FIGS.  961-963.  —  Tendrils  of  the  Japan 
ivy  (Psedera  tricuspidata):  961,  a  portion  of 
a  stem,  showing  a  young  tendril  (/),  the 
branches  having  small  globular  swellings  (s) 
at  the  tipj  962,  a  similar  tendril  at  maturity, 
the  swollen  tips  (s1)  being  much  larger,  and 
also  flattened  on  the  side  in  contact  with  the 
support;  963,  a  mature  tendril  tip,  somewhat 
magnified,  showing  the  adhesive  disk  (d), 
which  may  fasten  even  to  a  smooth  surface. 


Tendril  climbers.  —  Scarcely 
second  to  twiners  in  specializa- 
tion are  the  tendril  climbers,  the 
tendril  commonly  being  an  organ 
homologous  with  leaves  or  leaflets 
(figs.  959,  960,  939,  943),  or  with 
branches  (as  in  the  passion  flowers 
and  grapes,  figs.  961,  962);  or 
tendrils  may  be  organs  of  doubt- 
ful homology  (as  in  the  pumpkin 


STEMS 


653 


family,  fig.  958).  Tendrils  may  be  simple  or  forked  and  commonly 
are  sensitive  to  contact,  coiling  about  their  support  ;  in  Sicyos  the 
sensitiveness  is  so  great  that  the  slightest  friction  incites  differential 
growth,  resulting  after  a  few  moments  in  conspicuous  curvature. 

The  region  of  sensitiveness  may  be  somewhat  extensive,  or  (as  in  the  pumpkin 
family)  confined  to  certain  areas,  known  as  tactile  spots,  where  there  may  be  thin 

places   or    slight    elevations   in 

the  outer  epidermal  wall;  the 
tendrils  that  react  most  quickly 
usually  are  those  that  have  lo- 
calized sensitive  regions.  Soon 
after  tendrils  become  attached 
to  a  support,  spiral  coils  appear 
in  the  unattached  portions  (fig. 
958),  and  the  tendril-bearing 
plant  is  drawn  close  to  its  sup- 
port ;  mechanical  tissues  also 
may  develop,  increasing  the 
strength  of  the  tendril  (p.  699). 
One  of  the  most  specialized  of. 
tendrils  is  that  of  the  Japan 
ivy  (P  seder  a  tricuspidata);  the 
tendril  branches  terminate  in 
knobs,  which  upon  contact 
broaden  out  into  disk-shaped 
suckers  that  secrete  a  mucilagi- 
nous substance  and  thereby  ad- 
here most  tenaciously  to  walls 
or  bark  (figs.  961-963).  A 
variety  of  the  Virginia  creeper 


FIG.  964.  —  A  fern  (N ephrolepis}  as  a  rhizome 
climber  on  the  palmetto  (Sabal  Palmetto) ;  note  that 
the  fern  occurs  at  the  upper  part  of  the  trunk  where 
the  leaf  bases  persist;  the  rhizome  dies  below  and 
ascends  pari  passu  with  the  developing  palm ;  Miami, 
Fla.  —  Photograph  by  E.  W.  COWLES. 


(Psedera  quinquefolia)  has 
somewhat  similar  tendrils  with 
adhesive  disks. 

Plants  which  climb  by  roots  or 
rhizomes.  —  Root  climbers  have 
been  considered  elsewhere,  and 
it  has  been  noted  that  anchoring  roots,  like  tendrils,  often  are  sensitive  to  mechanical 
stimulation,  and  that  such  roots  frequently  grow  horizontally  about  their  support 
instead  of  growing  downward  as  do  most  roots.  Root  climbers  (as  the  English 
ivy)  are  quite  as  able  to  adhere  to  vertical  walls  as  are  those  plants  whose  ten- 
drils have  adhesive  disks.  The  Virginia  creeper  sometimes  climbs  by  roots  as 
well' as  by  tendrils  with  adhesive  disks.  The  elongating  rhizomes  of  various 
tropical  ferns  (as  Nephrolepis)  often  come  in  contact  with  tree  trunks,  which 
they  may  ascend,  especially  if  the  bark  is  spongy  and  easily  penetrated  (as  in  the 
palmetto).  If  the  rhizome  continues  to  ascend,  its  ground '  connections  may  be 


654 


ECOLOGY 


severed  through  the  decay  of  the  posterior  portions,  thus  resulting  in  the  transforma- 
tion of  a  soil  plant  into  an  epiphyte  or  an  epiphytic  liana,  though  the  roots,  whether 
in  the  soil  or  in  the  bark,  are  true  soil  roots,  which  absorb  water  and  salts  from  the 
substratum  (fig.  964).  Such  climbing  rhizomes  often  keep  pace  with  the  ascent  of 
the  supporting  tree,  and  in  the  palmetto  they  are  found  most  commonly  in  the 
enlarged  spongy  region  just  below  the  leaf  crown.  The  rhizome  of  Polypodium 
aureum  often  creeps  around  palmetto  trunks  in  a  slowly 
ascending  spiral.  Rhizome  climbers  do  not  always  start 
from  the  ground,  since  spores  may  germinate  at  any 
level  on  the  trunk.  Outside  of  the  tropics  climbing  by 
rhizomes  is  illustrated  by  several  species  of  Polypodium ; 
various  creeping  mosses  and  liverworts  ascend  tree  trunks 
in  similar  fashion,  especially  in  swamps,  and  embedded  in 
the  mosses,  there  occasionally  may  be  seen  such  rhizomes 
as  those  of  Maianthemum. 

Plants  which  climb  by  hooks  or  thorns.  —  In  hook  climbers 
and  thorn  climbers  attachment  or  connection  with  a  support 
is  in  a  sense  accidental,  involving  no  special  growth  features 
as  in  the  previous  groups  of  lianas.  Climbing  by  thorns  is 
seen  in  roses,  in  blackberries,  and  in  the  greenbrier;  the  last, 
however,  is  supported  much  more  effectively  by  tendrils. 
Hooks  that  point  downward  characterize  various  bedstraws 
(Galium)  and  the  hop  (fig.  965),  and  are  superior  to  thorns 
in  that  they  prevent  slipping  backward. 

Leaner s.  —  A  transition  between  lianas  and  ordinary  erect 
plants  is  afforded  by  one  of  the  night-shades,  Solanum 
Dulcamara,  which  leans  on  neighboring  plants,  having  no 
means  for  climbing,  except  that  it  sometimes  twines  to  a 
slight  extent.  Such  a  plant  may  be  called  a  leaner.  The 
tall  nasturtium  (Tropaeolum)  is  another  such  leaner,  which 
also  may  climb  to  some  extent  by  petiole  twisting.  Many 
plants  that  are  prostrate  in  the  open  may  be  leaners  where 
the  vegetation  is  dense.  Closely  related  to  leaners  are  those 
submersed  aquatics  whose  stems  are  supported  by  the  water,  their  mechanical 
tissue  being  insufficient  to  keep  the  plants  erect. 


FIG.  965.  —  A  dia- 
grammatic longitudi- 
nal section  through  the 
outer  portion  of  a  hop 
stem  (Humulus  Lupu- 
lus),  showing  emer- 
gences with  obliquely 
oriented  barbs,  the 
lower  ones  (h)  point- 
ing downward  and  out- 
ward in  such  a  way  as 
to  hold  the  stem  to  a 
support;  somewhat 
magnified. 


The  advantages  and  disadvantages  of  the  liana  habit.  —  The  great 
advantage  possessed  by  lianas  is  their  favorable  position  for  foliage  dis- 
play without  the  construction  of  a  large  amount  of  supporting  tissue. 
When  a  vine  sprawls  over  a  hawthorn,  the  liana  may  have  a  foliage 
display  equal  to  that  of  the  tree.  However,  this  habit  may  be  accom- 
panied by  ultimate  disadvantage,  because  the  leaves  of  the  liana  cut 
off  the  light  from  the  leaves  of  the  supporting  tree ;  also  the  weight  of 
the  vine  may  become  too  great  for  the  tree,  resulting  in  the  downfall 
of  both  (fig.  966),  The  collapse  of  a  branch  may  suffice  for  the  down- 


STEMS 


6SS 


fall  of  a  tendril  climber;  the  stems  of  woody  twiners  sometimes  are 
ruptured  by  the  increasing  pressure  to  which  they  are  subjected  by 
the  diametral  enlargement  of  the  tree,  though  more  commonly  the 
twining  stem  becomes  embedded  in  the  growing  trunk. 

The  origin  and  distribution   of  lianas.  —  While   herbaceous  lianas 


FIG.  966. —A  grape-vine  (Vitis)  that  has  clambered  over  a  hawthorn  (Cmtaegus) 
and  occasioned  its  destruction;  in  the  left  foreground  is  a  cottonwood  (Populus  deltoides) 
and  in  the  left  background  is  an  elm  (Ulmus  americana) ;  River  Forest,  111.  —  Photograph 
by  LAND. 

may  occur  almost  anywhere,  woody  lianas  are  associated  peculiarly  with 
forests,  reaching  their  culmination  in  the  tropics,  where  there  exist  all 
but  impenetrable  tangles  of  intertwining  vines,  some  of  which  (as  the 
rattan  palm)  may  reach  the  great  length  of  250  meters,  that  is,  equal 
to  twice  the  height  of  the  tallest  trees  (fig.  967).  In  temperate  regions 
lianas  culminate  in  the  rich  soil  along  rivers,  the  greenbrier  thickets  with 


656 


ECOLOGY 


their  thorns  approaching  the  impenetrability  of  tropical  tangles.  It  is 
assumed,  and  probably  correctly,  that  lianas  have  come  from  erect 
ancestors,  and  that  their  evolution  was  subsequent  to  that  of  trees, 
although  potential  lianas  well  may  have  existed  before  trees  and  even 
may  have  climbed  over  rock  cliffs.  Probably  the  first  lianas  were 
leaners,  the  twiners  and  tendril  climbers  developing  later.  It  is  the 


FIG.  967.  —  A  tropical  mesophytic  forest,  rich  in  lianas;  at  the  left  is  a  sinistrorse  twiner; 
note  the  twisting  of  the  liana  stems  at  the  center;  the  dominating  liana  is  Agelaea  Wal- 
lichii,  but  in  the  undergrowth  are  many  rattan  palms  (Calamus},  which  at  first  are  ordinary 
palm  rosettes,  but  later  develop  into  lianas  of  extraordinary  length ;  Lamao  Forest  Reserve, 
Philippine  Islands.  —  From  WHITFORD  (Courtesy  of  the  Philippine  Bureau  of  Forestry). 

prevalent  view  that  lianas  have  resulted  from  the  "  struggle  for  light  " 
in  the  forest.  Of  this  there  is  no  evidence  whatever,  except  in  so  far  as 
stems  in  general  tend  to  elongate  where  there  is  decreased  light  (p.  726).1 

1  Recently  evidence  of  the  inception  of  a  twining  habit  has  been  discovered  in  a  race  of 
snapdragons  (Antirrhinum  Majus),  the  new  form  appearing  to  be  a  mutant.  This 
form  has  the  characteristic  anatomical  features  of  twiners,  such  as  a  small  pith  region, 
compact  vascular  tissues,  and  cortical  differences  on  the  convex  and  concave  surfaces. 
Furthermore,  the  twining  variants,  however  caused,  come  true  to  seed.  It  is  difficult  to 


STEMS 


657 


Pendulous  plants.  —  Various  vines,  as  the  Virginia  creeper,  if  rooted  at  the  top  of 
a  narrow  canon,  may  hang  down  over  the  wall  ;  however,  such  vines  are  prophoto- 
tropic  and  apogeotropic,  as  may  be  seen  by  the  recurved  tips,  and  by  the  petioles 
which  point  upward  toward  the  light  (fig.  968).  Leaners  sometimes  behave  similarly, 
the  most  remarkable  feature  being  that  the  pendulous  stems  are  several  times  as 
long  as  are  erect  stems  in  the  same  species  (as  in  Rubus 
occidentalis  and  Ribes  Cynosbati).  The  cause  of  this 
elongation  is  unknown,  though  it  seems  possible  that  it  is 
in  some  way  associated  with  the  fact  that  growth  is  in 
the  direction  of  the  gravity  pull,  instead  of  against  it,  as 
!n  erect  stems.  It  has  been  found  that  when  growing  stems 
are  subjected  to  tension,  cell  elongation  takes  place  in  the 
direction  of  the  pull. 

Epiphytes.  —  General  remarks.  —  Epiphytes  are 
nutritively  independent  plants,  which  are  given 
complete  mechanical  support  by  other  plants,  dif- 
fering from  parasites  in  not  deriving  food  or  water 
from  the  supporting  plant,  and  from  lianas  in  having 
no  soil  connections.  Although  all  gradations  exist 
between  lianas,  epiphytes,  and  ordinary  soil  plants 
(as  in  Nephrolepis,  p.  653),  the  most  representative 
epiphytic  forms  occur  only  as  epiphytes.  In  regions 
with  winters  or  with  prolonged  dry  periods,  true 
epiphytes  are  limited  essentially  to  algae,  lichens, 
liverworts,  and  mosses;  in  the  moist  tropics,  these 
forms  are  supplemented  by  many  ferns  and  seed 
plants,  especially  orchids  and  bromelias  (fig.  969). 
In  many  tropical  forests  the  epiphytes  are  arranged 

/  r    J 

in  stones  or  strata  ;  those  in  the  treetops  (such  as  gjnia  creeper  (Psedera 
species  of  Tillandsia)  are  very  xerophytic  in  struc-  guinquefoiia),  showing 
ture,  while  farther  down  are  more  mesophytic  forms,  the,  curvature  of  lat; 

eral  branches  upward 

such  as  the  orchids  and  ferns.  Near  the  ground,  toward  the  light;  note 
where  desiccation  rarely  takes  place,  there  occur  the  conspicuous  leaf 
extreme  mesophytes,  such  as  the  filmy  ferns.  The  scars(r)  of  the  previous 

...>•?',  -  season. 

xerophytic  forms  of  the  treetops  of  moist  regions 

penetrate  farther  into  dry  regions  than  do  other  epiphytes.  Even  in 
northern  forests  a  similar  stratification  exists,  xerophytic  lichens  occur- 
ring in  the  branches,  while  farther  down  are  mosses  and  liverworts. 

see  much  advantage  in  the  changed  habit,  since  the  new  forms  are  quite  as  erect  and 
strong  as  ordinary  individuals,  and  coils  often  occur  in  positions  where  they  scarcely  can 
be  of  use,  as  at  the  base  of  a  shoot. 


f  IG>  968* 

dulous  stem  of  the  Vir- 


658  ECOLOGY 

Probably  the  absence  of  epiphytic  seed  plants  and  ferns  in  most  dry 
and  cold  climates  is  due  to  the  long  period  of  excessive  transpiration 
with  little  or  no  aerial  absorption,  characteristic  of  such  climates.  The 
most  northern  of  such  epiphytes  in  the  eastern  United  States,  Tillandsia 
usneoides  (fig.  903)  and  Polypodium  polypodioides,  in  their  structural 


FIG.  969.  —  Epiphytes  on  a  live  oak  (Quercus  virginiana\  the  dominating  form  being  a 
species  of  Tillandsia;  Miami,  Fla.  —  Photograph  by  MEYERS. 

features  and  life  habits  are  most  pronounced  xerophytes.  Even 
mosses  and  lichens  as  epiphytes  are  much  more  abundant  in  humid 
climates  than  elsewhere. 

Structures  characterizing  epiphytes.  —  Epiphytes  commonly  are  char- 
acterized by  highly  specialized  organs  of  absorption,  or  by  structures 
which  effectively  reduce  transpiration  or  accumulate  large  quantities 


STEMS 


659 


of  water.  The  absorptive  organs  of  epiphytes,  as  seen  in  the  lichen 
thallus,  in  the  aerial  roots  of  orchids,  and  in  the  leaves  of  mosses  and 
bromelias,  have  been  treated  elsewhere.  Most  orchids  and  bromelias 
have  highly  cutinized  epidermal  walls,  which,  with  other  protective 
features,  reduce  transpiration  to  such  an  extent  that  the  plants  do  not 
dry  out  for  weeks.  In  many  orchids  the  leaves  or  stems  or  both  are 
greatly  thickened  and  contain  large  quantities  of  water,  the  stems  fre- 
quently showing  bulbous  enlargement.  In  most  bromelias  the  leaves 
form  a  sort  of  cistern,  which  retains  water  for 
weeks  after  a  rain;  since  these  cistern  epiphytes 
absorb  water  almost  entirely  through  the  leaves, 
the  advantage  of  the  habit  is  obvious. 

Epiphylls  ;  injury  due  to  epiphytes.  —  Epiphytes 
occurring  on  leaves  are  known  as  epiphylls  (fig.  970), 
and  are  especially  characteristic  of  the  moist  tropics; 
lichens,  mosses,  and  even  vascular  plants  occur  in  this 
strange  position.  Epiphylls  are  very  injurious  to  the 
leaves  on  which  they  grow,  cutting  off  light  and  im- 
peding gas  exchange.  Epiphyllous  lichens  show  all 
gradations  between  epiphytism  and  parasitism,  some 
forms  being  strictly  external  to  the  leaf,  while  other 
forms  destroy  the  cuticle  and  thus  have  a  position 
directly  on  the  outer  wall  of  the  pidermis ;  in  still  other 
cases  (as  in  Strigula)  the  lichen  tissues  may  penetrate 
the  mesophyll.  Stem  epiphytes  also  often  injure  their 
supporting  plants,  checking  gas  exchanges  through  the 
bark  or  breaking  the  branches  by  their  weight.  Even 
in  cold  climates  the  beard  lichens  (such  as  U  nea  and 
Alectoria)  may  enfold  the  leafy  twigs  of  conifers  to  such 
an  extent  as  to  cause  their  death ;  in  some  cases  the  hyphae  of  Usnea  penetrate  the 
living  cells  of  the  supporting  plant. 

Subordinate  categories  of  epiphytes.  —  Plants  epiphytic  for  only  a  part  of  their 
existence  (as  Ficus,  p.  515)  are  known  as  hemi-epiphytes.  In  temperate  regions  many 
ordinary  soil  plants  are  found  in  the  crotches  of  trees,  where  a  little  soil  has  collected; 
such  plants  may  be  called  pseudoepiphytes.  Various  algae  occur  as  epiphytes  in 
the  water,  but  most  forms  grow  equally  well  when  attached  to  rocks  or  shells; 
however,  for  species  requiring  considerable  light,  attachment  to  plant  organs  which 
float  near  the  surface  may  be  advantageous.  Various  epiphytic  lichens  and  mosses 
occur  also  as  lithophytes,  and  Tillandsia  may  grow  even  on  telegraph  wires. 

Restriction  of  epiphytes  to  particular  supports.  —  Most  true  epiphytes  are  re- 
stricted to  trees,  some  being  confined  to  particular  species.  Among  the  lichens 
some  crustose  species  (as  Graphis  scriptd)  grow  chiefly  on  smooth-barked  trees. 
The  palmetto  has  certain  characteristic  epiphytes  that  rarely  if  ever  grow  on  other 
trees ;  probably  this  is  because  its  soft  spongy  bark  especially  facilitates  attachment. 


Fio.  970.  —  Epiphyllous 
foliose  lichens  (Strigula 
complanata)  on  a  leaf  of 
Ocotea;  a,  fruit  dots  (apo- 
thecia) ;  such  lichens  prop- 
agate radially  and  the 
largest  one  here  figured 
already  has  disorganized  at 
the  older  central  portion, 
the  thallus  becoming  ring- 
shaped  instead  of  solid. 


66o 


ECOLOGY 


Possibly  some  epiphytes  have  chemical  as  well  as  mechanical  relations  with  trees, 
certain  bark  substances,  perhaps,  furnishing  a  necessary  food  element  or  neutralizing 
root  excreta;  perhaps  the  bark  of  some  trees  may  contain  substances  injurious  to 
the  roots  of  certain  epiphytes. 

The  advantages  and  disadvantages  of  epiphytism.  —  It  is  difficult 
to  see  any  great  advantage  in  the  epiphytic  habit  other  than  that 
epiphytes  are  relatively  exempt  from  the  extreme  overcrowding  which 
characterizes  soil  plants.  It  is  usual  to  class  epiphytes  with  lianas  as 
a  group  developed  in  the  "  struggle  for  light,"  but  there  is  no  evidence 
therefor;  furthermore,  epiphytes  occur  chiefly  on  trunks  and  branches 
which  are  lighted  scarcely  better  than  the  ground.  The  disadvantages 
of  the  epiphytic  habit,  namely,  restricted  absorption  and  exposure  to 
high  transpiration,  are  very  obvious.  The  underlying  causes  of  epiphy- 
tism are  unknown,  though  facultative  epiphytes, 
such  as  Nephrolepis,  suggest  possible  beginnings. 
The  bromelia  series  (p.  616)  is  continuous  from  the 
non-epiphytic  pineapple  through  the  leafy  species 
of  Tillandsia  (as  T.  utriculata)  to  the  leafless  T. 
usneoides  and  may  represent  a  line  of  progress 
toward  obligate  xerophytic  epiphytism. 

Carbohydrate  synthesis  and  aeration  in  stems.  — 
Stem  chlorophyll.  —  In  addition  to  being  organs  of 
foliage  display,  stems  are  important  food  making 
organs.  Young  woody  stems,  as  well  as  herba- 
ceous stems,  commonly  are  green,  though  the  total 
expanse  of  chlorophyll  tissue  in  stems  is  much  less 
than  in  leaves.  The  chlorophyll  occurs  in  the 
cortex,  gradually  decreasing  inwards,  as  in  thick 
leaves;  sometimes  (as  in  most  leafless  stems)  the 
outermost  layers  are  differentiated  into  palisade 
cells.  As  in  leaves,  there  are  internal  air  chambers 
and  stomata,  which  act  as  passageways  for  gases. 
In  many  water  plants  there  are  capacious  air 
chambers,  which  are  separated  from  one  another 
by  diaphragms  (fig.  792)  or  by  solid  nodes  that 
strengthen  the  stem.  During  the  first  year  a  thick 
bark  develops  in  woody  stems,  largely  through  the 
formation  of  new  cells  and  the  subsequent  modification  of  their  walls. 
The  stems  in  most  instances  no  longer  appear  green;  nevertheless  be- 


FIG.  971.  —  A  por- 
tion of  the  stem  of 
Bryophyllum  caly- 
cinum,  showing  an 
abundant  development 
of  warty  emergences, 
the  lenticels;  note  also 
the  leaf  scars  at  the 
nodes  (n). 


STEMS 


661 


neath  the  outer  bark  there  is  green  tissue  that  is  of  great  interest  because 
of  the  relative  darkness  in  which  chlorophyll  is  developed  and  food  is 
manufactured  (figs.  972,  1033).  As  the  bark  increases  in  thickness, 
the  chlorophyll  gradually  decreases,  finally  disappearing  from  old  stems. 
It  has  been  suggested 
that  the  food-making 
activity  of  deciduous 
trees  may  not  cease 
upon  leaf  fall,  but 
that  the  stem  chloro- 
phyll, like  the  leaf 
chlorophyll  of  coni- 
fers, may  manufac-  972 
ture  food  at  tempera- 
tures favorable  for 
synthesis  during  the 
late  autumn,  winter, 
and  early  spring. 

The  structural  fea- 
tures of  lenticels.  — 
During  the  first  vege- 
tative period  of  a 
woody  stem,  a  cork 
cambium  or  phello- 
gen  layer  (p.  705) 
makes  its  appearance 


FIGS.  972,  973.  —  Lenticels:  972,  a  young  lenticel  as  seen 
in  a  cross  section  through  the  outer  part  of  a  growing  stem 
of  the  privet  (Ligustrum  vulgar  e) ;  s,  a  stoma  beneath  which 
a  lenticel  is  developing;  p,  the  phellogea  layer  from  which 
closing  layer  of  cork  later  develops;  x,  chlorenchyma, 
in  the  cortex,  giving  composed  of  thin-walled  cells  beneath  the  lenticel  and  of 
to  a  protective  thick-walled  cells  elsewhere;  h,  epidermal  hairs;  973,  a 
cross  section  through  the  outer  part  of  a  stem  of  an  elder 
(Sambucus  nigra),  showing  a  lenticel  during  the  summer  oi 
its  second  year;  e,  epidermis;  c,  cork  layer  of  the  preceding 
year,  which  has  been  ruptured  at  a  by  the  development 
of  complementary  tissue  (£');  c',  developing  cork  layer 
of  the  current  season ;  x,  chlorenchyma ;  both  figures  highly 
magnified.  —  Fig.  973  from  HABERLANDT. 


rise 

cylinder  of  cork, 
which  cuts  off  com- 
munication between 
the  cortex  and  the 
stomata.  Under- 


neath   some    of    the 

stomata,  however,  cork  does  not  develop  for  a  time,  the  phellogen 
giving  rise  instead  to  a  loose  tissue  composed  of  rounded  cells,  the 
complementary  cells,  between  which  are  conspicuous  air  spaces  (figs. 
972,  973).  This  tissue,  by  reason  of  the  cell  turgidity  and  the  air 
spaces,  takes  up  much  more  space  than  does  the  cork,  rarely 


662 


ECOLOGY 


having  adequate  room  for  full  development  beneath  the  epidermis. 
Consequently  the  epidermis  is  soon  ruptured  and  the  complementary 
tissue  protrudes,  forming  with  the  upturned  ruptured  edges  a  char- 
acteristic emergence  visible  to  the  unaided  eye  (figs.  971,  1057- 
1059) ;  the  entire  structure,  of  which  the  loose  complementary  tissue 
forms  the  most  significant  part,  is  known  as  a  lenticel.  Though  lenticels 
in  some  species  remain  open  for  many  years  through  the  continued 
formation  of  complementary  cells  from  phellogen,  in  most  plants  the 

cork  cylinder  eventually  be- 
comes continuous  through 
cork  formation  underneath 
the  lenticels;  often  this  cork 
is  more  permeable  than  ordi- 
nary cork,  on  account  of  the 
presence  of  intercellular  air 
spaces.  In  many  plants 
closure  occurs  in  the  autumn, 
the  lenticels  being  essentially 
cut  off  from  the  cortex  before 
winter.  Such  lenticels  may 
remain  permanently  closed, 
or  the  following  spring  the 
development  of  complemen- 
tary cells  underneath  the  cork 
may  cause  it  to  burst,  thus 
reopening  the  lenticels  to  gas 
exchange.  In  most  woody 
plants  the  lenticels  are  round- 
ish or  slightly  elongated  struc- 
tures which  disappear  after  a 

few  years  by  reason  of  bark  exfoliation,  but  in  the  birches  and  in 
some  cherries  they  remain  for  many  years,  elongating  horizontally  as 
the  trunk  increases  in  diameter  (fig.  974).  In  some  woody  stems,  as 
in  the  grape,  lenticels  do  not  occur. 

The  causes  of  lenticel  development.  —  The  development  of  cork  from 
phellogen  is  favored  by  desiccation  (p.  706),  while  the  presence  of  abun- 
dant water  causes  the  development  of  a  loose  tissue  with  prominent 
air  spaces,  the  aerenchyma  (p.  553).  The  complementary  tissue  of  lenti- 
cels, like  aerenchyma,  is  derived  from  phellogen,  and  the  cause  of  its  de- 


FIG.  974.  —  Trunks  of  the  paper  birch 
(Betula  alba  papyrifera),  showing  numerous 
transversely  elongated  permanent  lenticels; 
Salisbury,  Conn.  —  Photograph  by  E.  W. 
COWLES. 


STEMS  663 

velopment  has  been  discovered  to  be  similar,  while  subsequent  closure 
through  cork  development  is  due  to  desiccation.  In  some  plants  cork 
and  complementary  tissue  develop  alternately,  the  reason  apparently 
being  that  lenticel  closure  checks  transpiration  and  permits  water  accu- 
mulation and  the  consequent  development  of  complementary  tissue, 
which  bursts  through  the  cork  and  opens  the  lenticel.  Then  active 
transpiration  again  takes  place,  desiccation  ensues,  and  a  cork  layer 
once  more  develops. 

When  a  woody  stem  (as  in  the  willow)  is  submerged,  the  tissues  usually  be- 
come surcharged  with  water,  and  complementary  cells  develop  from  the  phellogen  to 
such  an  extent  that  they  burst  th  ough  the  bark  (particularly  at  the  lenticels,  be- 
cause they  are  the  points  of  least  resistance),  forming  whitish  patches,  the  so-called 
•water  lenticels,  which  differ  from  ordinary  lenticels  only  in  that  the  greater  water 
supply  causes  larger  emergences. 

The  rdle  of  lenticels.  —  Lenticels  are  regions  of  gas  exchange,  taking 
the  place  of  stomata  in  stems  after  the  inception  of  secondary  growth, 
and  making  possible  the  continued  activity  of  the  chlorophyll  after  cork 
formation  has  begun.  Only  a  somewhat  structureless  organ  such  as  a 
lenticel,  consisting  of  an  indefinite  patch  of  loose  cells,  is  fitted  for  gas 
exchange  in  bark,  where  growth  and  rupture  occur  continually.  Lenti- 
cel closure  involves  a  decrease  in  gas  exchange,  hence  reducing  syn- 
thesis and  perhaps  retarding  respiration;  however,  as  closure  occurs 
chiefly  at  the  inception  of  dry  or  cold  weather,  it  is  likely  that  the  gain 
from  the  reduction  of  transpiration  is  greater  than  the  loss  resulting 
from  decreased  synthesis.  Probably  the  small  amount  of  oxygen  used 
in  respiration  is  obtained  in  part  through  the  bark,  which  is  not  wholly 
impermeable;  the  lenticels,  even  when  closed,  are  more  permeable 
than  other  regions  of  the  bark. 

After  the  lenticels  disappear,  the  bark  continues  to  increase  in  thickness,  gas  ex- 
change becoming  less  and  less,  until  it  ceases  to  be  appreciable,  except,  perhaps, 
beneath  furrows,  where  the  protective  layers  are  thin  and  frequently  ruptured  by 
stem  enlargement.  It  has  been  suggested,  however,  that  the  death  of  trees  partly 
submerged  by  water  or  by  dune  sand  is  due  to  the  checking  of  stem  respiration,  thus 
assuming  that  gas  exchange  through  bark  may  not  be  inconsequential,  though  in 
such  cases  death  may  be  due  to  the  cutting  off  of  oxygen  from  the  roots;  in  any 
event,  continued  vigor  characterizes  trees  like  the  willows  and  poplars,  in  which  sub- 
mergence by  water  or  sand  incites  the  development  of  adventitious  roots  on  the 
buried  stems,  while  oaks  and  pines,  having  no  capacity  to  develop  such  adven- 
titious roots,  soon  die. 

The  distribution  of  lenticels.  —  No  systematic  study  has  been  made  of  lenticel 
distribution.  Usually  lenticels  are  most  abundant  just  beneath  the  nodes.  In  some 


ECOLOGY 


FIG.  975.  —  Ephedra  tnfurca,  a  desert  switch  plant,  with  numerous  green  leafless 
switchlike  branches;  the  structures  on  the  branches  are  reproductive  organs;  Mesilla, 
New  Mexico  —  Photograph  supplied  by  LAND. 


STEMS 


665 


trees,  as  Gleditsia,  they  are  more  numerous  on  the  under  side  of  horizontal  branches, 
while  occurring  equally  on  all  sides  of  vertical  branches. 

Carbohydrate  synthesis  in  leafless  stems.  —  From  the  standpoint  of 
synthesis  the  most  important  stems  are  those  on  which  leaves  are  insig- 
nificant or  wholly  absent,  for  here  the  stem  becomes  the  chief  food- 
making  organ.  This  habit  is  well  illustrated  by  the  cacti,  whose  stems 


FlG.  976.  —  A  colony  of  bulrushes  (Scirpus  validus) ;  though  the  habit  of  an  individual 
shoot  is  that  of  an  extreme  xerophyte  well  fitted  for  protection  from  intense  light,  a  crowded 
colony  of  such  shoots  is  fitted  for  optimum  lighting;  Selkirk,  Manitoba. —  Photograph 
by  E.  W.  COWLES. 

may  be  flattened  (Opuntia,  figs.  1040-1042),  cylindrical  (Cereus,  fig. 
1035),  or  spherical  (Echinocactus,  fig.  1063);  the  stem  of  Echinocactus 
with  its  small  surface  in  proportion  to  its  volume  represents  the  extreme 
antithesis  of  thejorest  mesophyte  with  its  expanded  leaves.  Such  stems 
contrast  with  ordinary  leaf-bearing  green  stems  in  possessing  strong 
palisades  in  place  of  weak  palisades  or  sponge  tissue.  The  chlorophyll 
also  extends  much  deeper  than  in  most  stems  and  leaves,  reaching  the 


666 


ECOLOGY 


great  depth  of  6.5  mm.  in  Cereus  giganteus.  Synthetically  comparable  to 
cacti  are  many  switch  plants,  with  numerous  leafless  switchlike  stems 
(as  in  Ephedra,  fig.  975).  Some  switch  plants,  as  Spartium  and  Cy- 
tisusj  have  small  leaves,  which  have  been  shown  to  equal  or  surpass  the 
stems  in  synthetic  activity. 

Asparagus  and  Casuarina  have  numerous  slender  branches,  which  give  the  aspect 
of  delicate  foliage.  Equisetum  (figs.  1054,  1055)  is  a  characteristic  leafless  herb. 
In  Muehlenbeckia  the  stems,  though  vertical,  are  much  flattened,  and  in  Myrsiphyl- 
lum,  Ruscus,  and  Phyllodadus  they  quite  resemble  leaves  and  are  called  phylloclades. 
Many  such  plants  have  prominent  leaves  in  the  seedling  stages,  suggesting,  accord- 
ing to  the  recapitulation  theory,  that  various  leafless  xerophytes  may  have  been 
derived  from  a  mesophytic  leafy  ancestry;  the  phylloclade  forms  have  been  thought 
to  represent  a  subsequent  return  to  more  mesophytic  structures. 

Not  all  leafless  stems  are  xerophytic.  Such  representative  swamp 
plants  as  the  rushes  (Juncus,  Scirpus,  Eleocharis)  often  are  essentially 
leafless,  the  synthesis  of  foods  being  here  a  stem  function,  as  in  the 
cacti  (fig.  976).  In  their  high  cutinization  and  prominent  palisades 

such  plants  resemble  xerophytes, 
but  they  are  quite  unlike  them  in 
their  abundance  of  air  spaces  and 
in  their  high  transpiration .  It  has 
been  suggested  that  their  ex- 
posure to  intense  light,  reflected 
as  well  as  direct,  makes  vertical- 
ity  almost  as  advantageous  as 
in  xerophytic  habitats.  It  is 
much  more  likely  that  in  rushes, 
as  in  swamp  grasses  and  in  flags, 
verticality  is  advantageous,  be- 
cause it  permits  a  maximum 
display  to  light  where  growth  is 
dense.  Whatever  may  be  the 
causes  or  advantages,  it  certainly 
is  striking  that  leafless  stems 
FIG.  977. -The  upper  part  of  a  gametophytic  with  a  relative  maximum  of  stem 

stem  of  Sphagnum,  above  which  is  displayed  a  synthesis  occur  in  such  opposite 

terminal  cluster  of  sporophytes  with  their  spore-  habitats  as  deserts  and  swamps, 
bearing  organs;  note  the  descending  gameto-  .  .  . 

phytic  branches,  which  facilitate  the  ascent  of  and  that  the  vertical  hablt  whlch 

water  by  capillarity.— From  COULTER  (Part  I),     means  minimum  light  exposure 


STEMS  667 

and  maximum  protection  for  the  desert  individual  means  maximum  light 
exposure  for  the  mass  of  vegetation  in  the  swamp. 

The  display  of  reproductive  organs  by  sterns.  —  In  the  lower  groups, 
stems  seem  to  be  associated  with  the  display  of  reproductive  rather  than 
synthetic  organs,  the  obvious  advantage  being  the  facilitation  of  spore 
dispersal  by  wind  and  by  other  agents.  In  the  fungi,  where  carbohy- 
drate synthesis  does  not  take  place,  there  are  prominent  stalks  or  stipes 
tipped  by  the  spore-bearing  organs  (as  in  various  molds  and  toadstools, 
figs.  1078,  1 122,  197).  While  the  leafy  shoots  and  thalli  of  liver- 
worts generally  are  closely  appressed  to  the  substratum,  most  species 
have  stalked  spore-bearing  organs  that  facilitate  dispersal  (fig.  235). 
In  the  mosses  the  organ  (seta)  that  bears  the  capsule  with  its  asexual 
spores  is  elevated  above  the  rest  of  the  plant  (fig.  977).  In  the  pterido- 
phytes  the  sporangia  may  be  borne  on  ordinary  leaves  or  on  special 
leaves  or  stems  (as  in  Osmunda,  Equisetum,  Lycopodium,  fig.  266).  In 
the  seed  plants  the  display  of  reproductive  organs  may  be  still  more 
advantageous  than  in  the  lower  groups,  by  reason  of  the  important 
part  played  by  insects  in  pollination  and  by  birds  in  seed  dispersal. 
Even  rosette  plants  (as  Taraxacum  and  Agave,  figs.  869,  922)  usually 
have  tall  stalks  on  which  the  reproductive  organs  are  borne.  In  some 
aquatics  the  flowers  are  erected  above  the  water  on  special  stems. 
Colorless  seed  plants  (as  Monotropa)  resemble  fungi  in  having  stems 
that  display  only  reproductive  organs. 

2.  STEMS  AS  REPRODUCTIVE  ORGANS 

General  remarks.  — Plants  spread  in  various  ways,  involving  ordinary 
vegetative  organs  or  specialized  organs,  such  as  seeds  and  spores. 
While  the  latter  are  the  more  effective  dispersing  agents  over  great  dis- 
tances, vegetative  reproduction  is  much  more  effective  as  a  means  of 
occupying  the  ground  in  the  immediate  vicinity  of  the  parent  plant. 
Among  such  organs  of  vegetative  reproduction  stems  take  the  foremost 
place. 

Rhizomes  or  rootstocks.  —  General  features.  —  Rhizomes  or  root- 
stocks  are  horizontally  elongated  underground  stems,  which  occasionally 
compose  the  entire  stem  system  of  the  plant  (as  in  various  violets  and  in 
most  ferns),  but  which  more  commonly  give  rise  to  erect  annual  stems 
that  bear  foliage  leaves  and  flowers  (fig.  978).  In  the  latter  case  the 
rhizome  bears  only  scale  leaves  (figs.  979,  980),  in  whose  axils  erect 


668 


ECOLOGY 


stems  may  originate,  while  in  the  former,  foliage  leaves  and  even  flowers 
(as  in  some  violets)  issue  directly  from  the  rhizome.  In  some  species 
of  Viola  and  Polygala  underground  flowers  develop  from  rhizomes 
(fig.  1191). 

The  reactions  of  rhizomes  to  changes  of  soil  level.  —  The  horizontality 
of  rhizomes  has  led  to  the  general  conception  that  they  are  diageotropic 
organs,  as  probably  is  the  case  under  ordinary  conditions.  How- 
ever, rhizome  ac- 
tivities are  too 
complicated  to  be 
accounted  for 
solely  by  diageo- 
tropism.  The  un- 
derground stems 
of  most  species 
have  a  definite 
position  in  the 
soil,  varying  from 
a  considerable 


FIG.  978.  —  The  horizontal  underground  stem  (rhizome  or 
rootstock)  of  a  false  Solomon's  seal  (Smilacina  stellatd) ;  note  the 
erect  stalk  of  the  current  season  (e)  and  the  bud  (£)  which  gives 
rise  to  a  similar  erect  shoot  the  following  season ;  note  also  the 
lateral  branch  (£>'),  the  beginning  of  another  potential  plant;  r, 
adventitious  roots. 


depth,  as  in  Equi- 
setum,  Asparagus, 
and  many  xero- 
phytes,  to  a  slight 
depth,  as  in  Jun- 

cus  balticus  and  many  swamp  plants  and  mesophytes.  When  a  rhizo- 
matous  plant  is  transplanted,  it  grows  up  or  down  to  its  specific 
soil  level,  thenceforth  growing  horizontally.  If  soil  is  added,  the 
rhizome  begins  to  ascend,  sometimes  almost  vertically,  as  when  Juncus 
balticus  is  submerged  by  a  dune  (fig.  982);  if  soil  is  removed,  the 
rhizome  descends.  Thus  a  rhizome  grows  parallel  to  the  soil  surface; 
this  phenomenon  is  known  as  maintenance  of  soil  position  and  is  said 
to  illustrate  the  law  of  level.  The  cause  of  this  extraordinary  behavior 
is  complex  but  it  is  believed  that  the  nature  of  the  geotropic  reaction 
varies  with  the  depth.  Experiments  on  Polygonatum  show  that  the 
distance  separating  the  rhizome  from  the  place  where  the  aerial  shoot 
emerges  into  the  light  is  the  chief  depth-determining  factor;  if  .the 
aerial  shoot  is  obliged  to  pass  through  a  darkened  layer  of  air,  the 
rhizome  ascends,  precisely  as  if  a  soil  layer  of  equal  depth  were  added. 
Variations  in  the  soil  moisture  or  in  the  oxygen  content  of  the  soil  also 


STEMS 


669 


may  aid  in  determining  the  position  of  rhizomes.  Furthermore,  be- 
havior may  vary  with  the  season,  stems  often  appearing  progeotropic  or 
diageotropic  in  the  autumn  (fig.  981)  and  apogeotropic  in  the  spring 
(figs.  979,  980).  Experiments  have  shown  that  many  shoots  are  diageo- 
tropic at  low  temperatures  and  apogeotropic  at  high  temperatures. 
Obviously,  progeotropic  and  diageotropic  reactions  result  in  increased 


FIGS.  979,  980.  —  Rhizomes  of  the  cord  grass  (Spartina  cynosur aides):  979,  the  basal 
portion  of  a  shoot,  as  seen  in  summer,  showing  the  origination  of  new  rhizomes  (r),  which 
are  completely  ensheathed  by  overlapping  scale  leaves  (s) ;  980,  a  similar  shoot,  as  seen  early 
in  the  following  spring;  note  the  sharp  change  in  the  growth  direction  of  the  rhizomes 
(r')  from  progeotropic  to  apogeotropic. 

protection  from  transpiration  and  from  cold.  In  Circaea  and  in  Sola- 
num  tuberosum,  the  rhizome  develops  into  an  erect  shoot,  when  the  aerial 
stem  is  removed.  This  experiment  appears  to  show  that  such  rhizomes 
are  apogeotropic  organs  whose  customary  horizontality  is  due  to  the 
inhibition  of  verticality  by  the  erect  stem,  much  as  the  growth  direc- 
tions of  lateral  stems  and  roots  are  due  to  the  inhibition  of  verticality 
by  the  primary  organs. 

Linear  and  radial  migration.  —  In  the  simplest  rhizomes,  branching  is 
comparatively  slight,  and  each  season  there  is  a  new  growth  of  several 


670 


ECOLOGY 


centimeters  at  the  anterior  end,  while  at  the  posterior  end  a  correspond- 
ing portion  may  decay.  Thus  the  plant  occupies  a  new  position  in  the 
soil  each  year,  and  if  the  advance  is  in  lines,  as  in  Polygonatum  and  in 
Juncus  balticus,  the  phenomenon  is  known  as  linear  migration.  In 
Juncus  several  erect  shoots  develop  in  a  line  each  year,  making  linear  mi- 
gration very  obvious,  even  to  the  casual  observer.  In  Solomon's  seal 
(Polygonatum,  fig.  983)  a  single  erect  shoot  develops  each  year,  and,  as 
it  leaves  a  definite  scar,  it  is  possible  to  determine  the  age  of  the  un- 


FIG.  981. — The  basal  portion  of  a 
shoot  of  the  ditch  stonecrop  (Penthorum 
sedoides),  showing  the  development  of  new 
rhizomes;  note  that  the  upper  rhizomes 
(r.  r')  are  more  pronouncedly  progeotropic 
than  are  the  lower  rhizomes  (r");  s,  scale 
leaves. 


FIG.  982.  — An  obliquely  ascend- 
ing rhizome  of  Juncus  balticus, 
illustrating  the  reaction  of  this 
species  to  sand  submergence;  st 
scale  leaves.  —  From  SCHOLZ. 


decayed  portion  of  the  rhizome ;  it  is  possible  also  to  learn  something  of 
the  life  conditions  of  each  season  on  account  of  variations  in  the  annual 
increment.  More  commonly  the  radiation  and  branching  of  rhizomes 
in  all  directions  from  the  original  center  result  when  isolated  in  sym- 
metrical colonies  of  ever  increasing  circumference.  Sometimes  (as  in 
the  flags)  the  death  of  the  older  portions  in  the  interior  of  such  a  colony 
results  in  the  formation  of  a  ring,  comparable  to  the  "  fairy  rings  "  of 
various  fungi  (p.  807),  though  continued  branching  is  more  likely  to 
keep  the  entire  space  occupied  (fig.  984). 

The  role  of  rhizomes.  —  The  rhizome  habit,  perhaps  more  than  any 
other,  facilitates  the  occupation  of  space  by  plants,  especially  because 


STEMS 


671 


adventitious  roots  and  erect  shoots  develop  at  various  points  on  the 
radiating  stems.  The  older  parts  gradually  decay,  so  that  the  branches 
become  isolated  as  separate  plants;  consequently  there  is  an  increase  in 
the  number  of  individuals  as  well  as  in  the  space  occupied.  In  this 
phenomenon  of  vegetative  reproduction,  however,  it  is  not  a  matter  of 
particular  importance  whether  or  not  the  actual  number  of  individuals  is 
increased  by  isolation.  The  important  matter  is  the  occupation  of  new 
space,  for  in  any  case  a  rhizome  colony  or  rhizome  complex  represents  a 
number  of  potential  individuals,  as 
is  well  shown  after  the  plowing  of 
a  field  partially  occupied  by  such 
plants  (e.g.  the  couch  grass,Agropyrum 
repens) ;  the  rhizomes  are  dislodged 
and  broken  and  the  scattered  frag- 
ments form  new  centers  of  migration. 
The  great  advantage  of  rhizomes  as 
organs  of  propagation  is  due  partly 
to  their  horizontality,  partly  to  their 
elongation,  and  partly  to  the  fact  that 
they  are  soil  structures  and  thus  are 
able  to  invade  regions  already  oc- 
cupied. Seeds  fall  in  numbers  every- 
where, but  hundreds  die  where  one  Fig.  983.— A  rhizome  of  the  Solomon's 

develops,     because    of     the    difficulty     seal  (Polygonatum  biflorum)\   note  the 


tuberous  enlargements  of  the  rhizome 
with  the  conspicuous  scars  (s)  left  by 
the  fall  of  the  erect  stems  of  previous 
seasons;  r,  adventitious  roots;  e.  erect 
stem  of  the  current  season. 


of  striking  root  in  ground  already 
preempted;  even  plants  with  runners 
propagate  with  difficulty  where  vege- 
tation is  dense.  Rhizomes,  however, 
penetrate  the  soil  of  forests  or  of  grasslands  scarcely  less  readily  than 
that  of  open  grounds.  The  advantage  of  the  rhizome  habit  is  well 
illustrated  in  fields  that  have  lain  fallow ;  during  the  first  year,  annuals 
usually  dominate,  but  little  by  little  rhizome  plants  and  other  perennials 
increase  their  area,  invading  the  space  previously  occupied  by  annuals, 
until  finally  all  of  the  latter  are  excluded. 

Often  it  is  thought  that  plants  exhaust  the  soil  in  which  they  grow,  and  therefore 
that  rhizomes  are  additionally  advantageous  in  that  the  plant  has  a  means  of  mi- 
grating into  new  soil  richer  in  food  materials.  However,  it  is  now  known  that 
plants  rarely  exhaust  the  soil,  at  least  in  nature.  It  might  be  supposed  that  migra- 
tion is  advantageous  as  a  means  of  withdrawal  from  soil  regions  in  which  deleterious 


672  ECOLOGY 

root  excretions  have  accumulated,  but  the  persistent  occupation  of  the  same  spot 
makes  this  view  rather  untenable,  except,  possibly,  for  those  rhizomes  or  rhizome 
colonies  (e.g.  in  Polygonatuni)  which  migrate  as  a  whole  into  new  areas;  this  con- 
sideration weighs  equally  against  the  soil  exhaustion  theory.  Maintenance  of  soil 
level  probably  is  advantageous,  since  great  depth  interferes  with  the  development 
of  aerial  organs,  while  extreme  shallowness  would  mean  lessened  soil  protection  and 
greater  difficulty  in  the  invasion  of  new  areas,  because  of  the  increasing  occupation 
of  space  towards  the  surface.  Rhizomes  are  advantageous  further  as  organs  of 
persistence  through  unfavorable  seasons  (p.  716)  and  as  organs  of  food  accumula- 
tion; in  Psilotum  and  in  Corallorhiza  they  replace  roots,  and  even  may  bear  "  root 
hairs." 

Runners.  —  General  features.  —  Runners  are  horizontal  stems  at  or 
above  the  ground  level,  taking  root  in  the  soil,  and  differing  from 


FIG.  984.  —  A  plant  of  the  creeping  juniper  (Jumperus  horizontalis\  illustrating  radial 
migration;  horizontal  branches  advance  in  all  directions,  rooting  in  the  sand;  in  the 
background  are  numerous  plants  of  Artemisia;  Waukegan,  111.  —  Photograph  by 

McCALLUM. 


rhizomes  chiefly  in  not  being  subterranean.  Stolons  are  essentially 
identical  with  runners,  though  the  term  sometimes  is  applied  to  certain 
rhizomes.  Runners  usually  are  much  slenderer  than  rhizomes,  and  often 
their  internodes  are  greatly  elongated;  chlorophyll  commonly  is  present. 
If  runners  come  into  contact  with  moist  soil,  roots  and  buds  develop 
at  the  nodes,  thus  giving  rise  to  potential  or  actual  new  plants  (fig.  985). 
A  representative  runner  is  that  of  the  strawberry  in  which  there  is  a  con- 
tinuation of  horizontal  elongation  accompanied  by  repeated  rooting, 
thus  giving  rise  to  a  number  of  potential  plants.  Sometimes  (as  in 
Sempervivum  and  Saxifraga)  the  runner  ceases  to  elongate  after  a  bud 
has  developed;  in  such  cases  the  term  offset  may  be  used  (fig.  1165). 
Creeping  stems  (as  in  white  clover)  are  runners  which  lie  close  to  the 


STEMS 


673 


ground,  rooting  copiously  at  the  fre- 
quent nodes  (fig.  712).  Also  to  be 
classed  with  runners  are  such  stems 
as  those  of  Decodon  and  Rubus  which 
bend  over  and  take  root  if  the  tip 
comes  into  contact  with  moist  soil. 
Prostrate  stems  differ  from  runners 
in  not  taking  root,  though  stems 
classed  as  prostrate  (Arclostaphylos, 
Juniperus  horizontalis,  fig.  984)  fre- 
quently develop  roots  under  favor- 
able conditions.  When  the  stems 
of  Decodon  or  Rubus  bend  over  and 
root  at  the  tip,  probably  it  is  because 
there  is  not  sufficient  mechanical 
tissue  to  hold  the  stems  erect.  Pos- 
sibly the  same  is  true  of  many 
runners,  since  they  often  appear 
phophototropic  at  the  outset.  In 
any  case,  the  development  of  ad- 
ventitious roots  is  likely  thenceforth 
to  cause  a  downward  pull  on  the 
older  parts  of  the  runner.  Ne- 
phrolepis  is  somewhat  unique  in 
possessing  both  rhizomes  and  run- 
ners; the  rhizomes  develop  first 
and  give  rise  freely  to  adventitious 
roots.  The  runners  are  peculiar 
leafless  organs  without  a  conspicuous 
role;  if  they  are  removed,  some  of 
the  rhizomes  develop  into  aerial 
shoots. 

The  advantages  of  runners. — Run- 
ners compare  favorably  with  rhi- 
zomes as  organs  of  vegetative  re- 
production, as  may  be  seen  in  the 
development  of  a  colony  of  straw- 
berries or  of  white  clover.  They 
commonly  elongate  more  than  do 


985 


FIGS.  985-988.  — Satureja  glabra:  985. 
the  basal  portion  of  a  plant,  showing  a 
runner  (s)  in  which  all  of  the  leaves  appear 
to  issue  from  the  upper  side,  because  of 
stem  twisting  and  petiole  curvature;  at  the 
nodes  adventitious  roots  (r)  issue  from  the 
under  side  of  the  runner,  fastening  it  to 
the  ground ;  note  the  unfastened  ascending 
tip  (/);  if  the  runner  is  severed  at  a.  a',  or 
a",  the  rooted  portions  develop  into  in- 
dependent plants;  986.  a  short,  broad  leaf, 
characteristic  of  a  runner,  developed 
especially  in  the  autumn,  and  remain- 
ing over  winter;  987,  an  elongated,  nar- 
row leaf,  characteristic  of  the  erect  stem; 
988,  a  short,  narrow  leaf,  characteristic 
of  the  floral  region :  986-988  equally  mag- 
nified. 


674 


ECOLOGY 


rhizomes,  possibly  because 
resistance;  such  elongation 


FIG.  989.  —  A  tuber  of  the 
potato  (Solanum  tuberosum) ;  note 
the  remains  of  the  rhizome  (r)  at 
the  end  of  which  the  tuber  grew 
and  of  which  it  is  merely  an  en- 
largement; the  "  eyes  "  (e)  are 
buds,  some  or  all  of  which  sprout 
and  develop  into  stems,  when  the 
tuber  germinates. 


the  medium  in  which  they  grow  offers  less 
favors  more  rapid  migration  and  the  develop- 
ment of  new  individuals  at  a  greater  distance 
from  the  old.  Creeping  stems  appear  to  be 
almost  as  advantageous  as  rhizomes,  since 
they  grow  close  to  the  ground  in  a  position 
favorable  for  the  development  of  the  adven- 
titious roots  that  are  necessary  for  propa- 
gation. In  Decodon  and  Rubus,  and  even 
in  the  strawberry,  it  is  much  more  difficult 
for  the  stems  to  come  into  contact  with  the 
ground.  While  most  creeping  stems  re- 
main alive  for  some  years,  as  do  rhizomes, 
the  internodes  of  strawberry  runners  soon 
die,  so  that  the  new  potential  plants  soon 
become  actual  individuals. 


Tubers,  bulbs,  and  corms.  —  Tubers.  —  A  tuber  is  essentially  a  rhizome  in  which 
elongation  is  replaced  by  stem  enlargement.,  as  in  the  potato,  in  which  the  under- 
ground stem  begins  as  a  rhizome,  but  later  develops  at  the  growing 
tip  into  a  tuber  (fig.  989).  In  Sculellaria  parvula  and  in  Juncus 
Torreyi  the  rhizome  alternately  elongates  and  enlarges,  thus  de- 
veloping a  beadlike  (moniliform)  chain  of  tubers  (figs.  990,  983, 
1069).  In  nature  each  tuber  usually  gives  rise  to  an  erect  shoot, 
forming  one  potential  plant ;  or  an  actual  plant  may  result,  if  the 
rhizome  portions  die,  as  in  the  potato.  As  a  matter  of  fact,  tubers 
represent  at  least  as  many  potential  plants  as  there  are  buds  or 
"eyes,"  potato  tubers 
commonly  being  cut 
into  several  pieces 
for  planting.  Even 
in  tuber  fragments 

FIG.  990.  —  The  rhizome  (f)  of  a  rush  (Juncus  Torreyi) 
with  tuberized  portions  (*),  from  which  the  erect  shoots  arise; 
e,  the  erect  stem  of  the  current  season;  e',  e"t  the  remnants 
of  similar  shoots  of  former  seasons. 

I 

without  "  eyes  "  a  bud  may  organize  and  grow  into  a  plant.  In  one  of  the 
yams  (Dioscorea  saliva),  there  are.  aerial  tubers,  which  are  of  importance  in 
reproduction. 

Bulbs.  —  Bulbs  are  underground  stems  differing  from  tubers  and  rhizomes  in 
their  almost  total  lack  of  stem  elongation,  and  in  the  fact  that  the  main  axis  is  ver- 
tical and  is  enclosed  by  a  number  of  relatively  large  overlapping  scale  leaves  in 


STEMS 


675 


which  food  accumulates  (as  in  the  lilies,  fig.  991)  Sometimes,  as  in  the  tiger  lily 
(fig.  992)  and  in  Lycopodium,  bulbs  develop  on  aerial  stems;  in  some  species  of 
Sedum,  ordinary  axillary  buds  drop  off  and  develop  into  independent  plants.  Bulbs 
usually  give  rise  to  a  single  potential  plant. 

Corms.  —  Corms  usually  agree  with  bulbs  in  having  vertical  orientation,  slight 
stem  elongation,  and  prominent  apical  buds,  while  they  agree  with  tubers  in  their 
inconspicuous  scale  leaves  and  in  the  accumulation  of  food  in  the  stem  (fig.  993). 
Transitions  exist  between  the  various  kinds  of  under- 
ground stems,  for  example,  in  Trillium. 

The  advantages  of  tubers,  corms,  and  bulbs.  —  Tubers, 
corms,  and  bulbs  are  much  inferior  to  rhizomes  and  run- 
ners as  propagative  organs,  particularly  because  their  slight 
elongation  limits  the  number  of  potential  plants  developed 
and  permits  but  slight  migration  from  the  original  center. 
The  advantages  of  these  organs  are  found  rather  in  food 
accumulation  (p.  719)  and  in  protection  (p.  716).  When 
a  tuber  (as  in  the  potato)  or  a  bulb  (as  in  Erythronium) 
is  borne  at  the  end  of  a  rhizome,  the  possibilities  of 
effective  migration  and  reproduction  are  unsurpassed,  but 
this  is  due  chiefly  to  the  rhizome  element  in  the  under- 
ground stem,  the  tuberous  or  bulbous  portion  being  of 
significance  mainly  in  furnishing  an  abundant  supply  of 


FIG.  99 1. — A  median 
longitudinal  section 
through  the  bulb  of  a 
hyacinth  (Hyacinthus 
onentalis),  showing  the 
overlapping  scale  leaves 
(s)  which  compose  the 
bulb;  the  innermost 
leaf  primordia  develop 
into  foliage  leaves  (/); 
/,  flower  stalk ;  (r),  ad- 
ventitious roots. 


FIG.  992.  —  A  portion  of  a  shoot  of  the  tiger  lily  (Lilium 
tigrinum),  showing  the  bulbils  (b)  which  develop  in  the  axils 
of  sessile  foliage  leaves;  these  bulbils  readily  become  detached 
and  fall  to  the  ground. 


food  to  the  new  plant.  Tubers,  corms,  and  bulbs,  like  rhizomes,  have  a  definite 
soil  level  varying  with  the  species,  the  largest  forms  commonly  having  the  greatest 
depth.  Because  of  their  slight  migratory  powers  it  takes  these  organs  much  longer 
than  rhizomes  to  reach  the  proper  level  when  displaced ;  however,  in  such  forms  as 
Erythronium  the  rhizomes  (sometimes  called  droppers')  which  bear  the  new  bulbs 
grow  up  or  down  as  the  case  may  be,  placing  the  new  plant  at  the  proper  level  (figs. 
718,  719).  In  Erythronium  albidum  there  has  been  recorded  a  descent  of  ten 
centimeters  in  one  season.  The  soil  position  of  tubers,  corms,  and  bulbs  is  be- 
lieved to  depend  chiefly  upon  their  distance  from  the  synthetic  organs,  and  to  a 
smaller  degree  upon  variations  in  soil  moisture. 


ECOLOGY 


Land  plants  with  little  capacity  for  vegetative  reproduction.  —  Many  herbs  with 
persistent  primary  roots  (e.g.  dock,  dandelion,  vervain)  die  down  to  the  ground  in 
autumn,  appearing  essentially  stemless  in  winter.  In  reality  there  is  a  short  thick 
stem  which  elongates  and  enlarges  slightly  each  year;  at  first  only  one  bud  is  formed, 
growing  into  a  single  leafy  shoot,  but,  as  the  size  increases,  a  number  of  buds  are 

formed,  growing  into  several  leafy  shoots. 
Such  a  perennial  stem  is  known  as 
multicipital  (figs.  994-996,  716,  717). 
Plants  with  multicipital  stems  do  not 
migrate,  and  vegetative  reproduction  is 
very  limited,  on  account  of  the  lack  of 
lateral  ground  stems  with  adventitious 
roots;  new  stem  increments  are  as  de- 
pendent as  the  old  upon  the  persistent 
primary  root.  Annual  and  biennial 
herbs,  many  shrubs,  and  most  trees  have 
persistent  primary  roots  and  are  without 
lateral  ground  stems  bearing  adventitious 
roots;  like  multicipital  herbs,  they  have 
little  or  no  capacity  for  vegetative  repro- 
duction. While  some  trees  exhibit  propa- 
gation by  roots  (p.  505),  others  (as  the 
linden  and  the  redwood)  produce  suckers 
at  the  base,  thus  resembling  multicipital 
herbs  except  in  the  persistence  of  the 
aerial  stems;  basal  shoots  of  this  sort, 
however,  are  of  little  reproductive  sig- 
nificance. In  various  trees  and  shrubs, 
especially  the  willows,  cuttings  placed  in 
the  soil  develop  into  new  plants;  repro- 
duction of  this  kind  is  rare  in  nature, 
though  employed  artificially  in  many 
plants.  Of  all  the  common  trees  the 


FIG.  993.  — A  plant  of  the  dwarf  ginseng 
(Panax  trifolium),  showing  a  corm  or  solid 
bulb  (c) ;  note  the  whorl  of  three  palmately 
compound  leaves  and  the  umbel  of  pistillate 
flowers  («). 


conifers  have  the  least  capacity  for  vege- 
tative reproduction,  but  a  fallen  Torreya 
tree  develops  adventitious  roots  and  erect 
shoots  along  the  trunk  almost  as  readily 
as  do  the  willows. 


Reproduction  by  stems  in  water  plants.  —  General  features.  — 
Aquatics  rooted  in  the  soil  reproduce  by  underground  stems  and  runners, 
exactly  as  do  land  plants,  except  that  migration  usually  is  more  rapid, 
perhaps  because  of  the  easy  penetration  of  the  oozy  slime  at  the  bottom 
of  ponds.  Some  rhizomatous  plants  (as  Hippuris,  Limnanthemum,  and 
species  of  Potamogeton)  may  advance  as  much  as  a  meter  a  year,  soon 
filling  a  small  pond  (fig.  1165).  The  water  lilies  have  gigantic  rhizomes 


STEMS 


677 


in  which  considerable  surplus  food  accumulates;  the  rhizomes  of  bul- 
rushes, cattails,  and  flags,  though  much  smaller,  are  large  as  compared 
with  most  rhizomes,  yet  they  migrate  with  some  rapidity. 

The  remarkable  capacity  of  water  plants  for  vegetative  reproduction 
is  due  chiefly  to  the  ready  detachability  of  aquatic  stems,  whose  frag- 
ments float  to  a  more  or  less  distant  locality,  where  a  new  growth  center 
is  established.  A  striking  case  of  vegetative  reproduction  among  hydro- 
phytes is  seen  in  Eichhornia  (the  water  hyacinth),  which  in  recent  years 
has  filled  various  streams  in  Florida  to  such  an  extent  as  to  impede  navi- 
gation; another  remarkable  example  is  afforded  by  Elodea  canadensis 


996 


FIGS.  994-996.  —  Multicipital  stems  in  the  dandelion  (Taraxacum  officinale):  994,  a 
young  plant,  showing  a  simple  tap  root  (r)  crowned  with  a  single  rosette  of  leaves;  995, 
an  older  plant,  showing  a  larger  root,  crowned  with  a  multicipital  stem  (s)  with  three  leaf 
rosettes;  996,  a  much  older  plant,  showing  how  a  dandelion  plant  eventually  may  break 
up  into  several  individuals. 

(often  called  the  water  pest,  because  of  its  rapid  filling  of  ponds),  which, 
from  a  single  plant  introduced  from  America,  spread  over  Europe 
within  half  a  century ;  this  rapid  spread  probably  was  due  entirely  to 
vegetative  reproduction,  since  only  pistillate  plants  of  Elodea  are  known 
in  Europe.  In  Elodea,  spreading  is  due  to  the  fragmentation  of  ordi- 
nary shoots,  but  in  Eichhornia  there  are  aquatic  runners  which  give  rise 
to  readily  detachable  new  plants.  Another  rapid  spreader  is  the  water 
cress,  which  soon  fills  a  spring  brook  when  planted  at  the  head.  In  the 
duckweeds  new  thalli  develop  from  the  old  (figs.  997,  727),  becoming  de- 
tached with  such  rapidity  that  the  vegetative  offspring  of  a  few  plants 
may  fill  a  small  pond  in  a  short  time.  In  rivers,  migration  is,  of 
course,  much  more  rapid  downstream  than  up,  yet  water  plants  may 


678 


ECOLOGY 


advance  upstream,  and  even  may  migrate  from  one  river  or  pond  to 
another;  wind  and  fish  may  be  agents  of  dispersal  upstream,  and  it  is 
the  current  belief  that  the  feet  of  wading  birds  are  important  agents 
in  carrying  plant  fragments  from  one  pond  to  another.  Clearly  the 
culmination  of  vegetative  reproduction  among  seed  plants  is  to  be  seen 
in  fragmenting  hydrophytes,  where  stem  detachability  facilitates  dis- 
persal as  much  as  does  seed  production. 

Winter  buds.  —  In  many  aquatics  there  develop  special  winter  buds  or  hibernacula, 
which  readily  become  detached  and  drop  to  the  bottom  of  the  pond  because  they  are 
heavier  than  water.  In  the  duckweeds  this  heaviness  is  due  to  the  relative  lack  of 


FIG.  997. — Plants  of  one  of  the  smallest 
of  the  duckweeds  (Wolffia),  whose  body 
is  reduced  to  a  thallus  (/);  note  that 
vegetative  reproduction  occurs  through 
the  development  of  a  bud  (b\  which  be- 
comes detached  at  maturity,  as  at  m; 
note  also  that  a  part  of  the  thallus  is 
below  and  a  part  above  the  water,  the 
former  having  to  do  chiefly  with  the  ab- 
sorption of  water  and  salts  and  the  latter 
with  food -making  and  transpiration ;  con- 
siderably magnified. 


FIGS.  998,  999.  —  Winter  buds  (hiber- 
nacula) of  the  bladderwort  (Uiricularia) : 
998,  a  portion  of  a  shoot  of  the  land  form 
of  Utricularia  intermedia,  showing  a  hori- 
zontal axis,  terminating  in  a  winter  bud 
(h) ;  note  also  that  the  progeotropic  earth 
shoots  have  terminal  winter  buds  (h')  and 
conspicuous  bladders  (6) ;  999,  a  winter 
bud  of  Utricularia  vulgaris.  —  From 
GLUCK. 


air  spaces,  while  in  Utricularia,  it  is  due  to  the  compact  growth  of  either  the  ter- 
minal or  the  lateral  buds,  the  stem  ceasing  to  elongate  and  the  leaves  being  very 
closely  imbricated;  the  whole  forms  a  somewhat  globular  structure  (figs.  998,  999). 
In  spring  most  hibernacula  develop  roots,  and  the  stem  elongates  into  an  ordinary 
vegetative  shoot;  however,  in  the  duckweeds  and  in  Utricularia,  soil  roots  do  not 
appear,  and  the  developing  plants  soon  become  lighter  than  water  and  rise  to  the 
surface.  In  Utricularia,  winter  buds  may  be  induced  experimentally  at  any  season. 
The  chief  advantage  of  hibernacula  is  the  preservation  of  the  species  over  inclement 
seasons;  however,  they  represent  new  plants  that  have  arisen  vegetatively  from  the 
parent  individual  and  thus  are  reproductive  structures. 


3.  CONDUCTIVE  TISSUES 

General  remarks.  —  Water  and  solutes  may  move  from  any  cell  to  any 
adjoining  cell,  if  the  walls  are  permeable  to  the  diffusing  substances.  In 
most  small  plants  special  conductive  tissues  are  absent ;  also  they  are 


STEMS 


679 


absent  or  poorly  developed  in  submersed  aquatics,  where  the  food- 
making  organs  are  likewise  organs  of  water  absorption.  But  in  all 
large  land  plants,  where  the  organs  of  synthesis 
are  remote  from  the  organs  of  absorption,  con- 
ductive tissues  are  well  developed.  The  sub- 
stances that  migrate  from  cell  to  cell  are  either 
raw  materials  (water  and  inorganic  salts)  or 
organic  foods  commonly  manufactured  by  the 
plants  themselves,  water  being  quantitatively 
much  the  most  important  migrating  substance. 
These  materials  move  from  regions  of  higher 
to  regions  of  lower  pressure,  water  and  soil  salts 
ascending  from  the  roots  to  the  leaves,  and 
organic  foods  moving  in  various  directions  from 
the  seat  of  manufacture  in  the  leaf. 

The  structure  of  the  conductive  elements.  — 
Tracheids  and  tracheae.  —  When  conductive 
cells  begin  to  differentiate  from  other  cells, 
their  chief  distinctive  feature  is  elongation, 
which  remains  the  most  fundamental  char- 
acteristic common  to  all  conductive  elements. 
The  commonest  conductive  elements  are  tracheae 
(also  called  vessels  or  ducts)  and  tracheids,  which 
together  are  sometimes  called  hydroids,  a  term 
suggestive  of  their  role  in  water  conduction. 
Tracheids  arise  through  the  differentiation  of 
certain  parenchyma  cells  which  elongate  and 
enlarge,  the  walls  also  becoming  lignified  (i.e. 
woody);  at  maturity  they  commonly  are  pro- 
senchymatous ,  that  is,  their  ends  are  pointed, 
owing  to  the  development  of  oblique  walls  from 
originally  transverse  terminal  walls  (figs.  907, 
936).  Tracheae  differ  from  tracheids  in  being 
cell  fusions  or  syncytes,  arising  through  the  re- 
sorption  of  the  end  walls.  Thus  a  tracheid  may 
represent  only  a  stage  in  the  development  of  a 
trachea,  as  in  young  angiosperm  tissue,  all 
transitions  sometimes  being  observed  (figs.  1000- 
1002).  However,  in  ferns  and  conifers  and  in 


FIGS.  1000,  looi.- — A 
portion  of  a  scalariform 
vessel  (trachea)  from  the 
root  of  the  prickly  lettuce 
(Lactuca  scanola):  1000, 
a  longitudinal  view,  show- 
ing how  tracheae  arise 
from  tracheids  through  the 
resorption  of  the  cross 
walls;  note  that  these 
walls  have  been  resorbed 
except  for  rings  of  tissue 
(r)  next  to  the  longitudinal 
walls;  note  also  the  trans- 
versely elongated  pits  (/>), 
characteristic  of  scalari- 
form vessels ;  100 1 ,  a  cross 
section  of  such  a  scalari- 
form vessel;  both  figures 
highly  magnified. 


68o 


ECOLOGY 


the  bundle  termini  of  angiosperm  leaves,  tracheids  usually  remain  as 
such,  that  is,  the  end  walls  are  not  resorbed  (fig.  1003). 

Tracheids  are  much  shorter  than  tracheae,  rarely  exceeding  a  millimeter  in  length, 
though  sometimes  attaining  a  length  of  twelve  centimeters  (as  in  Nelumbo)  or  even 
a  meter  (as  in  some  conifers).  '  Tracheae  rarely  are  longer  than  ten  centimeters, 
though  they  may  attain  a  length  of  one  or  two  meters  in  Quercus  and  three  to  six 
meters  in  lianas  (as  Wistaria)  and  in  Eucalyptus.  While  isolated  tracheids  and 
tracheae  sometimes  occur,  they  usually  are  grouped  in  continuous  strands  traversing 
the  entire  plant  body. 

Individual  tracheids  or  tracheae,  even  within  a  single  conductive  strand,  vary 
widely  in  wall  sculpturing,  owing  to  differential  lignification.  When  the  thickenings 


m 


FIG.  1002.  —  A  diagrammatic  longitudinal  section 
of  a  young  xylem  strand;  c,  cambium: -y,  a  young 
undifferentiated  trachea  with  cross  walls  as  yet  un- 
resorbed;  p,  a  trachea  with  transversely  elongated 
pits;  s,  spiral  tracheae;  o,  an  annular  trachea; 
m,  pith;  highly  magnified.  —  From  BARNES  (Part 
II). 


FIG.  1003.  —  Tracheids  of 
a  gymnosperm  with  bordered 
pits;  highly  magnified. — Alter 
CHAMBERLAIN. 


are  annular  or  spiral,  the  wall  area  largely  remains  thin  (figs.  1002,  1004,  1005). 
In  reticulated  vessels,  the  strengthening  fibers  form  a  network,  and  in  the  pitted  tra- 
cheids and  tracheae,  most  of  the  wall  becomes  thickened,  the  thin  places  appearing  as 
small  pits  (fig.  1006).  Pitted  and  reticulated  vessels  usually  are  larger  than  those 
with  annular  or  spiral  thickenings. .  Still  another  form  of  thickening  is  seen  in  the 
scalariform  vessels,  in  which  transversely  elongated  pits  occur  in  parallel  rows  (fig. 
1000).  Bordered  pits  occur  in  many  plants,  especially  in  the  conifers,  where  they  are 
arranged  in  longitudinal  rows,  a  surface  view  disclosing  two  concentric  rings  or  el- 
lipses (fig.  1003) ;  in  section  there  are  seen  a  thickened  central  portion,  the  torus,  and 


STEMS 


681 


1004 


1005 


1006 


FIGS.  1004-1008. — Vascular  elements  of  an  angiosperm:  1004,  spiral  vessels;  1005, 
spiral  and  annular  vessels;  1006,  a  pitted  vessel;  1007,  a  sieve  tube  with  companion  cells; 
1008,  a  cross  section  of  a  sieve  tube  at  the  sieve  plate,  showing  also  a  companion  cell  in 
cross  section;  1004-1007  are  from  longitudinal  sections  ;  all  figures  highly  magnified.  — 
1004  and  1005  after  BONNIER  and  LECLERC  DU  SABLON;  1006  after  DEBARY;  1007  and 
1008  after  STRASBURGER. 


a 


a  thin  margin,  the  margo,  the  latter  representing  the  space  between  the  rings  as 
seen  in  surface  view.  When  the  thickening  of  tracheids  and  tracheae  is  completed, 
the  cell  contents  die,  after  which  the  lumen  contains  water  and  solutes,  together 
with  numerous  air  bubbles.  In  old  wood  the  lumina  contain  largely  air,  although 
various  excreted  products  often 
accumulate. 


Sieve  tubes  and  conductive 
parenchyma. — Sieve  tubes  are 
syncytes,  occurring  in  con- 
tinuous rows,  like  tracheae, 
but  differing  therefrom  in 
their  thin  cellulose  walls  and 
in  their  living  and  highly 
albuminous  viscous  contents 
(fig.  1007).  They  rarely  ex- 
ceed two  millimeters  in  length, 
in  this  respect  resembling  tra- 
cheids rather  than  tracheae. 
A  unique  feature  of  sieve 


FIG.  1009.  —  Cross  sections  of  sieve  tubes  of 
a  gourd  (Lagenaria  vulgaris),  showing  sieve 
plates  with  large  pores  (a)  and  with  small  pores 
(6);  highly  magnified.  —  From  DEBARY. 


682  ECOLOGY 

tubes,  giving  rise  to  their  name,  is  the  presence  at  their  widest  parts 
of  perforated  oblique  or  transverse  sieve  plates,  which  sometimes  occur 
also  on  the  side  walls  (figs.  1008,  1009).  The  sieve  plates  may  be  en- 
wrapped by  a  highly  refrangible  callus,  which  is  easily  soluble,  disappear- 
ing when  the  cell  contents  are  dilute,  but  reappearing  and  closing  the 
sieve  plate  when  the  contents  become  less  dilute,  as  in  winter. 

In  angiosperms  sieve  tubes  are  accompanied  by  companion  cells,  elements  of 
smaller  caliber,  whose  abundant  cytoplasmic  contents  are  connected  prominently 
with  the  cytoplasm  of  the  sieve  tubes  (figs.  1007,  1008).  Besides  specialized 
elements,  conductive  areas  contain  many  parenchymatous  cells  that  remain  un- 
differentiated  except  for  elongation;  such  cells  make  up  the  so-called  conductive 
parenchyma,  but  they  do  not  differ  essentially  from  cortical  cells. 

Primary  conductive  tissues.  —  The  arrangement  of  conductive  ele- 
ments into  strands  or  bundles.  —  Sometimes  tracheids  occur  as  isolated 
cells  or  idioblasts  (as  in  Salicornia,  fig.  772),  but  in  such  cases  they  are  not 
to  be  regarded  as  conductive  cells.  Sometimes  there  are  simple  strands, 
such  as  isolated  bundles  of  sieve  tubes  and  the  finer  leaf  veins,  which 
are  composed  chiefly  of  tracheids.  But  in  most  cases  all  the  conductive 
elements  are  grouped  into  compound  bundles,  notably  in  the  ferns  and 
seed  plants,  although  suggestions  of  conductive  bundles  as  well  as  of 
conductive  cells  are  found  in  many  non-vascular  plants.  A  strand 
made  up  of  conductive  elements  is  known  as  a  vascular  bundle  or  as 
mestome;  mechanical  elements  usually  are  closely  associated  with  the 
conductive  elements,  the  two  making  up  a  fibrovascular  bundle. 

Xylem  and  phloem.  —  Usually  there  are  two  more  or  less  distinct  re- 
gions within  the  bundle,  namely,  the  xylem  which  contains  tracheae  or 
tracheids,  and  the  phloem  which  contains  sieve  tubes  and  their  asso- 
ciated elements  (fig.  760).  Sometimes  xylem  and  phloem  are  indistin- 
guishable from  one  another,  as  in  certain  hydrophytes.  Both  phloem 
and  xylem  may  contain  mechanical  elements,  and  just  as  the  conductive 
elements  as  a  whole  are  known  as  the  mestome,  so  the  mechanical 
blements  as  a  whole,  either  within  of^wijhout  the  fibrovascular  bundle, 
are  known  as  the  stereome.  The  conductive  portion  of  the  xylem  is 
known  as  hadrome  (or  hydrome),  and  the  conductive  portion  of  the 
phloem  is  known  as  leptome;  for  example,  phloem  may  contain  such 
stereome  elements  as  bast  fibers  and  such  leptome  elements  as  sieve 
tubes,  while  secondary  xylem  may  contain  such  stereome  elements  as 
wood  fibers  and  such  hadrome  elements  as  tracheae  or  tracheids. 


STEMS 


683' 


The  arrangement  of  the  mestome  elements.  —  In  stems  the  hadrome  generally  is 
within  the  leptome.  In  dicotyls  the  mestome  strands  are  arranged  in  a  broken 
cylinder  (fig.  541),  which  later  may  become  a  complete  cylinder  through  cambium 
activity,  while  in  monocotyls  the  bundles  are  scattered,  though  more  abundant  out- 
ward (fig.  550).  The  most  common  stem  arrangement  is  collateral,  the  leptome 
and  hadrome  being  side  by  side  on  the  same  radius,  the  leptome  outermost  (figs. 
541,  550);  in  Cucurbita  the  arrangement  is  bicollateral,  there  being  a  leptome  strand 
inward  from  the  hadrome,  as  well  as  outward.  In  some  plants,  notably  the  pterido- 
phytes,  the  arrangement  of  the  mestome  elements  is  concentric,  the  leptome  commonly 
forming  a  cylinder  about  the  hadrome  (hadrocentric  arrangement,  fig.  1010),  although 
there  are  many  cases,  as 
in  monocotyl  rhizomes, 
where  the  leptome  is 
surrounded  by  hadrome 
(leptocentric  or  amphi- 
vasal  arrangement,  fig. 
551).  In  young  roots 
there  are  alternating 
plates  of  hadrome  and 
leptome  in  the  vascular 
cylinder  (radial  arrange- 
ment, fig.  555);  if  there 
are  three  xylem  rays 
alternating  with  three 
phloem  rays,  the  root 
is  called  triarch,  while 

such  terms   as   tetrarch,         ^  IOI0.  _  A  partial  cross  section  of  a  stem  of 
pentarch,    hexarch,    and     ndla^  showing    a    hadrocentric   (or   xylocentric)   vascular 
polyarch    mean    respec-     bundle;   note  that  the   thick- walled   hadrome   (or   xylem) 
tively,  four,  five,  six,  and     cells  are  surrounded  by  thin-walled  leptome  (or  phloem) 
many  rays  of  both  xylem     cells;  highly  magnified.  —  From  COULTER  (Part  I), 
and  phloem.    In  the  col- 
lateral bundles  of  leaves  -the  hadrome  is  uppermost,  even  in  ferns,  in  spite  of  the 
hadrocentric  arrangement  in  the  stems. 

Sheaths  encircling  the  mestome.  —  In  most  but  not  in  all  cases,  there  are  one  or 
two  sheaths  or  layers  of  cells  surrounding  the  vascular  tract.  The  inner  sheath, 
regarded  as  the  outermost  layer  of  the  vascular  region,  is  known  as  the  pericycle  or 
pericambium  (also  as  the  parenchyma  sheath  or  phloem  sheath),  and  commonly  is 
made  up  of  delicate  parenchyma  cells  (fig.  555).  Outside  the  pericycle,  and  regarded 
as  the  innermost  layer  of  the  cortex,  is  the  endodermis  (also  known  generally  or  in 
special  cases  as  the  protective  sheath,  bundle  sheath,  starch  sheath,  or  phloeo- 
terma,  fig.  555).  The  cells  are  closely  packed,  and  in  roots  and  rhizomes  the  lateral 
and  inner  walls  are  suberized  (i.e.  thickened  with  suberin,  as  in  cork)  and  relar 
tively  impermeable;  in  aerial  stems  the  layer  is  less  definite  and  often  the  celjs 
are  rich  in  starch  (whence  the  name  starch  sheath).  In  some  roots  occasional 
cells  opposite  the  hadrome  plates  remain  unsuberized  for  a  time  and  are  known 
as  passage  cells. 


684 


ECOLOGY 


Secondary  conductive  tissues.  —  The  cambium  ring.  —  Soon  after 
the  formation  of  the  primary  vascular  tissues,  renewed  growth  takes 
place,  especially  in  conifers  and  dicotyls;  this  results  in  the  formation 
of  secondary  tissues,  the  active  element  in  their  formation  being  the  cam- 
bium, a  layer  of  cells  that  retain  a  capacity  for  active  growth  and  hence 
known  as  meristematic  (fig.  ion).  In  addition  to  the  fascicular  cam- 
bium, representing  the  meristematic  progeny  of  the  procambium  from 
which  the  primary  xylem  and  phloem  were 
developed,  there  is  an  interfascicular  cambium 
which  develops  in  the  rays  between  the  primary 
bundles.  The  most  active  division  of  the  cam- 
bium cells  is  tangential,  the  new  cells  arising 
inward  developing  into  secondary  xylem,  and 
.those  .arising  outward  into  secondary  phloem. 
Radial  division  also  takes  place,  resulting  in  the 
merging  of  the  fascicular  and  interfascicular 
cambium  into  the  cambium  ring,  whose  circum- 
ference is  subject  to  constant  enlargement,  owing 
to  the  outward  growth  of  the  secondary  xylem 
cells,  and  to  the  tangential  growth  of  the  ring 
itself.  This  circumferential  enlargement  is 
responsible  for  the  continued  rupture  and 
exfoliation  of  the  bark. 

Mature  secondary  tissues.  —  Secondary  xylem 
remains  as  permanent  tissue  in  shrubs  and 
trees,  thus  bringing  about  an  annual  increase 
in  diameter ;  secondary  phloem  is  relatively 
ephemeral,  being  subject  to  yearly  renewal  within 
and  exfoliation  without.  An  old  tree  has  thicker 
bark  than  a  young  tree,  partly  because  each  year  the  new  secondary 
phloem  a  little  more  than  offsets  the  amount  exfoliated,  and  partly  be- 
cause of  the  activity  of  the  phellogen  (p.  705).  As  in  primary  tissues,  the 
phloem  is  made  up  of  sieve  tubes,  companion  cells,  parenchyma,  and 
mechanical  elements  (mostly  bast  fibers),  while  secondary  xylem  con- 
sists of  tracheids  or  tracheae,  parenchyma,  and  characteristic  mechanical 
elements  known  as  wood  fibers ;  all  gradations  exist  between  these  fibers 
and  ordinary  tracheids  or  tracheae.  Rays  of  parenchymatic  cells,  the 
medullary  rays,  traverse  the  secondary  tissues  radially  (fig.  1012).  In 
woody  monocotyls  and  lianas  secondary  growth  diverges  considerably 


FIG.  ion.  —  A  cross 
section  of  a  vascular  bundle 
in  which  secondary  thick- 
ening is  in  progress;  pt 
phloem;  c,  cambium  from 
which  secondary  phloem 
and  xylem  are  forming;  x, 
xylem,  composed  of  prim- 
ary xylem  (x^)  and  second- 
ary xylem,  (#2);  highly 
magnified. —  From  BARNES 
(Part  II). 


STEMS 


685 


from  that  just  described.  In  the  secondary  wood  the  tracheids  and 
tracheae  soon  die,  though  much  of  the  parenchyma  remains  alive  for 
some  years.  The  living  portion  of  the  wood  is  known  as  sap-wood  or 
alburnum,  while  the  dead  portion  is  known  as  heart-wood  or  duramen. 
Usually  the  alburnum  and  dura- 
men differ  in  color  and  other- 
wise, owing  to  the  accumulation 
of  excreta  in  the  latter. 


Conductive  cells  and  tissues  in  non- 
vascular  plants.  —  Thallophytes  and 
liverworts.  —  In  vascular  plants  the 
first  step  in  the  development  of  con- 
ductive cells  from  parenchyma  is  elon- 
gation in  the  direction  of  maximum 
conduction.  In  many  lower  plants 
similarly  elongated  cells  more  or  less 
grouped  into  tissues  are  by  no  means 
infrequent  (as  in  the  stipes  of  the  larger 
fungi,  fig.  198).  Many  fungi  have  elon- 
gated rootlike  organs  which  conduct 
foods  for  long  distances.  Much  the 
most  remarkable  conductive  system  in 
thallophytes,  however,  is  that  in  the 
brown  and  the  red  algae,  where  in  the 
larger  species  there  are  central  strands 
of  elongated  cells  with  viscous  albu- 


FiG.  1012.  —  A  diagrammatic  cross  section 
of  the  stem  of  a  dicotyl,  the  box  elder  (Acer 
Negundo),  illustrating  secondary  growth; 
there  are  shown  three  annual  growth  rings 
of  xylem  formed  from  the  cambium;  inside 
of  the  first  ring  is  pith;  the  lines  traversing 
the  growth  rings  represent  medullary  rays 
and  the  outer  layer  represents  the  bark.  — 
From  COULTER  (Part  I). 


minous  contents  and  with  transverse 

sieve  plates  at  the  widest  portions,  resembling  those  found  in  the  leptome  of  seed 
plants;  there  is  also  a  characteristic  callus.  Tracheids  do  not  occur  in  the  algae, 
but  water  plants  in  general  have  a  better  development  of  sieve  tubes  than  of  tra- 
cheids or  tracheae.  In  liverworts  there  is  but  a  slight  suggestion  of  conductive 
tissues,  though  Porella  exhibits  elongated  cells,  and  the  thallus  of  Pallavicinia 
contains  a  distinct  midrib  composed  of  elongated  and  pitted  cells. 

Mosses.  —  In  the  mosses  elongated  cells  are  frequent,  especially  in  the  stems, 
though  they  occur  also  in  the  midribs  of  many  leaves.  In  the  stem  of  the  Poly 
trichaceae  the  structural  differentiation  approaches  that  found  in  ferns,  the  elements 
having  a  concentric  arrangement;  the  central  region  consists  largely  of  thick- walled 
cells,  which,  like  tracheids,  are  prosenchymatous  dead  cells  containing  air  and 
water  (figs.  1013-1016).  Often  two  or  more  of  these  central  tracheid-like  cells  are 
associated  in  a  group  with  a  common  thick  wall,  while  the  walls  between  the  indi- 
vidual cells  remain  thin,  suggesting  an  approach  toward  lateral  fusion  (fig.  1014). 
This  central  xylem-like  tissue  is  surrounded  by  a  cylinder  of  living  cells  with  al- 
buminous contents  (fig.  1015).  The  underground  stem  or  "  rhizome  "  of  the  Poly- 
trichaceae  possesses  a  suberized  "  endodermis,"  a  "  pericycle,"  and  a  radial  triarch 


636 


ECOLOGY 


arrangement  of  the  "  xylem  "  and  "  phloem,"  as  in  a  triarch  root.  In  the  Polytri- 
chaceae  the  leaf  and  stem  bundles  join,  though  they  are  not  connected  in  most 
mosses.  The  bundle  of  Polyirichum  certainly  is  more  complex  in  structure  than 
that  of  the  simpler  seed  plants.  The  "  vascular  bundles  "  of  algae  and  mosses 

doubtless  have  no  genetic 
I  connection  with  the  vascular 
bundles  of  higher  plants,  but 
they  are  of  great  interest  as 
showing  possible  early  steps 
in  the  differentiation  of  con- 
ductive tracts. 


J 


FIGS.  1013-1016. — The  "vascular  strand"  of 
the  leafy  stem  of  a  moss,  Polyirichum:  1013,  a  dia- 
gram, showing  the  stem  regions,  as  seen  in  cross  sec- 
tion; c,  cortical  region;  x,  leptome-like  cylinder  sur- 
rounding the  central  hadrome-like  strand  (A);  1014, 
cells  from  the  hadrome-like  central  strand ;  note  that 
there  are  groups  of  thin-walled  cells,  suggesting  lateral 
fusion;  1015,  cells  from  the  leptome-like  cylinder; 
note  the  abundant  cell  contents;  1016,  a  longitudinal 
section  through  a  part  of  the  central  strand,  showing 
elongated  prosenchymatic  cells;  all  figures  highly 
magnified. 


Variations  in  primary 
conductive  tissues  due  to 
external  factors.  —  Gen- 
eral remarks.  —  The  vas- 
cular system  often  has 
been  regarded  as  essenti- 
ally invariable,  so  far  as 
external  conditions  are 
concerned.  Recent  in- 
vestigations, however,  show 
that  vascular  tissues  react 
to  external  changes  quite 
as  do  other  tissues,  varia- 


tions being  brought  about 

readily,  not  alone  in  the  shape,  size,  and  number  of  the  cells  or  cell 
fusions,  but  also  in  the  position  of  the  vascular  tract  and  in  the  arrange- 
ment of  its  members.  It  is  possible  in  certain  cases  even  to  inhibit 
the  development  of  entire  vascular  strands,  or  to  stimulate  the  appear- 
ance of  others  in  unusual  positions.  Contrary  to  earlier  views,  internal 
tissues  appear  to  react  to  external  changes  about  as  readily  as  do  such 
external  tissues  as  the  epidermis. 

Water  and  vascular  development.  —  In  amphibious  plants  different 
individuals  of  the  same  species,  or  even  different  parts  of  the  same 
plant,  display  more  extensive  and  more  specialized  conductive  tissue 
in  organs  exposed  to  transpiration  than  in  similar  organs  that  are  sub- 
mersed. Similarly,  in  land  plants  of  a  given  species  the  conductive 
tracts  are  much  better  developed  in  dry  air  or  dry  soil  than  in  moist  air 
or  moist  soil.  Thus  desiccation,  whether  brought  about  by  increased 
transpiration  or  by  diminished  absorption,  appears  to  stimulate  increased 


STEMS 


687 


vascular  development.  In  dry  cultures  the  vessels  are  more  numerous, 
larger,  longer,  and  have  thicker  walls  than  in  moist  cultures ;  some  of 
the  smaller  veins  present  in  the  dry  cultures  are  absent  in  the  moist 
cultures,  remaining  as  undifferentiated  parenchyma.  Furthermore,  in 
the  drier  cultures  lignification  begins  earlier  and  is  much  more  pro- 
nounced, and  the  differential  thickening  of  the  walls  is  more  conspicuous; 
also  the  endodermis,  which 
often  retains  its  cellulose  walls 
in  water  or  in  moist  air,  has 
thicker  and  more  completely 
suberized  walls.  Finally,  the 
hadrome  elements  die  much 
sooner  in  dry  than  in  moist 
cultures.  Where  growth  and 
transpiration  are  pronounced 
from  the  outset  (as  in  bulbous 
plants),  the  first  new  vessels 
often  are  larger  than  where 
growth  is  slow  (as  in  many 
seedlings) . 

Observation  tends  to  con- 
firm experiment  regarding  the 
influence  of  water  upon  vas- 
cular development.  In  sub- 
mersed hydrophytes,  such  as 
Elodea  and  Ceratophyllum, 
the  vascular  elements  occupy 


Fig.  1017.  — A  cross  section  through  the  vascu- 
lar bundle  of  a  stem  of  the  waterweed  (Elodea 
canadensis);  note  that  the  vascular  tract  (v)  is 
not  obviously  differentiated  into  leptome  and 
hadrome,  and  that  the  vascular  cells  have  thinner 
walls  than  the  cortical  cells  (c) ;  s,  starch  grains ; 


a,  intercellular  air  chamber  within  the  vascular 
tract;  a',  similar  chambers  in  the  cortex;  highly 
magnified. 


a  much  smaller  space  and  are 
much  less  differentiated  than 
in  land  plants  of  similar  size, 
the  leaf  bundles  often  being  so  small  as  easily  to  escape  detection  (figs. 
1017,  1018).  Although  the  duckweeds  are  regarded  as  vascular  plants, 
their  conductive  tissues  are  much  less  developed  than  are  those  of  mosses 
like  Poly  trie  hum,  entire  organs  sometimes  having  no  vascular  tissue,  as 
in  the  roots  of  Lemna.  In  hydrophytes  the  leptome  generally  is  reduced 
less  than  the  hadrome,  though  in  rare  cases  there  may  be  but  a  single 
row  of  sieve  tubes  within  a  bundle.  In  contrast  to  hydrophytes,  mpst 
xerophytes  and  alpine  plants  have  highly  developed  conductive  systems 
with  large  thick-walled  elements,  and  an  endodermis  that  is  strongly 


688 


ECOLOGY 


suberized.     Branch  veins  and  veinlets  usually  are  much  more  numerous 
in  xerophytic  leaves  than  in  the  leaves  of  shade  plants  and  hydrophytes. 

Parasitism  and  vascular  development.  —  When  Orobanche  fasciculata  grows 
parasiticaily  on  an  Artemisia  root  (fig.  1083),  the  latter  often  is  stimulated  to  unusual 
development,  the  hadrome  in  particular  being  subject  to  extensive  enlargement.  In 
the  haustoria  of  Melampyrum,  tracheids  develop  only  after  attachment  to  a  host 
plant.1  Leaves  infested  by  the  parasitic  fungus,  Peronospora,  sometimes  develop 
entirely  new  bundle  tracts,  certain  primordia  that  commonly  grow  into  mesophyll 
developing  instead  into  vascular  tissue.  In  insect  galls  of  Vitis  (fig.  823)  there  is  a 
vast  increase  in  hadrome,  there  being  about  the  larval  chamber  a  festoon  of  this 

tissue  developed  from  the  cortex.  Some- 
times in  vascular  tracts  infested  by  para- 
sites, parenchyma  cells  adjoining  the 
enlarged  vessels  become  hypertrophied, 
bulging  out  into  the  vessels  as  tyloses 

(P-  695). 

Miscellaneous  reactions  of  vascular 
tissue.  —  A  potato  tuber  usually  decays 
after  giving  rise  to  new  tubers  or  rhi- 
zomes, which  withdraw  the  food  it  had 
accumulated.  But  if  a  tuber  is  planted 
at  the  ground  level  in  such  a  way  that 
sprouts  developing  in  the  air  are  con- 
nected with  the  developing  roots  only 
through  the  old  tuber  (fig.  1046),  the 
latter  not  only  lives  another  season,  but 


V 


FIG.  1018.  —  A  cross  section  of  a  leaf 
segment  of  the  hornwort  (Ceratophyllum 
demersum),  showing  an  extremely  simple 
conductive  bundle  (•y),made  up  of  small  un- 
differentiated  cells ;  note  the  capacious  air 
chambers  (a)  and  the  mesophyll  (m),  com- 
posed of  nearly  uniform  cells  which  con- 
tain scattered  chloroplasts  (c),  while  the 
epidermis  (e)  contains  densely  packed 
chloroplasts;  highly  magnified. 


many  of  its  mature  cells  become  once 
more  meristematic.  Among  such  re- 
juvenating tissues  conductive  elements 
play  the  most  important  part,  many 
parenchymatic  cells  growing  into  tra- 
cheids and  becoming  of  importance  in  conducting  water  and  salts  to  the  growing  shoots. 
Similarly,  when  a  leaf  of  Torenia  is  placed  upright  in  the  soil,  it  gives  rise  to  a  shoot ; 
the  dorsiventral  petiole  develops  into  a  radially  symmetrical  organ,  and  new  vascular 
bundles  develop  from  parenchyma,  forming  a  vascular  cylinder  comparable  to  that 
of  the  stem.  In  many  similar  instances,  where  there  is  an  increased  flow  of  sub- 
stances through  the  parenchyma,  some  of  the  cells  in  the  latter  may  become  trans- 
formed into  tracheids;  for  example,  when  vascular  bundles  are  severed,  tracheids 
and  even  tracheae  may  develop  from  parenchyma  cells,  in  some  cases  forming 
connecting  "bridges "  of  conductive  tissue  between  the  severed  bundles  and  neigh- 
boring intact  bundles.  When  scale  primordia  grow  into  leaves  in  air  and  light,  there 

i  In  the  stem  of  Cuscuta  and  in  parasites  generally,  the  hadrome  but  not  the  leptome 
is  Jess  developed  than  in  green  plants  of  similar  size.  Occasionally,  also,  parasitism 
checks  the  growth  of  the  organs  infested  by  the  parasite,  the  conductive  tissues  as  well 
as  the  others  haying  a  reduced  development. 


STEMS  689 

is  a  much  greater  development  of  vascular  tissue  than  when  they  grow  into  scales 
in  soil  or  darkness.  When  seedlings  are  deprived  of  leaves,  or  when  they  have, 
smaller  leaves  than  usual,  or  are  otherwise  poorly  nourished,  the  bundles  are 
smaller  than  when  the  seedlings  are  well-nourished. 

So  far  as  known,  changed  conditions  cause  much  less  variation  in  leptome  than 
in  hadrome.  However,  an  increase  in  sieve  tubes  has  been  observed  in  Ipomoea 
and  Raphanus  grown  in  solutions  of  saccharose  or  glucose ;  sometimes  in  such  con- 
ditions sieve  tubes  appear  even  in  the  hadrome.  Remarkable  variations  have  been 
observed  in  roots  subjected  to  lateral  pressure;  for  example,  in  Pisum  the  primary 
root,  commonly  triarch,  becomes  tetrarch  under  pressure,  and  the  side  roots  become 
polyarch;  the  commonly  pentarch  roots  of  Vicia  Faba  similarly  become  hexarch. 
In  the  pine  the  number  of  bordered  pits  increases  with  the  altitude. 

The  vascular  tissues  of  lianas.  —  The  relative  area  occupied  by  the 
vascular  tract  in  lianas  usually  is  greater  than  in  other  stems  of  similar 
proportions,  and  the  individual  elements  possess  unusual  length  and  size ; 
the  Cucurbitaceae  with  their  capacious  vessels  and  sieve  tubes  furnish  a 
familiar  illustration.  Many  woody  lianas  exhibit  peculiar  secondary 
tissues;  for  example,  Bignonia  capreolata  has  radial  plates  of  phloem 
penetrating  far  into  the  xylem,  thus  appearing  like  a  cross  in  section 
(whence  the  name  cross  vine).  In  Mucuna  there  are  alternating  rings  of 
phloem  and  xylem,  and  in  Rhus  Toxicodendron  a  cross  section  of  the 
climbing  stem  is  strikingly  eccentric,  owing  to  the  much  greater  wood 
development  on  the  side  toward  the  support.  That  the  characteristic 
structural  features  of  lianas  may  be  due  in  part  to  external  factors  is 
clear  from  the  fact  that  in  Vitis  vinifera  the  vascular  tract  in  climbing 
stems  is  much  more  differentiated  than  in  stems  that  do  not  climb. 
However,  lianas  have  been  inadequately  studied,  and  little  is  known 
concerning  the  cause  or  significance  of  their  peculiar  secondary  tissue. 

Variations  in  secondary  wood  due  to  external  factors.  —  The  annual 
ting.  —  In  most  trees  and  shrubs  of  temperate  climates  growth  is  much 
more  vigorous  in  spring  than  later,  the  spring  wood  being  characterized 
by  large  thick-walled  vessels,  and  the  summer  or  autumn  wood  by 
small  thin-walled  vessels  (fig.  1019).  The  contrast  between  the  spring 
wood  and  the  autumn  wood  often  is  the  chief  circumstance  which 
makes  it  easy  to  discern  the  growth  rings  of  trees.  The  theory  has 
been  advanced  that  the  decreasing  size  of  vessels  from  spring  to 
autumn  is  due  to  the  gradual  increase  of  pressure  to  which  the  growing 
tissues  underneath  the  bark  are  subjected.  A  more  tenable  theory, 
harmonizing  better  with  conditions  in  primary  conductive  tissues,  is 
that  the  size  and  number  of  the  cells  and  the  thickness  of  the  walls  are 


690 


ECOLOGY 


greatest  in  spring,  because  the  ascent  of  sap  is  more  active  then  than 
later.  The  new  cells  take  part  in  this  movement,  and  from  the  ascend- 
ing materials  derive  the  substances  used  in  their  development.  Here, 
then,  as  in  primary  hadrome,  maximum  growth  is  correlated  with  a 
large  movement  of  materials. 

Variations  in  the  width  of  annual  rings.  —  The  width  of  the  annual 
ring  is  subject  to  considerable  variation,  which  is  dependent  in  part 
upon  the  age  of  the  tree  and  in  part  upon  seasonal  conditions.  In 

early    life    there    is    a 

f  n  period    of    acceleration, 

during  which  the  width 
of  the  rings  usually  in- 
creases from  year  to  year, 
probably  because  of  the 
increased  absorptionand 
food  supply  which  are 
made  possible  by  a  more 
extensive  root  system 
and  by  a  greater  ex- 
panse of  foliage.  Even 
if  the  rings  are  of  equal 
width  year  by  year, 
there  is  an  increasing 
increment  of  tissue, 


FIG.  1019  — A  cross  section  through  an  annual  ring 
in  the  secondary  wood  of  the  Austrian  pine  (Pinus 
Laricio),  showing  the  large -calibered  vessels  of  the  spring 
wood  (s)  and  the  small-calibered  vessels  of  the  preceding  owing  to  the  enlarging 
autumn  wood  (a);  note  the  relatively  sharp  line  (/)  be-  circumference.  After 
tween  the  autumn  wood  and  spring  wood;  b,  bordered 
pits  in  cross  section;  highly  magnified. 


some  years,  which  may 
be  few  or  many  accord- 
ing to  the  species,  there  comes  a  period  of  maturity,  characterized  by 
approximate  constancy  in  the  annual  increment.  Finally,  there  is 
a  period  of  retardation,  which  is  marked  by  an  actual  decrease  in  the 
amount  of  tissue  laid  down  year  by  year,  the  annual  increment 
approaching  zero  in  extreme  old  age.  While  new  roots  and  branches 
develop  yearly  throughout  life,  the  loss  of  old  branches  by  death 
finally  exceeds  the  gain,  and  it  is  possible  also  that  in  an  aging  tree  a 
given  amount  of  leaf  or  root  surface  becomes  less  effective.  In  long- 
lived  trees  (such  as  the  oak,  chestnut,  or  yew)  the  final  period  may  not 
begin  for  150  or  200  years,  and  may  continue  some  hundreds  of  years 
thereafter. 


STEMS  691 

In  young  and  middle-aged  trees  there  often  are  such  large  fluctuations 
in  the  width  of  the  annual  rings  that  the  progressive  phenomena  out- 
lined above  are  not  easily  recognized.  Careful  observation  makes  it  ap- 
pear that  the  growth  rings  are  wider  and  the  cell  caliber  greater  when  the 
season  is  warm  and  dry  than  when  it  is  cold  and  wet,  thus  appearing  to 
indicate  that  strong  transpiration  facilitates  the  development  of  second- 
ary as  of  primary  wood.  Stem  elongation,  on  the  other  hand,  is  greatest 
in  wet  seasons,  being  facilitated  by  weak  transpiration  (p.  736). 

Annual  rings  and  climate.  —  Annual  rings  are  much  more  sharply 
marked  in  periodic  than  in  uniform  climates,  the  greatest  difference 
being  found  where  there  is  a  well-defined  winter  and  summer,  or  where 
wet  and  dry  seasons  alternate.  In  uniform  climates  annual  rings  are 
poorly  marked  or  even  absent,  especially  in  the  most  pronounced  ever- 
greens (such  as  the  conifer,  Araucaria).  The  trees  of  eastern  Java,  which 
has  alternate  wet  and  dry  seasons,  have  much  more  prominent  growth 
rings  than  those  of  the  uniform  climate  of  western  Java;  even  species 
that  are  common  throughout  (as  the  teak,  Tectona  grandis)  have  uni- 
form wood  in  the  latter  region  and  growth  rings  in  the  former.  In  the 
teak  the  difference  produced  directly  by  climate  is  accentuated  by  the 
fact  that  the  tree  is  evergreen  in  western  and  deciduous  in  eastern  Java. 
That  the  deciduous  habit  favors  ring  formation  is  shown  generally  by 
the  presence  of  more  prominent  rings  in  deciduous  than  in  evergreen 
trees  in  the  same  climate.  Probably  the  sudden  cessation  and  renewal 
of  activity  in  deciduous  trees  as  compared  with  the  less  interrupted 
growth  of  evergreen  trees  sufficiently  account  for  their  more  pronounced 
ring  development.  The  continued  appearance  of  annual  rings  in  certain 
European  trees  transferred  to  uniform  tropical  climates  shows  that 
hereditary  as  well  as  environmental  factors  may  be  of  influence. 

Where  the  climate  is  detrimental  to  tree  growth,  as  in  alpine  and  arctic  regions, 
the  annual  rings  frequently  are  eccentric  rather  than  concentric,  and  are  exceedingly 
narrow,  sometimes  being  discernible  only  upon  microscopic  examination;  a 
Juniperus  stem  has  been  reported  as  having  a  diameter  of  only  thirty  centimeters 
and  yet  exhibiting  four  hundred  rings.  Some  of  the  large  polar  kelps  have  a  tissue 
differentiation  suggesting  annual  rings,  there  being  alternating  regions  whose  differ- 
ences probably  are  due  to  seasonal  variations. 

Rings  other  than  annual.  —  Sometimes  two  rings  are  developed  in  a  single  year,  as 
when  a  prolonged  summer  drought  is  followed  by  a  pronounced  wet  period,  or  when 
a  tree  puts  forth  new  leaves  after  defoliation  by  insects  or  by  storms,  the  new 
foliage  being  accompanied  by  a  second  cylinder  of  "spring  wood."  Some  tropical 
trees  shed  their  leaves  two  or  three  times  a  year,  and  in  such  cases  the  number 


692  ECOLOGY 

of  rings  coincides  with  the  number  of  times  the  leaves  are  shed;  for  example,  a 
Theobroma  tree  known  to  be  only  seven-and-a-half  years  old  had  twenty-two  rings. 
In  Dioon  and  perhaps  in  other  cycads,  rings  are  not  formed  annually,  but  once  in 
every  ten  or  twenty  years. 

Concluding  remarks  on  vascular  variation.  —  Differences  in  the  flow 
of  materials  through  conductive  tissues  appear  to  be  the  chief  cause  for 
their  structural  differentiation,  both  in  primary  and  in  secondary  wood. 
Nearly  all  cases  of  increased  vascular  development,  whether  involving 
an  increase  in  the  length,  caliber,  or  number  of  the  elements  or  in  the 
thickness  of  the  walls,  can  be  referred  to  increased  conduction;  this 
may  result  either  from  high  transpiration,  as  in  desert  regions  and  in 
other  situations  that  are  exposed  to  desiccation,  or  from  an  increased 
flow  of  structural  materials,  as  in  secondary  wood  and  in  plants  at- 
tacked by  parasites.  In  the  weak  development  of  wood  in  alpine  and 
polar  regions,  unfavorable  conditions  for  absorption  seem  to  outweigh 
the  otherwise  favorable  influence  of  strong  transpiration.  The  conduc- 
tive tissues  appear  to  furnish  the  best  evidence  found  in  the  plant  king- 
dom in  favor  of  the  idea  that  organs  increase  through  use ;  however,  it 
is  a  more  tenable  assumption  that  an  increased  flow  provides  more 
adequately  the  materials  requisite  for  enlargement  and  perhaps  also  the 
physical  stimulus  needed  for  continued  growth. 

The  role  of  vascular  tissues.  —  The  hadrome.  —  The  hadrome  forms 
the  pathway  of  ascending  water,  as  is  evident  from  the  quick  wilting  of 
the  leaves  when  a  complete  section  of  wood  is  removed,  and  from  their 
continued  turgescence  when  a  cylinder  of  bark  is  removed.  The  ascent 
in  tracheids  and  tracheae  of  water  colored  with  eosin  has  been  micro- 
scopically observed,  and  the  cessation  of  such  movement,  when  these 
tissues  are  infiltrated  with  cocoa  butter,  gelatin,  or  paraffin,  has  been 
demonstrated.  In  the  hadrome  also  there  ascend  inorganic  salts  in 
solution  in  the  water.  While  it  is  generally  believed  that  the  move- 
ment of  water  is  through  the  lumina,  some  observers  have  held  that  the 
lignified  walls  are  the  chief  paths  of  conduction.  There  is  no  adequate 
disproof  of  wall  conduction ;  the  stoppage  of  movement  by  paraffin  in- 
filtration is  often  cited  as  such,  but  it  is  probable  that  the  walls  as  well 
as  the  lumina  are  infiltrated.  If,  as  is  now  generally  believed,  the 
ascending  water  forms  a  continuous  column  involving  the  entire  had- 
rome, the  walls  would  appear  to  play  an  important  part  in  conduction, 
though  doubtless  subordinate  to  the  lumina. 

Structural  advantages  of  tracheae  and  tracheids.  —  The  great  elon- 


STEMS  693 

gation  of  the  hadrome  elements  probably  facilitates  conduction,  move- 
ment through  elongated  lumina  being  supposed  to  be  more  rapid  than 
through  a  series  of  short  lumina  separated  by  walls.  Tracheae  thus 
may  be  better  conductive  elements  than  tracheids,  though  the  latter 
certainly  are  efficient,  since  they  alone  are  present  in  the  conifers,  which 
include  the  tallest  known  trees.  In  some  conifers,  as  the  larch,  con- 
duction has  been  shown  to  be  essentially  as  rapid  as  in  dicotyls.  Per- 
haps the  conductive  efficiency  of  conifer  wood  is  due  in  part  to  its 
relatively  long  tracheids.  Doubtless  the  large  caliber  of  the  hadrome 
elements  also  facilitates  conduction.  The  large  and  long  hadrome  ele- 
ments of  lianas  may  be  regarded  as  very  advantageous,  since  the  length 
of  the  stems  is  so  great  in  proportion  to  their  diameter.  Oblique  end 
walls  have  been  thought  to  be  more  advantageous  than  horizontal  end 
walls  because  they  present  a  greater  diffusion  surface. 

The  advantages  of  differential  lignification.  —  Lignification  is  highly  beneficial, 
since  lignin,  which  is  the  chief  factor  in  giving  rigidity  and  strength  to  woody  tissues, 
is  at  the  same  time  permeable  to  water  and  solutes.  The  slightly  lignified  spiral 
and  annular  vessels  are  not  so  strong  and  rigid  as  are  the  larger  and  more  lignified 
pitted,  reticulated,  and  scalariform  vessels,  which  may  be  regarded  as  the  main 
conductive  elements.  Though  the  entire  wall  is  permeable,  the  thin  spots  or  pits 
probably  represent  more  permeable  regions  which  facilitate  rapid  lateral  transfer, 
without  interfering  with  the  mechanical  efficiency  of  the  wall.  Bordered  pits  are 
supposed  to  act  somewhat  in  the  manner  of  a  valve,  opening  when  the  pressure  on 
the  two  sides  is  equal  and  closing  when  it  is  unequal,  a  pressure  difference  of  one 
fifteenth  of  an  atmosphere  being  sufficient  to  cause  closure;  thus  bordered  pits  may 
either  facilitate  or  impede  lateral  transfer. 

The  advantages  of  dead  tissues  in  conduction:  —  One  of  the  chief 
advantages  ot  tracheids  and  tracheae  is  associated  with  their  early 
death.  Water  and  solutes  pass  through  living  cells  by  osmosis,  their 
rate  and  direction  of  movement  being  independent  of  one  another;  the 
movement  of  any  given  solute  is  conditioned  by  the  permeability  of  the 
cell  walls  and  the  protoplasmic  membranes  for  that  particular  solute, 
and  by  differences  in  its  pressure  in  adjoining  cells.  In  the  dead  had- 
rome, however,  water  and  solutes  may  move  together  for  great  distances 
without  passing  protoplasmic  membranes  and  only  occasionally  trav- 
ersing cell  walls,  movement  being  much  more  rapid  than  by  diffusion 
through  living  cells.  Except  for  a  few  cells  in  the  epidermis  and  cortex 
of  the  root  and  in  the  mesophyll  of  the  leaf,  the  entire  course  of  water 
through  a  plant  is  in  dead  tissues.  The  streaming  of  protoplasm,  a 


694  ECOLOGY 

phenomenon  especially  common  in  water  plants,  sometimes  may  facilitate 
conduction  in  living  cells. 

The  role  of  sieve  tubes.  —  The  sieve  tubes  generally  are  regarded  as 
organs  of  protein  conduction,  partly  because  they  are  living  elements 
rich  in  protein,  partly  because  they  form  a  continuous  system  of  tubes 
comparable  to  the  water-conducting  elements,  partly  because  the  porous 
sieve  plates  seem  well  fitted  for  the  passage  of  viscous  albuminous  ma- 
terials, and  partly  because  no  other  tissues  are  known  that  are  peculiarly 
fitted  for  protein  conduction.  The  girdling  of  trees  often  results  in 
increased  growth  above  the  girdled  area  (fig.  665),  and  under  favorable 
conditions  roots  originate  from  that  region.  Such  phenomena  appear 
to  indicate  that  there  is  in  the  bark  a  downward  movement  of  foods  and 
that  they  tend  to  accumulate  above  the  girdled  portion,  furnishing  the 
materials  used  in  the  extra  growth.  Trees  often  are  killed  by  complete 
girdling,  probably  because  of  the  inability  of  the  food  to  reach  the  roots. 
In  those  dicotyls  which  have  sieve  tubes  inside  the  hadrome,  none  of 
the  above-noted  effects  of  girdling  are  seen. 

Sieve  tubes  have  been  variously  regarded  as  organs  of  protein  accumulation,  as 
organs  of  protein  manufacture,  and  as  structures  that  are  well  fitted  for  the  propa- 
gation of  stimuli.  The  conduction  theory,  however,  seems  most  tenable,  though 
the  actual  movement  of  material  has  not  been  well  observed.  Movement  is  from 
regions  of  high  to  regions  of  low  pressure  and  may  be  downward  toward  the  roots, 
outward  toward  the  branches,  or  upward  toward  the  flowers  and  fruits.  Owing  to 
the  chemical  similarity  between  the  protoplasm  and  the  conducted  protein,  it  may 
be  regarded  as  advantageous  or  even  necessary  that  the  conducting  cells  be  living. 
The  closing  of  the  sieve  plate  in  winter  by  a  callus  may  be  advantageous,  but  more 
probably  it  is  of  no  particular  significance.  Nothing  is  known  concerning  the  role 
of  the  companion  cells,  which  in  angiosperms  persistently  accompany  the  sieve 
.tubes  and  even  extend  beyond  them  in  the  bundle  termini. 

Conduction  in  the  lower  plants. —  The  sieve  tubes  of  the  larger  algae  commonly 
are  supposed  to  be  organs  of  protein  conduction,  but  this  has  not  been  proved. 
The  rise  of  colored  fluids  has  been  observed  in  the  "  vascular  "  tract  of  moss 
stems ;  it  is  not  known  that  movement  is  more  rapid  there  than  in  the  other  stem 
tissues,  though  the  greater  cell  elongation  makes  it  seem  probable. 

The  path  of  carbohydrate  conduction.  —  Sugar  passes  readily  from 
cell  to  cell  by  osmosis,  and  it  is  probable  that  the  cortical  parenchyma 
is  the  chief  tissue  involved  in  its  conduction,  though  carbohydrate  as 
well  as  protein  may  pass  along  the  sieve  tubes;  the  ill  effects  of  girdling 
may  be  due  as  much  or  more  to  the  cutting  off  from  the  roots  of  carbo- 
hydrates as  to  the  cutting  off  of  proteins.  The  endodermis,  because  of 
its  rich  starch  content,  has  been  regarded  as  a  region  of  carbohydrate 


STEMS  695 

conduction,  but  it  is  much  more  probable  that  it  is  a  layer  in  which 
surplus  starch  accumulates.  Much  of  the  carbohydrate  that  descends 
in  the  cortex  and  leptome  during  the  summer  and  autumn  may  ascend 
in  the  wood  with  water  and  soil  salts  the  following  spring,  as  in  the  sugar 
maple,  whose  sap  may  contain  as  much  as  three  per  cent  of  sugar. 

Advantages  associated  with  various  bundle  arrangements.  —  The 
grouping  of  conductive  elements  into  strands  is  advantageous,  partly 
because  of  the  consequent  economy  in  space  and  structural  material  in 
the  protective  stereome  as  well  as  in  the  hadrome  and  leptome,  but  more 
particularly  because  continuity  thus  is  made  possible.  In  the  bundle  the 
hadrome  seems  to  have  the  place  of  advantage.  In  the  stem  it  is  inside 
the  leptome,  where  there  is  greater  freedom  from  mechanical  strains, 
and  where  dangers  incident  to  transpiration  are  more  remote.  In  the 
leaf  (even  in  ferns,  where  the  stem  bundles  are  concentric)  the  hadrome 
is  uppermost,  and  therefore  closest  to  the  most  active  food-making  re- 
gion. Even  in  roots  the  hadrome  occupies  equally  with  the  leptome 
the  place  of  advantage,  that  is,  nearest  the  cortex,  from  which  food 
materials  come  and  to  which  organized  foods  go. 

There  has  been  advanced  the  very  dubious  hypothesis  that  a  "  struggle  for  ex- 
istence "  has  taken  place  between  the  tissues,  and  that  the  stronger  element,  the 
hadrome,  has  "  won "  the  place  of  advantage.  In  secondary  tissues  it  is  highly 
advantageous  for  the  xylem,  which  gives  rise  to  the  permanent  tissue,  to  be  inner- 
most, since  the  less  permanent  tissue  may  then  exfoliate  as  the  other  develops. 
Where  other  conditions  obtain,  as  in  some  lianas  (e.g.  Mucuna)  and  in  monocotyls  (as 
Cocos),  temporary  and  permanent  tissues  are  intercalated  in  such  a  way  as  to  make 
impossible  the  development  of  a  solid  cylinder  of  permanent  tissue;  in  Cocos  the 
decay  of  the  temporary  elements  leaves  the  wood  in  disconnected  strands,  whence 
the  name,  porcupine  wood: 

The  slight  reduction  of  leptome  in  hydrophytes  is  advantageous,  for  while  sub- 
mergence results  in  reduced  water  conduction,  protein  conduction  is  as  prominent  as 
ever.  The  large  development  of  leptome  in  lianas  having  bicollateral  bundles  may 
facilitate  protein  conduction  in  the  long  slender  stems  just  as  the  elongated  and 
enlarged  hadrome  elements  facilitate  water  conduction.  The  great  development 
of  hadrome  in  xerophytes  appears  disadvantageous  rather  than  advantageous. 

In  secondary  wood,  conduction  is  mainly  in  the  alburnum,  where  living  paren- 
chymatous  cells  are  intercalated  among  the  dead  vessels,  the  medullary  rays  fur- 
nishing lateral  connections  between  the  alburnum,  cambium,  and  phloem.  The 
dead  duramen  often  is  a  region  of  accumulation  of  waste  products  (p.  725),  the 
vessels  being  filled  with  tannins,  gums,  and  other  excreta.  Sometimes  vessels  are 
closed  by  tyloses  which  arise  through  the  intrusive  growth  of  neighboring  paren- 
chyma cells.  Tylosis  formation  characterizes  especially  the  transformation  of 
alburnum  to  duramen,. representing. the  final  phase  of  activity  in  the  wood  paren- 


/696 


ECOLOGY 


chyma.  Tyloses  develop  also  at  wounded  surfaces  and  in  the  vascular  tracts  of 
certain  plants  attacked  by  parasites;  in  these  cases  they  clearly  are  advantageous 
in  checking  conduction,  but  no  particular  advantage  or  disadvantage  is  apparent 
in  the  clogging  of  duramen  tissues. 

The  role  of  the  pericycle  and  of  the  endodermis. 
—  The  chief  role  of  the  pericycle,  which  is  a 
meristematic  layer,  is  the  origination  of  new 
lateral  organs,  especially  roots.  The  endodermis 

I     MVWrSS^§s^^=  ^^        often  is  a  Protective  layer>  particularly  in  roots 
•"•      njE*s4B«r    ^^^ssmr^sSSSs^SS  ,      ..  .,  ,       •       ,          , , 

and    rhizomes,  its  suberized  walls    serving    to 

check  local  losses  of  water  from   the  bundle. 
In  aerial  stems   it  may  serve   as  a  region  of 
starch  accumulation,  whence  the  name  starch 
W  sheath. 


4.  MECHANICAL  TISSUES 

Introductory  remarks.  —  In  small  land 
plants  and  in  water  plants  of  all  sizes 
mechanical  tissues  are  poorly  if  at  all 
developed,  their  rigidity  when  present 
being  due  to  the  cell  walls  and  to 
turgor.  Large  aerial  and  soil  organs, 
however,  are  subjected  to  considerable 
strain,  and  conspicuous  mechanical 
tissues  commonly  are  present,  insuring 
the  maintenance  of  form  and  position. 

The  mechanical  elements  or  stereids. 
—  Bast  and  wood  fibers.  —  The  most 
representative  mechanical  elements  are 
the  bast  fibers,  which  are  especially  char- 
acteristic of  the  phloem.  They  are  elon- 
gated prosenchymatic  structures  with 
attenuated  points,  and  thus  have  long 
regions  of  contact  with  adjoining  fibers 
in  the  same  longitudinal  row,  insuring 
a  degree  of  dovetailing  perhaps  un- 
equaled  elsewhere  (fig.  1020,  B).  The 
fibers  commonly  average  one  or  two 
millimeters  in  length,  but  in  ramie 
(Boehmeria),  one  of  the  most  important 
fiber  plants,  the  length  may  be  two 


FIG.  1020.  —  Bast  fibers  of  the 
century  plant  (Agave  americana) : 
A,  a  cross  section  through  a  bundle 
of  bast  fibers,  showing  the  uniform 
stratification  of  the  walls  (iv),  and 
the  small  size  of  the  lumina  (b)  in 
proportion  to  their  thickness;  B,  a 
longitudinal  section  through  such  a 
bundle,  showing  prosenchymatous 
cell  ends  (p)  dove-tailing  with  one 
another;  s,  spirally  arranged  wall 
slits;  other  lettering  as  in  A  ;  highly 
magnified. 


STEMS 


697 


hundred  millimeters  or  more.  Bast  fibers  are  multinucleate  from 
the  outset  (i.e.  they  are  coenocyles)  and  they  remain  alive  much  longer 
than  do  tracheids  or  tracheae.  The  walls  are  highly  thickened  with 
cellulose  or  occasionally  with  lignin,  the  material  being  deposited  in 
regular  centripetal  layers;  the  stratification  incident  to  differential 
or  periodic  deposition  often  is  very  evident  (fig.  1020,  A).  In  the  mature 
fiber  the  lumen  is  extremely  small,  the  volume  of  the  wall  being  many 
times  greater.  The  walls  are  marked  by  spirally  arranged  slits,  sug- 
gesting that  the  ultra-microscopic  particles  (micellae)  also  are  spirally 
arranged  like  the  strands  of  a  rope.  Wood  fibers  (also  called  libriform 
elements)  resemble  bast  fibers,  differing  therefrom  in  their  restriction  to 
secondary  wood,  and  in  their  lignified  walls  and  early  death;  further- 
more, they  are  not  coenocytes.  There  are  all  gradations  between  wood 
fibers  and  tracheae,  whereas  bast  fibers  are  contrasted  rather  sharply 
with  other  phloem  elements. 

Collenchyma.  —  Collenchyma  is  a  term  applied  to  living  mechanical 
tissues  made  up  of  elongated  cells  whose  walls  are  unequally  thickened 


FIG.  102 1.  —  A  cross  section  of  a 
Begonia  petiole,  showing  collenchy- 
matic  thickening  (c)  of  the  walls  at 
the  angles  of  the  outermost  cortical 
cells;  t,  cuticle;  highly  magnified. 


FIG.  1022.  —  A  stone  cell  or  sclereid 
from  the  petiole  of  the  wax  plant  (Hoya 
carnosa);  note  the  lines  of  stratification 
(s),  representing  successive  periods  of 
wall  construction,  and  also  the  canals 
(c)  which  connect  the  lumina  of  adjoin- 
ing cells,  and  which  have  remained  open 
during  wall-building;  highly  magnified. 


with  cellulose  of  high  water  content  (sixty  to  seventy  per  cent,  as  against 
twenty  to  forty  per  cent  in  bast),  the  thickening  often  being  most  pro- 
nounced at  the  cell  angles  (fig.  1021).  Unlike  bast,  Collenchyma  is 


698  ECOLOGY 

capable  of  growth  elongation,  and  the  walls  are  much  more  refrangible. 
In  addition  to  the  permanent  collenchyma,  which  is  characteristic  of  the 
cortical  region  of  stems  and  petioles,  temporary  collenchymatic  thicken- 
ing often  occurs  in  bast  primordia. 

Sclerenchyma. —  Representative  sclerenchyma  cells  or  sclereids  are 
mechanical  cells  that  cannot  be  classified  as  collenchyma  or  as  bast, 
though  bastlike  fibers  in  the  cortex  often  are  called  sclerenchyma.  The 
most  characteristic  sclereids  are  the  hard,  stiff,  and  relatively  isodia- 
metric  stone  cells  with  brownish  lignified  walls,  which  are  found  in  the 
secondary  bark  of  trees  and  in  the  shell  of  the  hickory  nut,  and  which 
arise  through  the  sclerosis  of  ordinary  parenchyma  cells  (fig.  1022). 
Sclereids  of  this  sort  have  stratified  walls  due  to  differential  centripetal 
growth,  the  walls  being  traversed  by  more  or  less  branched  canals,  along 
which  the  structural  materials  probably  have  passed.  The  cells  die 
soon  after  stratification  ceases. 

The  stellate  mechanical  cells  of  water  lily  leaves  (fig.  805)  and  the  T-shaped 
prop  cells  of  Osmanthus  (fig.  937)  may  be  classed  with  sclereids.  The  rigidity  of 
stems  is  not  due  entirely  to  bast,  collenchyma,  and  sclerenchyma,  other  tissues  with 
thickened  walls,  such  as  the  wood  and  the  cutinized  epidermis,  playing  an  important 
part.  In  Equisetum  and  in  various  grasses  (notably  the  cereals),  considerable  silica 
is  deposited  in  the  walls,  giving  them  considerable  rigidity. 

The  distribution  of  stereome  in  plants.  —  Many  sclereids  occur  as 
idioblasts,  being  scattered  irregularly  through  various  tissues,  as  in  the 
bark  of  trees.  More  commonly  the  mechanical  elements  are  grouped  in 
strands,  the  most  usual  condition  being  the  association  of  mechanical 
and  conductive  elements  into  a  fibrovascular  bundle.  As  a  rule,  com- 
pact strands  of  bast  fibers  occur  just  outside  the  leptome  (fig.  760),  and 
sometimes  there  is  a  mechanical  cylinder  entirely  surrounding  the  vas- 
cular bundle  (fig.  1028),  or  even  the  whole  vascular  region  (fig.  1027). 
In  monocotyls  (especially  in  xerophytic  species)  there  usually  is  a  pro- 
gressive decrease  of  bast  strands  inward,  the  outer  bundles  having 
conspicuous  mechanical  strands  outside  the  leptome,  while  often  the 
inner  bundles  are  without  them.  In  many  xerophytic  leaves  and  stems 
there  often  are  cortical  strands  of  bastlike  fibers,  especially  just  beneath 
the  epidermis;  similar  strands  are  frequent  also  in  the  bark.  Wood 
fibers  rarely  are  grouped  in  compact  strands,  but  individual  fibers  with  all 
intergrading  stages  are  scattered  here  and  there  among  the  conducting 
elements.  •  ^  ifa^  :;i«J  i 


STEMS 


699 


The  influence  of  external  factors  upon  mechanical  tissues.  —  Mechan- 
ical stimuli.  —  An  unattached  tendril  has  less  tensile  strength  than  one 
that  is  attached,  the  latter  sometimes  being  two  to  five  times  stronger  than 
the  former.  Frequently  this  change  in  tensile  strength  is  associated  with 
a  structural  change,  more  mechanical  cells  being  developed  or  further 
wall  thickening  appearing  in  those  already  present  (figs.  1023-1026).  In 
some  cases  collenchyma 
develops  into  bast  upon 
the  attachment  of  the 
tendril.  It  has  been 
claimed  that  growing 
stems  of  Helleborus  sub- 
jected to  tension  develop 
bast  in  regions  where 
ordinarily  none  is  pres- 
ent; furthermore,  stems 
subjected  to  tension  for 
three  days  have  been 

found  able  to  support  a  SA  *    IOB 

weight  of   3500  grams, 

,.,                ,                     .  FIGS.  1023-1026.  —  Cross  sections  of  tendrils  of  Cyclan- 

™niie                                       1  thera  explodens:    1023,  a  diagrammatic   cross  section  of 

of   the  experiment   only  a  tendril  that  has  not  come  in  contact  with  a  support; 

4OO  grams  COUld  be  SUP-  I024>  a  diagrammatic  cross  section  of  a  tendril  that  has 

rpi                .,  .  come  in  contact  with  a  support;  note  the  larger  size  of  the 

=>  tendril,  the  increased  development  of  the  chief  mechanical 
strand  (m),  and  the  development  of  accessory  mechanical 

QUentlv  tested    have  not  s*rands  (m'}\   v,  vascular  tracts;    1025,  a  portion  of  the 

K                   fi         H        °th  mechanical  strand  of   1023,   highly  magnified,   showing 

ea»    e          r  collenchymatic  wall  thickening;    1026.  a  portion  of  the 

in  Helleborus  or  in  Other  chief  mechanical  strand   of    1024,   similarly   magnified ; 

plants      though     it     ap-  note  l^e  highly  thickened  walls,  the  elements  being  bast 

'    .  rather  than  collenchyma.  —  From  HABERLANDT. 
pears  that  certain  stems 

when  exposed  to  tension  for  some  days  or  weeks  become  somewhat 
stronger;  furthermore,  the  bast  fibers  and  hadrome  elements  become 
somewhat  more  numerous  and  have  slightly  thicker  walls  than  in 
controls  not  so  exposed.  In  many  cases  tension  appears  to  cause  no 
appreciable  change. 

In  pendent  fruits  (as  the  apple)  the  tension  on  the  fruit  stalk  increases  constantly 
as  the  fruit  gains  in  weight,  also  the  strength  of  the  stalk  increases  from  week  to  week, 
although  it  has  been  shown  that  the  increasing  pull  of  the  fruit  is  not  a  determin- 
ing factor  of  importance.  The  fruitstalks  of  Cucurbila,  however,  have  more 


ported. 

results,    though    fre 


yoo  ECOLOGY 

mechanical  tissue  when  the  fruits  hang  free  in  the  air  than  when  they  rest  on  the 
ground.  It  is  claimed  by  some  investigators  that  compression  is  more  effective  than 
tension  in  stimulating  the  growth  of  mechanical  tissue.  Roots  react  to  mechanical 
stimuli  more  readily  than  do  stems,  tension  resulting  in  a  conspicuous  increase  in 
the  number  of  mechanical  elements  and  in  the  thickness  of  the  walls,  and  com- 
pression resulting  in  a  decreased  cell  size  and  in  an  increased  wall  thickness. 

If  a  young  tree  is  fastened  so  that  it  can  sway  in  but  one  plane,  its  diameter  in 
this  plane  will  be  greater  than  in  any  other.  Probably  the  elliptical  cross  section 
seen  in  trunks  of  seacoast  trees  is  caused  by  wind,  the  long  diameter  being  per- 
pendicular to  the  coast  line.  In  the  spruce  the  upper  side  of  a  horizontal  branch  is 
composed  largely  of  white  wood  and  the  lower  side  of  red  wood,  the  former  having 
about  twice  the  tensile  strength  of  the  latter;  the  red  wood,  whose  elements  have 
thicker  walls,  possesses  the  greater  compression  strength.  The  upper  part  of  a 
branch  is  subjected  to  tension  and  the  lower  part  to  compression,  and  the  differ- 
ences observed  may  thus  be  explained;  gravity  also  is  believed  to  be  an  important 
causative  factor.  Red  wood  occurs  also  on  the  lee  side  of  branches  exposed  to 
wind.  In  dicotylous  trees  the  differences  of  wood  are  less  marked  than  in  conifers, 
and  mechanical  tissues  are,  if  anything,  better  developed  on  the  upper  than  on  the 
under  side  of  horizontal  branches. 

Desiccation.  —  While  the  influence  of  mechanical  stimuli  is  not  clearly 
understood,  desiccation  is  known  to  favor  the  increased  development  of 
mechanical  tissue.  In  dry  air  a  mechanical  cylinder  is  developed  in 
the  stem  cortex  of  Mentha  aquatica,  while  in  moist  air  the  cells  remain 
parenchymatous.  In  the  stem  of  Ficus  scandens  the  cells  that  become 
collenchyma  in  moist  air  become  bast  in  dry  air.  In  deserts  plants  of 
irrigated  soil  show  less  mechanical  tissue  than  do  those  of  dry  soil. 
Aquatic  and  terrestrial  stems  of  the  same  species  (as  Polygonum  amphi- 
bium,  figs.  821,  822)  differ  widely  in  the  amount  of  mechanical  tissue 
developed,  the  water  form  being  too  weak  to  stand  alone,  whereas  the 
air  form  is  very  stiff  and  rigid.  Fern  roots  in  moist  soil  have  a  slight 
development  of  mechanical  tissue,  the  cells  being  few  and  the  walls  thin. 
Probably  it  is  generally  true  that  bast,  collenchyma,  and  sclerenchyma 
cells  increase  in  number  and  in  wall  thickening  as  the  tissues  in  which 
they  are  developing  are  exposed  increasingly  to  desiccation.  The  mini- 
mum of  mechanical  tissue  occurs  where  transpiration  is  reduced  to  zero, 
namely,  in  the  water.  Observation  confirms  experiment,  for  mechanical 
tissues  are  less  developed  in  hydrophytic  than  in  other  habitats  (figs.  791, 
1018),  reaching  their  culmination  in  xerophytes. 

The  role  of  mechanical  tissues.  —  The  mechanical  features  of 
stereids.  —  The  fact  that  ropes  and  cables  are  made  from  the  bast  fibers 
of  hemp,  ramie,  etc.,  indicates  the  tensile  strength  of  bast  strands. 
While  the  strength  of  ordinary  parenchyma,  as  pith,  is  but  one  kilo- 


STEMS 


701 


FIG.  1027.  —  A  diagrammatic  cross 
section  of  a  carnation  stem  (Dianthus 
Caryophyllus),  showing  the  develop- 
ment of  a  mechanical  cylinder  (m)  out- 
side the  vascular  tract  (v)  ;  c,  c',  cortical 
parenchyma;  p,  central  pith. 


gram  per  square  millimeter,  some  of 

the    strongest    bast    has    within    its 

limits  of  elasticity  a  tensile  strength 

of  twelve  to  twenty-five  kilograms  per 

square  millimeter,  the   higher  figure 

being  twice  that  of  wrought  iron  and 

about  equal  to  that  of  German  steel ; 

however,  the  breaking  point  of  steel 

is  much  beyond  that  of  bast.     Bast 

differs    widely    from    the    metals   in 

having    a    considerable     degree    of 

elastic  elongation,  and  is  itself  ex- 
ceeded in  this  respect  by  the  me- 
chanical strands  of  lichens,  those  of 

Usnea   being  capable  of  elongating 

sixty  to  one  hundred  per  cent  (see  fig.  1113).     Bast  is  considerably 

stronger  when  desiccated  than  when  it  contains  moisture,  though  it  is 

more  elastic  in  the  latter  condition;  ligni- 
fication  usually  decreases  the  tensile 
strength.  The  great  tensile  strength  of 
bast  is  due  to  the  amount  and  quality  of 
the  wall  thickening,  to  the  dovetailing  of 
the  prosenchymatic  cells,  and  probably, 
also,  to  a  spiral  arrangement  of  the  wall 
micellae.  The  fact  that  the  limit  of  elas- 
ticity is  so  near  the  breaking  point  is  not 
disadvantageous,  since  any  elongation  of 
bast  beyond  the  limit  of  elasticity  would 
be  harmful.  Collenchyma  is  almost  as 
strong  as  bast,  though  it  has  a  lower 
limit  of  elastic  elongation.  Having  the 
power  of  growth  elongation,  it  is  espe- 
cially suited  to  growing  tissues. 


FIG.  1028.  —  A  diagrammatic 
cross  section  of  the  erect  aerial 
stem  of  Juncus  balticus  littoralis, 
showing  an  interrupted  mechanical 
cylinder,  composed  of  alternating 
outer  (0)  and  inner  (i)  fibrovas- 
cular  bundles,  in  which  a  bast 
cylinder  (6)  surrounds  the  vas- 
cular tract  (v) ;  note  that  the  bast 
is  more  strongly  developed  out- 
ward than  inward;  c,  cortex;  a, 
large  central  chamber,  originally 
occupied  by  pith  cells;  consider- 
ably magnified. 


The  role  of  scattered  sclereids  is  less  obvious 
and  probably  less  important  than  is  that  of  bast 
and  collenchyma  though  it  is  of  significance  in 
certain  leaves  (p.  639).  Bast  and  collenchyma 
sometimes  have  been  regarded  as  advantageous 
in  checking  transpiration,  and  collenchyma  some' 


702 


ECOLOGY 


times  is  considered  to  be  a  sort  of  "  water  storage  "  tissue.  The  latter  theory 
may  be  dismissed  summarily,  and  the  former  theory  is  at  least  doubtful,  in  view 
of  the  permeability  of  the  walls. 

Flexile  strength.  —  The  flexion  of  a  stem  induces  tension  on  one  side 
and  compression  on  the  other,  each  strain  decreasing  to  zero  at  the  center. 
Consequently  a  stem  with  a  strong  cylinder  of  peripheral  mechanical  tis- 
sue may  have  a  central  region  of  pith  or  may  even  be  hollow  (as  in 
Equisetum  and  in  the  grasses)  and  yet  have  considerable  rigidity.  It 
has  been  computed  that  if  the  mechanical  periphery  forms  one  seventh 
of  the  diameter,  the  strength  will  be  sufficient  to  meet  all  usual  strains. 

Further  development  of  mechanical  tissue 
would  be  not  only  useless  but  actually 
disadvantageous,  because  it  would  re- 
quire a  considerable  amount  of  struc- 
tural energy  and  material,  and  also  be- 
cause space  would  be  used  that  might 
be  taken  by  other  tissues.  The  aver- 
age erect  herbaceous  stem  illustrates 
admirably  these  mechanical  principles. 
Sometimes  (as  in  Dianthus,  fig.  1027) 
there  is  a  mechanical  cylinder  sur- 
rounding the  leptome,  while  in  other 
cases  strands  of  bast  form  an  interrupted 
cylinder.  External  to  the  bast  is  the 
collenchyma  cylinder,  and  internal  to 
the  bast  is  the  cylinder  of  secondary 
wood  which  is  of  great  mechanical  im- 
portance. Although  the  conductive  and  mechanical  bundles  in  mono- 
cotyl  stems  are  scattered,  the  decrease  of  bast  elements  toward  the 
center  results  essentially  in  a  broken  peripheral  cylinder  of  mechan- 
ical tissue  (fig.  1028).  In  angled  stems  (notably  in  the  mints,  fig. 
1029)  strains  are  accentuated  at  the  angles,  where  there  is  consider- 
able collenchyma  just  inside  the  epidermis,  and  often  a  bast  crescent 
just  outside  the  leptome. 

The  advantage  of  flexile  strength  is  well  illustrated  on  mountain  slopes  that 
are  subject  to  snowslides  and  avalanches,  flexible  trees  and  shrubs  (such  as  alders 
and  willows)  being  uninjured,  whereas  some  rigid  trees  (as  conifers)  snap  off  like 
pipestems.  In  lowlands  the  accumulation  of  ice  during  a  cold  rain  frequently 
causes  the  rupture  of  branches  that  withstand  all  ordinary  strains. 


FIG  1029  —  A  diagrammatic 
cross  section  of  an  erect  aerial  stem 
of  a  mountain  mint  (Pycnanthemum 
virginianum),  showing  columns  of 
mechanical  tissue  (m)  at  the  stem 
angles;  c.  cortical  parenchyma;  vt 
vascular  cylinder;  p,  central  pith 
region. 


STEMS 


703 


v    e  c 

FIG.  1030.  —  A  diagrammatic  cross 
section  of  the  rhizome  of  Juncus  balticus 
littorahs,  note  the  symmetrically  ar- 
ranged air  chambers  (a)  in  the  cortex 
(c)\  e,  endodermis;  v,  vascular  tract, 
composed  of  a  matrix  of  mechanical 
cells  (w)  surrounding  groups  of  con- 
ductive vessels  (/). 


Tensile  strength.  —  Resistance  to  tension  depends  not  on  the  position 
but  on  the  size  of  the  mechanical  strand.  However,  since  tension  al- 
most always  is  unequal,  hence  involving  flexion,  a  massing  of  strands 
into  a  central,  solid  cylinder  is  most 
advantageous,  since  the  danger  of 
rupture  through  differential  strain 
is  reduced  to  a  minimum.  Roots  are 
subject  to  considerable  tension,  espe- 
cially when  the  stems  sway  in  the 
wind,  and  they  differ  from  erect  stems 
somewhat  generally  in  that  the  cen- 
tral portion  is  occupied  by  thick- 
walled  wood  cells  of  considerable 
strength  instead  of  by  a  core  of  pith. 
Rhizomes  (as  in  Lathyrus and  Juncus, 
fig.  1030)  are  subject  to  the  same 
tensions  as  are  roots,  and  like  them 
differ  from  erect  stems  in  the  absence 
or  weak  development  of  central  pith. 
The  relatively  central  vascular  tract  usually  has  thick-walled  wood 
elements,  and  there  may  be  a  mechanical  cylinder  or  scattered 
mechanical  strands  outside  the  wood. 

Lianas  are  subject  to  rather  extraordinary  strains,  the  combination  of  tension, 
flexion,  and  pressure  in  a  woody  twiner,  owing  to  the  growth  of  the  supporting  tree, 
being  especially  severe;  in  such  stems  there  is  a  large  development  of  bast  and  of 
thick-walled  wood.  In  tendril  climbers  much  of  the  strain  is  borne  by  the  relatively 
slender  tendrils,  which  often  have  central  mechanical  tissues  that  may  become 
strengthened  on  attachment.  In  flattened  and  spirally  twisted  tendrils  (as  in 
Echinocystis)  the  strains  are  similar  to  those  sustained  by  twining  stems. 

Submersed  stems,  especially  in  running  streams,  may  be  subject  to  considerable 
tension,  but  the  conditions  are  such  as  to  oppose  the  development  of  mechanical 
tissue;  however,  the  vascular  tract  usually  is  in  a  central  position,  as  in  roots  and 
rhizomes,  and  therefore  situated  where  the  strain  is  least  (fig.  791).  The  absence 
of  rigid  mechanical  tissues  is  distinctly  beneficial  in  such  plants  as  Nereocystis 
and  the  water  lilies,  since  the  floating  synthetic  organs  thus  are  enabled  to  rise  and 
fall  with  the  water.  Furthermore,  the  absence  of  mechanical  tissues  and  the  conse- 
quent easy  rupture  of  the  stems  favor  vegetative  propagation. 

Pendulous  organs,  such  as  fruit  stalks,  commonly  have  central  mechanical 
strands.  Thus  the  most  diverse  of  organs,  namely  roots,  rhizomes,  tendrils,  sub- 
mersed stems,  and  pendulous  stalks,  organs  which  agree  only  in  frequent  or  con- 
stant exposure  to  tension,  have  central  mechanical  tissues,  contrasting  with  the 
peripheral  mechanical  tissues  of  erect  herbaceous  stems. 


ECOLOGY 

Compression  strength.  —  The  trunks  of  trees  are  subject  to  compression,  owing 
to  the  weight  of  the  parts  above.  Longitudinal  pressure  of  this  sort  requires 
columnar  strength.  In  many  trees  (as  in  the  walnut,  red  cedar,  and  most  conifers) 
there  is  a  solid  supporting  column  whose  central  portions  are  the  hardest  and 
strongest,  but  in  some  instances  (as  in  the  sycamore)  the  supporting  column  may 
become  a  hollow  cylinder,  owing  to  the  decay  of  the  heartwood. 

Roots  and  rhizomes  are  subject  to  radial  pressure  as  they  grow  in  diameter.  A 
peripheral  mechanical  cylinder  is  the  most  effective  means  of  resisting  such  radial 
pressure,  and  this  is  present  in  many  rhizomes  in  addition  to  a  central  mechanical 
strand.  Prop  roots  (fig.  739)  are  subject  to  unusual  strains,  since  stem  flexion 
causes  severe  tension  on  one  side  of  the  plant  and  equally  severe  compression  on  the 
other.  The  prop  roots  of  corn  often  contain  both  central  strands  and  peripheral 
cylinders  of  mechanical  tissue,  so  that  they  are  well  fitted  to  withstand  strains  of 
every  sort. 

5.    THE   PROTECTIVE   RELATIONS   OF   STEMS 

Introductory  remarks.  — The  greatest  of  dangers  to  plants  is  excessive 
transpiration,  and  to  this  the  aerial  stem  is  especially  exposed;  indeed, 
in  deciduous  trees  and  shrubs  the  aerial  stem  is  the  most  exposed  of  all 
organs,  since  it  alone  is  subject  to  transpiration  and  to  other  aerial 
dangers  during  periods  of  drought  or  cold.  Aerial  stems  are  struc- 
turally the  best  protected  of  plant  organs,  save  only  seeds.  In  many 
cases  the  stems,  as  well  as  the  leaves,  are  deciduous,  and  in  such  plants 
protective  structures  usually  are  much  less  developed,  their  habit  of  life 
constituting  their  chief  protection. 

Epidermis.  —  In  aerial  herbaceous  stems  the  protective  tissues  resemble  those  of 
leaves  (p.  567),  the  outer  epidermal  walls  being  highly  cutinized,  while  waxy  coats 
and  hairy  coverings  occur  in  many  cases.  In  the  European  mistletoe  (Viscum 
album)  and  in  one  of  the  maples  (Acer  striatum),  the  epidermis,  through  continual 
radial  division,  remains  as  a  relatively  permanent  layer,  a  forty-year-old  trunk  of  the 
latter  sometimes  being  covered  with  a  true  epidermis. 

Bark.  —  In  most  trees  and  shrubs  the  inception  of  secondary  growth 
is  followed  by  the  rupture  and  exfoliation  of  the  epidermis  and  of 
portions  of  the  cortex.  Thenceforth  the  protective  tissues  generally  are 
called  bark,  a  term  including  the  heterogeneous  complex  of  living  and 
dead  cells  outside  the  cambium  ring.  Apart  from  remnants  of  the 
epidermis  and  the  primary  cortex,  the  bark  consists  chiefly  of  the 
phellogen  and  its  products  and  of  the  secondary  phloem,  the  latter 
including  new  living  cells  near  the  cambium  (sieve  tubes,  companion 
cells,  parenchyma),  dead  cells  of  similar  character  farther  out,  and 


STEMS 


705 


mechanical  elements  (largely  bast  fibers). 
The  development  of  the  cork  cylinder 
usually  occasions  the  death  of  all  cells 
external  to  it,  since  it  checks  the  move- 
ment of  material  from  within. 

Cork.  —  Structural  features.  —  The 
most  important  protective  tissue  of  the 
bark  is  the  cork,  which  is  developed 
from  a  meristematic  layer  known  as 
the  phellogen  or  cork  cambium.  Occa- 
sionally this  layer  arises  from  the  epi- 
dermis, as  in  some  Rosaceae  and  in 
many  herbs  (fig.  1031),  but  much  more 
commonly  the  phellogen  layer  arises  in 
the  primary  cortex  (figs.  1032,  1033),  as 
in  most  woody  stems  and  in  various 
underground  stems  (e.g.  potato  tubers).  The  region  usually  involved 
is  the  outermost  cortical  layer,  the  hypodermis,  but  phellogen  may 
develop  in  any  of  the  deeper  layers,  net  excluding  the  endodermis; 
even  the  pericycle  sometimes  gives  rise  to  cork.  Cork  is  developed 
outward  from  the  phellogen  layer,  which  toward  the  inside  may  give 
rise  to  phelloderm  or  cork  cortex;  the  phellogen,  cork,  and  phello- 


FIG.  1031.  —  A  cross  section  o\ 
the  outer  part  of  a  stem  of  the  bone- 
set  (Eupatorium  per/olialum),  show- 
ing the  development  of  epidermal 
cork  (c);  at  the  original  epidermal 
walls;  b,  later  cross  walls,  whose  ap- 
pearance indicates  the  inception  of 
cork  formation;  note  the  thick- 
walled  hypodermis  (h)  which  forms 
a  mechanical  cylinder  around  the 
cortical  parenchyma;  highly  mag- 
nified. 


1032  1033 

FIGS.  1032,  1033  —  1032,  a  partial  cross  section  of  a  stem  of  Jussiaea  peruviana  from 
a  dry  habitat,  showing  the  development  of  cork  tissue  (c)  underneath  a  stereome  bundle 
of  thick -walled  cells  (s) ;  from  SCHENCK;  1033,  a  cross  section  of  the  outer  part  of  a 
bur  oak  twig  (Quercus  macrocar pa) ,  showing  the  layers  of  the  periderm;  p,  the  phello- 
gen, from  which  cork  (c)  develops  externally  and  phelloderm  (d)  internally;  note  that 
the  phelloderm  contains  chloroplasts,  that  the  cork  layer  is  without  air  spaces,  and  that 
the  tissues  external  to  the  cork  are  rupturing;  both  figures  highly  magnified. 


706  ECOLOGY 

derm  together  are  known  as  periderm.  Eventually  this  phellogen 
layer  ceases  its  activity  and  a  new  phellogen  layer  develops  under- 
neath the  old,  though  in  some  cases  the  original  periderm  layer 
persists  for  years  (as  in  the  silver  fir),  or  even  throughout  life  (as  in 
the  beech),  the  lateral  growth  of  the  phellogen  keeping  pace  with 
the  annual  increase  of  the  secondary  wood. 

The  phellogen  consists  of  delicate  plasmatic  tabular  cells,  which  di- 
vide tangentially,  the  outer  half  becoming  cork,  while  the  inner  remains 
phellogen  and  divides  again.  The  cork  cells  also  are  tabular,  and, 
as  there  is  little  displacement  or  radial  division,  they  are  arranged  in 
radial  rows.  The  walls  soon  become  partly  or  wholly  suberized,  that 
is,  a  complex,  fatty  substance  known  as  suberin  replaces  or  is  added  to 
the  original  cellulose  after  which  the  cells  die.  The  walls  of  mature  cork 
cells  are  brownish,  the  lumina  containing  air,  and  sometimes  tannins, 
crystals,  and  other  excretions;  intercellular  air  spaces  are  practically 
wanting. 

In  the  cork  oak,  which  supplies  commercial  cork,  the  periderm  layers  are  very 
thick.  In  certain  trees  (as  in  the  winged  elm,  sweet  gum,  and  hackberry)  cork 
develops  irregularly,  forming  peculiar  warts,  wings,  or  thorns.  Often  there  are 
annual  growth  rings  in  cork  tissues  comparable  to  those  in  wood,  broad  layers  of 
large  cells  alternating  with  thin  layers  of  narrow  cells.  In  the  paper  birch  and 
in  various  other  trees  and  shrubs,  the  bark  is  exfoliated  in  thin  sheets  or  strips  by 
reason  of  an  alternation  of  dense  and  loose  layers  of  this  sort  (fig.  1034). 

The  influence  of  external  factors  upon  cork  development.  —  It  has  been 
shown  elsewhere  that  under  water,  phellogen  develops  into  aerenchyma 
rather  than  into  cork,  and  that  the  development  of  cork  layers  beneath 
lenticels  is  favored  by  desiccation.  Probably  cork  formation  is  favored 
generally  by  desiccation  resulting  from  transpiration,  the  cells  originating 
from  the  phellogen  tending  to  deposit  suberin,  just  as  epidermal  cells 
deposit  cutin  or  surface  wax  under  similar  conditions.  This  view  is 
supported  by  the  fact  that  on  the  inner  side,  where  less  exposed  to 
desiccation,  phellogen  cells  develop  into  phelloderm,  which  is  a  loose 
tissue  resembling  the  complementary  tissue  of  lenticels  and  the  products 
of  phellogen  when  developed  under  water.  In  the  sassafras  and  proba- 
bly in  trees  generally,  cork  formation  is  stronger  on  the  lighted  than  on 
the  shaded  side  of  the  branches. 

Possibly  the  strong  radial  growth  in  developing  phellogen  is  due  to  the  fact  that 
the  path  of  least  resistance  is  in  the  radial  direction.  When  a  branch  is  cut,  the  liv- 
ing cells  bordering  the  wound  are  incited  to  active  growth,  and  the  wound  is  covered 
by  a  so-called  callus,  one  of  the  most  prominent  elements  of  which  is  cork ;  such  cork. 


STEMS  707 

arising  from  a  phellogen  layer  originating  in  the  callus,  is  known  as  wound  cork. 
Probably  the  incitation  to  cambial  activity  is  given  by  the  increased  flow  of  material 
arising  from  greatly  accelerated  transpiration,  which  also  favors  cork  development. 
The  direction  of  growth  of  the  callus  tissues  is  in  the  path  of  least  resistance,  that , 
is,  toward  the  exterior.  When  the  cork  layer  of  a  potato  tuber  is  exposed  to  strong 
radial  pressure  during  its  development,  the  chief  cell  divisions  are  radial,  resulting 
in  strong  lateral  growth  instead  of  the  usual  radial  growth.1 

The  thickness  of  bark  varies  considerably  with  the  habitat,  being 
greatest  in  deserts  and  other  dry  situations  and  in  alpine  regions,  and  least 
in  the  tropical  rain  forest.  Individuals  of  a  species  common  to  two 
situations  have  the  thicker  bark  in  the  more  xerophytic  habitat;  alpine 
and  light  cultures  show  more  bark  than  lowland  and  shade  cultures. 
Probably  in  most  cases  thick  bark  is  associated  with  high  transpiration 
and  thin  bark  with  low  transpiration.  The  slight  development  of  bark 
in  roots  and  in  rhizomes  also  is  in  harmony  with  this  view.  Thick 
bark  in  submersed  stems  does  not  invalidate  this  view,  since  increased 
thickness  in  such  cases  is  due  to  the  large  development  of  air  spaces 
(as  in  Decodon,  p.  553). 

The  role  of  cork.  —  The  conduction  of  food  is  the  chief  role  of  living 
bark,  but  the  dead  bark  has  a  protective  role  of  great  importance  be- 
cause of  the  slowness  of  its  exfoliation.  The  thickness  of  the  bark  with 
its  large  number  of  dead  air-containing  cells  contributes  to  its  pro- 
tective efficiency,  but  the  cork  layer  is  by  far  the  most  significant 
feature.  Partly  because  of  the  absence  of  air  spaces,  but  more  because 
of  suberization,  cork  is  about  the  most  impermeable  of  tissues,  and  thus 
is  of  great  value  in  checking  transpiration.  A  potato  that  loses  but  0.04 
grams  in  weight  in  twenty-four  hours  if  unpeeled,  loses  2.56  grams  if 
peeled,-  or  sixty-four  limes  as  much,  and  yet  the  cork  layer  of  a  potato 
is  so  thin  as  to  appear  to  the  naked  eye  as  a  mere  film.  Twigs  with 
the  thinnest  of  cork  layers  transpire  with  almost  inappreciable  slowness, 
if  the  lenticels  and  cracks  are  artificially  sealed.  Probably  the  chief 
value  of  cork  is  in  checking  transpiration  in  periods  of  relative  in- 
activity, as  in  seasons  of  periodic  drought  or  of  winter  cold,  when  there 
is  little  or  no  absorption;  during  such  periods  the  exposed  parts  of 
deciduous  trees  and  shrubs  are  completely  mantled  by  an  almost  imper- 
vious coat  of  cork  and  bud  scales. 

1  Similarly,  in  the  spores  of  Equisetum  and  in  the  egg  of  Fucus,  the  usual  direction 
assumed  by  new  cell  walls  can  be  changed  through  the  influence  of  pressure;  comparable 
pressure  effects  have  been  reported  in  the  roots  of  Vicia  Faba,  but  recent  work  is  not 
confirmatory. 


708  ECOLOGY 

Cork  is  relatively  impermeable  to  air  as  well  as  to  water,  and  after  its  formation, 
carbohydrate  synthesis  and  aeration  in  subjacent  tissues  become  greatly  reduced, 
except  in  the  neighborhood  of  lenticels.  Cork  is  also  a  poor  conductor  of  heat,  so 
that  changes  of  temperature  are  slower  within  the  plant  than  outside.  Cork  pre- 
vents the  invasion  of  living  tissues  by  parasitic  fungi  and  bacteria;  a  freshly  cut 
surface  of  a  potato  tuber  develops  wound  cork  so  rapidly  that  after  twelve  hours 
bacterial  infection  is  impossible.  Cork  cells,  like  tracheids,  tracheae,  and  bast 
fibers,  are  more  efficient  dead  than  living.  Such  local  growths  of  cork  as  the  wings 
of  the  winged  elm  and  the  warty  projection  of  the  hackberry  are  probably  of  no 
advantage  to  the  trees  producing  them.  Sometimes  at  the  close  of  the  vegetative 
season,  bark  is  not  perfectly  "  ripened,"  that  is,  it  contains  considerable  water,  and 
the  various  protective  and  mechanical  elements  are  not  fully  formed.  In  many 
plants  transferred  from  warmer  to  colder  climates  the  shoots  die  back  in  the  winter, 
because  the  vegetative  season  is  too  short  to  permit  the  ripening  of  the  bark  and 
wood. 

Various  bark  features.  —  Color.  —  Young  bark  commonly  is  green,  because  the 
cortical  chlorophyll  is  evident  through  the  transparent  epidermis.  Soon  the  stem 
ceases  to  appear  green,  the  chief  cause  of  change  in  color  being  the  development  of 
the  cork  layer,  whose  opacity  makes  the  chlorophyll  invisible.  The  common  bark 
colors  are  gray,  brown,  and  black,  but  red  occurs,  as  in  some  dogwoods,  and  white, 
as  in  some  birches.  As  the  tree  matures,  the  characteristic  bark  color  may  be  seen 
only  on  the  younger  branches,  if  the  older  limbs  are  furrowed.  In  a  few  cases,  as  in 
the  mistletoe,  moonseed,  sassafras  and  greenbrier,  the  relative  freedom  from  cork 
formation  permits  the  green  color  to  remain  evident  longer  than  usual.  Such 
green-stemmed  trees  as  the  bamboo  and  the  banana  are  in  reality  gigantic  herbs,  in 
which  ordinary  bark  does  not  develop.  Often  the  exterior  and  the  interior  of  the 
bark  are  differently  colored,  as  in  the  hemlock,  where  it  is  black  without  and  red 
within,  and  as  in  the  yellow-barked  oak,  which  is  named  from  its  inner  bark,  the 
outer  bark  giving  rise  similarly  to  the  name,  black  oak.  Bark  colors,  especially 
interior  colors,  often  are  due  to  the  presence  of  various  excreted  products,  such  as 
tannins.  Advantages  in  the  various  colors  are  not  to  be  looked  for. 

Smoothness  and  roughness ;  exfoliation.  — •  While  the  epidermis  persists,  young 
stems  are  smooth,  except  in  the  neighborhood  of  lenticels  and  leaf  scars ;  the  latter 
are  of  various  shapes  and  sizes,  and  differ  widely  in  the  number  and  arrangement 
of  the  vascular  strands,  whose  s?vered  and  healed  surfaces  are  conspicuous  as  slight 
emergences  within  the  scar  (fig  1059).  For  a  few  years  most  stems  remain  smooth 
or  smoothish,  owing  to  the  development  of  bark  tissues  as  the  stem  increases  in 
diameter.  In  some  trees  (as  in  the  beech)  continued  lateral  growth  causes  the 
bark  to  remain  thin  and  smooth  throughout  life;  the  tropical  rain  forest  in  particu- 
lar is  rich  in  smooth-barked  trees.  In  various  palms  the  bark  is  soft  and  spongy, 
hence  affording  an  excellent  habitat  for  epiphytes.  In  most  trees  new  phellogen 
areas  develop  at  deeper  levels  or  lateral  growth  fails  to  keep  pace  with  diametral 
increase,  so  that  the  bark  splits  and  becomes  variously  roughened.  Some  trees, 
as  the  bur  oak,  become  furrowed  very  early,  while  others,  as  the  basswood,  remain 
smooth-barked  for  a  longtime,  but  ultimately  become  furrowed  "  Alligator  "  bark 
is  caused  by  the  division  of  the  bark  into  blocks  by  somewhat  equidistant  trans- 
verse and  longitudinal  furrows  (as  in  Nyssa). 


STEMS 


709 


In  a  number  of  trees  the  bark  exfoliates  in  definite  layers 
(fig.  1034),  the  separation  being  in  a  /one  of  weakness,  known 
as  the  separation  layer,  which  is  composed  of  loose  and  weak 
cells  that  alternate  with  the  denser  and  stronger  cork  layers. 
In  trees  with  scaly  bark  the  cork  layers  separate  into  patches 
or  arcs  (as  in  the  sycamore,  cherry,  and  pine),  while  in  plants 
with  ringed  bark,  the  cork  layers. form  concentric  cylinders 
and  the  bark  shreds  or  slivers  off  (as  in  the  grape  and  arbor 
vitae).  In  trees  with  shaggy  bark  the  exfoliating  masses  are 
elongated,  and  in  the  birches  the  bark  exfoliates  in  thin, 
papery  layers.  In  some  trees  the  bark  is  supplemented  in 
its  protective  role  by  dead-leaf  bases  (as  in  Yucca,  p.  588). 

The  protective  significance  of  different  stem 
habits.  —  Introductory  remarks.  —  The  reproductive 
activity  of  stems  appears  to  be  exhibited  somewhat 
equally  in  all  situations,  but  foliage  display  is  much 
more  prominent  in  mesophytic  than  in  xerophytic 
habitats,  probably  because  the  excessive  transpiration 
in  the  latter  makes  it  impossible  for  plants  to  de- 
velop greatly  elongated  stems;  nor  could  plants 
with  abundant  foliage  resist  desiccation  if  such 
development  were  possible. 

Tropical  evergreen  trees.  —  Only  in  the  tropical 
rain  forest  is  unrestricted  foliage  display  observed, 
for  there  alone  because  of  the  lack  of  drought  or 
cold  is  continued  activity  possible  without  protective 
structures  or  behavior.  Daily  synthesis  and  never- 
failing  moisture  combine  to  produce  the  most  luxuri- 
ant vegetation  (fig.  846).  An  excellent  example  of 
the  well-nigh  perfect  growth  conditions  of  the  rain 
forest  is  seen  in  the  bamboo,  whose  stems  sometimes  grow,  as  much  as 
eight  meters  in  a  single  month,  or  at  a  rate  of  more  than  twenty-five 
centimeters  per  day.  However,  the  very  excellence  of  tropical  condi- 
tions causes  such  a  superabundance  of  vegetation  that  only  an  occa- 
sional seed  of  any  given  species  can  fall  in  a  place  suitable  for  germi- 
nation, and  that  only  a  few  of  the  germinating  plants  can  ever  reach 
maturity. 

Sclerophyllous  evergreen  trees.  —  Except  in  the  rain  forest,  most  trees 
are  either  deciduous  or  sclerophyllous  (i.e.  having  stiff  evergreen  leaves), 
and  must  endure  inclement  seasons,  characterized  either  by  drought  or 


FIG.  1034.  —  A 
portion  of  a  stem 
of  the  nine-bark 
(Physocarpus  opuli- 
folius),  showing  the 
shredding  of  the 
bark  into  several 
thin  exfoliating 
layers. 


7  io  ECOLOGY 

by  cold.  In  such  regions  trees  have  thick,  impermeable  bark,  and 
the  leaves  of  sclerophyllous  species  are  heavily  cutinized  and  thus  well 
protected  against  excessive  transpiration  (figs.  809,  955).  The  advan- 
tages possessed  by  such  trees  are  extensive  foliage  display  and  the  pos- 
sibility of  synthetic  work  at  all  seasons.  However,  there  are  corre- 
sponding disadvantages,  since  much  energy  and  material  are  used  in 
the  construction  of  protective  tissues,  and  since  the  heavily  cutinized 
layers  which  reduce  transpiration  also  reduce  synthesis.  Sclerophyl- 
lous evergreens  may  be  broad-leaved,  as  in  the  olives,  oaks,  and  hollies 
of  warm  temperate  climates,  or  narrow-leaved,  as  in  the  conifer  which 
have  their  culmination  in  cold  climates. 

Leafless  evergreen  trees.  —  Leafless  trees,  such  as  Casuarina  and  the 
taller  cacti  (fig.  1035),  are  well  fitted  for  climates  in  which  all  seasons 
are  unfavorable,  there  being  a  relative  minimum  of  synthetic  surface 
and  a  relative  maximum  of  protection  from  excessive  transpiration, 
because  of  verticality,  leaflessness,  water  accumulation,  a  highly  cutin- 
ized epidermis,  and  slight  surface  exposure  in  proportion  to  volume. 
Restriction  in  foliage  display,  and  hence  in  synthesis,  generally  is  dis- 
advantageous, but  in  arid  regions  it  is  of  marked  advantage  since 
excessive  transpiration  entails  far  greater  danger  than  does  a  scanty 
food  supply.  The  cactoid  form  is  illustrated  not  alone  by  the  cacti 
of  American  deserts,  but  also  by  wholly  unrelated  plants  of  African 
deserts  having  almost  identical  form  (e.g.  Euphorbia  and  Stapelia). 
The  four  evergreen  habits  above  noted,  namely,  rain  forest  evergreens, 
broad-leaved  sclerophylls,  narrow-leaved  sclerophylls,  and  leafless  ever- 
greens, are  fitted  in  the  order  mentioned  for  conditions  that  are  in- 
creasingly xerophytic.  Evergreen  shrubs  may  be  divided  into  the 
same  four  classes  although  the  shrubs  in  each  class  extend  into  much 
severer  climates  than  do  the  trees. 

Deciduous  trees.  —  Different  as  are  the  classes  of  evergreens  and  the 
conditions  for  which  they  are  fitted,  they  agree  in  not  changing  their 
aspect  from  season  to  season.  Deciduous  trees  (figs.  843-845,  956) 
and  shrubs,  on  the  other  hand,  exhibit  leaf  abscission  at  the  inception  of 
dry  or  cold  periods,  thus  presenting  two  seasonal  aspects.  Such  trees 
in  the  vegetative  period  may  have  leaves  that  are .  as  expanded  and 
about  as  little  protected  as  are  those  of  rain  forest  evergreens,  while  in 
the  inclement  period  they  are  as  well  protected  as  are  the  cacti  and 
better  protected  than  are  the  sclerophylls.  The  obvious  advantages 
of  the  deciduous  habit  are  partially  offset  by  obvious  disadvantages; 


STEMS 


711 


FIG.  1035.  —  The  giant  cactus  (Cereus  giganteus),  whose  large  and  leafless  vertical 
stems  contain  great  quantities  of  water;  note  the  prominent  fluting  of  the  stems; 
mountains  near  Tucson,  Ariz.  —  From  MACDOUGAL  (Courtesy  of  the  Carnegie  Institu- 
tion of  Washington.) 


;i2  ECOLOGY 

considerable  energy  and  material  are  utilized  in  the  development  of 
protective  tissues  and  in  complete  leaf  renewal  each  year;  also  decidu- 
ous trees  are  inferior  to  sclerophylls  in  the  amount  of  synthetic  activity 
on  favorable  days  during  the  leafless  period  and  in  leaf  protection  on 
unfavorable  days  during  the  period  of  leafage. 

Evergreen  herbs.  —  Many  herbs,  especially  in  the  tropical  rain  forest, 
are,. like  the  trees,  evergreen,  presenting  the  same  aspect  at  all  seasons. 
There  are  some  such  plants,  even  in  periodic  climates,  as  the  scouring 
rush  (Equisetum  hyemale,  figs.  1054,  1055)  and  the  prickly  pear 
(Opuntia,  figs.  1040-1042),  which  are  leafless  evergreen  herbs  well- 
fitted  to  withstand  exposure.  Also  there  is  a  large  class  of  low  herbs 
writh  sclerophyllous  evergreen  leaves,  as  Linnaea,  Mitchella,  Cornns 
canadensis,  and  the  wintergreens  (Pyrola,  Gaultheria,  Chimaphild). 
Another  class  of  herbaceous  evergreens  includes  forms  which,  at  least 
in  winter,  are  without  aerial  stems  (as  Hepatica,  Mitella,  Geum,  and 
Polystichum) ;  in  this  group  are  a  number  of  species  with  relatively 
mesophytic  leaves  which  readily  survive  the  winter  if  transpiration  is 
prevented  by  coverings  of  leaves  or  snow.  Some  herbaceous  ever- 
greens are  more  xerophytic,  occurring  in  exposed  situations  (as  Ar- 
temisia, Taraxacum,  Lepidium,  and  Oenothera,  fig.  1036);  when  such 
plants  are  not  protected  by  coverings  of  leaves  or  snow,  many  of  the 
outer  leaves  die,  but  the  younger  leaves  within  are  uninjured.  Most 
of  the  xerophytic  evergreen  herbs  and  shrubs  of  cold  climates  form 
ground  rosettes  or  have  prostrate  stems,  and  in  many  cases  the  aerial 
organs  are  arranged  in  cushions  (fig.  1060) ;  in  all  such  cases  closeness 
to  the  ground  or  to  other  organs  reduces  transpiration  and  lessens  the 
detrimental  effect  of  sudden  changes  of  temperature.  Among  the 
evergreen  herbs  should  be  classed  most  of  the  lichens,  liverworts,  and 
mosses,  their  small  size  often  insuring  sufficient  winter  protection  by 
snow  and  leaves ;  furthermore,  most  lichens  and  many  mosses  are  quite 
unharmed  by  months  of  exposure  to  transpiration  without  absorption. 
Such  plants  as  the  melon  cacti  (Echinocactus ,  fig.  1063)  may  be  re- 
garded as  evergreen  herbs  which  are  the  extreme  antithesis  of  tropical 
evergreen  trees,  having,  on  account  of  their  spherical  shape,  the  least 
possible  transpiring  surface  in  proportion  to  volume,  and  therefore 
illustrating  the  culmination  of  protective  form  among  aerial  organs. 

Deciduous  herbs.  —  In  the  great  mass  of  herbs,  particularly  in 
periodic  climates,  aerial  stems  as  well  as  leaves  die  at  the  inception  of 
the  unfavorable  season.  The  death  of  the  stem  is  not,  as  in  the  case 


STEMS  713 

of  the  leaves  of  woody  plants,  associated  with  a  definite  process  of 
abscission  (except  rarely,  as  in  Polygonatum,  fig.  983),  but  the  death 
of  the  stem,  even  more  definitely  than  that  of  the  leaf,  is  occasioned  by 
exposure  to  severe  conditions.  Only  such  parts  survive  as  are  in  or 
near  the  soil.  In  stem  fall,  as  it  may  be  called,  the  stem  gradually  rots 
or  weakens  and  falls  to  the  ground  and  thus  is  comparable  to  leaf  fall 
in  ferns  or  in  such  trees  as  the  beech  and  many  oaks,  where  death 
ensues  without  the  development  of  a  well-defined  absciss  layer.  In 
periods  of  drought,  transpiration  becomes  too  great  to  permit  the  survi- 
val of  most  aerial  herbaceous  stems,  and  in  periods  of  cold  also,  tran- 
spiration becomes  relatively  excessive  in  proportion  to  the  reduced  rate 
of  absorption.  Aerial  organs  may  die  also  as  a  direct  result  of  freez- 
ing, though  many  plants,  especially  in  alpine  and  arctic  regions,  are 
quite  uninjured  thereby,  partly,  perhaps,  because  of  obviously  pro- 
tective structures,  but  more  because  of  low  water  content,  high  osmotic 
pressure,  or  other  and  mostly  unknown  features.  Tropical  plants, 
on  the  other  hand,  may  suffer  injury  or  even  death  before  the 
temperature  reaches  the  freezing  point  of  water.  Many  stems  die 
in  summer,  even  when  the  water  supply  is  adequate;  sometimes 
such  death  is  attributed  to  old  age,  but  old  age  is  hardly  a  cause 
of  death,  being  rather  a  result  of  certain  causes,  as  yet  imperfectly 
understood  (Part  II,  p.  480). 

Perennial  deciduous  herbs.  —  In  many  perennial  herbs  essentially  all 
stem  organs  are  aerial,  the  perennating  portions  being  just  above  the 
soil,  as  in  Lechea,  Satureja  glabra  (fig.  985),  and  Linaria  canadensis;  in 
these  plants  lateral  shoots  arise  at  the  base  of  the  erect  stem  and  develop 
into  short  runners  that  remain  over  winter,  while  the  erect  stem  dies.  In 
many  runner  plants,  as  Fragaria,  Hydrocotyle  (fig.  712),  and  Trifolium 
repens,  there  are  no  prominent  erect  stems,  the  summer  and  the  winter 
conditions  thus  being  essentially  alike.  An  herbaceous  group  with  more 
numerous  representatives  is  that  in  which  the  perennating  organ  is  a 
multicipital  stem  (as  in  the  dock  and  the  dandelion,  fig.  995) ;  here  the 
stem  seems  to  disappear  completely,  the  basal  lateral  shoots  remaining 
embryonic  through  the  inclement  season.  The  majority  of  herbs  in 
periodic  climates  perennate  by  means  of  underground  organs,  thus 
disappearing  from  view  during  the  season  of  inactivity;  the  erect  stem 
dies  to  the  ground  line  or  lower,  and  the  stem  is  represented  only  by 
the  rhizome  (figs.  978-981,  983),  tuber  (figs.  989,  990),  bulb  (fig.  991), 
or  corm  (fig.  993). 


ECOLOGY 


Annuals  and  biennials.  —  The  plants  hitherto  considered  may  be 
placed  in  three  general  classes:  those  with  uniform  seasonal  aspect;  those 
in  which  the  leaves  are  shed  at  the  inception  of  the  period  of  drought 
or  cold;  and  those  in  which  all  aerial  portions  are  lost  at  the  beginning 
of  the  inclement  season.  The  fourth  and  final  class  of  land  plants  is 
that  in  which  the  entire  plant  dies  at  the  inception  of  the  inclement 
season.  The  most  representative  members  of  this  class  are  the  annuals, 


FIG.  1036. — Winter  rosettes  of  an  evening  primose  (Oenothcra),  with  leaves  closely 
appressed  to  the  ground;  note  the  small  amount  of  leaf  overlap,  due  to  high-ranked 
phyllotaxy  and  to  variation  in  leaf  length;  Chicago,  111.  —  Photograph  by  LAND. 

which  are  plants  that  live  only  in  the  favorable  season,  and  which  have 
fewer  protective  structures  than  do  other  plants.  The  annual  alone 
among  plants  remains  through  seasons  of  severity  solely  in  the  form  of 
its  progeny,  the  seed.  Related  to  the  annuals  are  the  biennials,  which 
are  plants  that  live  in  two  vegetative  seasons.  In  the  first  season  most 
biennials  develop  a  rosette  (as  in  the  evening  primrose,  peppergrass,  and 
mullein,  figs.  1036,  840),  which  remains  as  such  through  the  period  of 
drought  or  cold.  During  the  second  vegetative  season  an  erect  shoot 
commonly  appears  and  develops  flowers  and  fruits,  death  ensuing  at  the 
inception  of  the  second  inclement  period. 

Annuals  and  biennials  do  not  perennate,  because  they  fail  to  develop 
lateral  basal  shoots ;  occasionally,  however,  some  individuals  of  species 


STEMS  715 

that  are  classed  as  biennials  develop  such  shoots  and  become  triennials 
or  even  quadrennials  (as  in  Artemisia  canadensis,  Arabis  lyrata,  and 
some  mutants  of  Oenothera  Lamarckiana,  figs.  716,  717).  Plants  that 
are  annuals  in  cold  climates  may  be  perennials  in  the  tropics  (e.g.  the 
castor  bean),  and  it  is  possible  to  cultivate  some  annuals  or  biennials 
•  as  perennials  by  exposing  them  to  favorable  conditions  during  periods 
of  severity  (as  in  the  pansy).  Many  annuals  and  biennials  die  rela- 
tively early  in  the  vegetative  season;  for  example,  the  staminate  plants 
of  the  hemp  (Cannabis  sativa)  die  in  late  summer,  while  the  pistillate 
plants  with  the  developing  seeds  still  are  green  and  in  full  vigor. 
Indeed,  most  annuals  and  biennials  that  fruit  early  in  the  season  die 
soon  thereafter.  The  causes  of  such  phenomena  are  unknown. 

Aquatic  herbs.  —  In  the  water  the  protective  habits  of  many  plants 
are  comparable  to  those  of  the  land,  especially  in  such  as  have 
rhizomes,  but  unattached  water  plants  form  a  class  by  themselves. 
Such  floating  plants,  which  include  numerous  large  and  small  algae 
and  some  ferns  and  seed  plants,  are  among  the  best  protected  of 
plants.  The  winter  buds  or  shoots  sink  in  autumn  and  rise  in  spring, 
thus  requiring  no  such  utilization  of  energy  and  material  as  in  the  deep 
placement  of  rhizomes  and  in  the  subsequent  emergence  of  erect  shoots. 
The  chief  disadvantages  inhering  in  aquatic  habitats  arise  from  the 
instability  of  the  water  which  makes  impossible  the  growth  of  tall  aerial 
organs  and  from  its  high  refrangibility,  which  permits  only  moderate 
synthetic  activity  in  submersed  organs. 

The  compensatory  relations  of  plant  habits.  —  General  remarks.  — 
In  the.  preceding  paragraphs,  reference  has  been  made  to  the  advantages 
and  disadvantages  of  the  various  classes  of  plant  habits,  and  it  may  be 
desirable,  partly  by  way  of  summary,  to  contrast  them  further.  In 
general,  the  principle  of  compensation  is  illustrated,  disadvantages  being 
offset  by  corresponding  advantages.  Erect  and  branching  stems,  whose 
habit  well  suits  them  for  optimum  display,  are  poorly  suited  for  vegeta- 
tive reproduction,  and  the  construction  of  their  protective  tissues  re- 
quires  a  great  consumption  of  energy  and  material.  On  the  other 
hand,  horizontal  ground  stems,  which  are  well  suited  for  vegetative 
reproduction  and  which  are  protected  with  a  minimum  consumption  of 
material,  are  unsuited  for  optimum  foliage  display. 

Annuals.  —  Annuals  as  a  rule  are  without  conspicuous  protective 
structures;  this  is  not  a  disadvantage,  because  the  non-existence  of 
these  forms  during  periods  of  severity  makes  protective  tissues  unneces- 


71 6  ECOLOGY 

sary  for  their  optimum  development.  Thus  an  obvious  advantage  of 
these  plants  is  that  all  their  energy  and  material  are  consumed  in  the 
development  of  organs  directly  concerned  with  nutrition  and  repro- 
duction. A  great  disadvantage  of  the  annual  habit  arises  from  the 
necessity  that  the  entire  race  start  anew  from  seed  each  year ;  thus  the 
shortness  of  the  season  excludes  annuals  from  alpine  and  arctic  regions,  • 
and  maximum  foliage  display  is  imp3ssible  for  such  forms  in  any 
region.  Probably  their  chief  disadvantage  is  that  ultimately  they  are 
excluded  from  most  habitats  by  perennials,  owing  to  the  increasing  pre- 
emption of  ground  by  the  perennating  organs  of  the  latter.  Annuals 
reach  their  culmination  in  open  situations,  as  in  deserts  and  in  culti- 
vated fields,  and  along  shores  and  roadsides. 

Ground  perennials.  —  Plants  whose  perennating  organs  are  rhizomes, 
bulbs,  corms,  tubers,  runners,  rosettes,  or  multicipital  stems  may  be 
termed  ground  perennials;  such  plants  have  a  high  measure  of  pro- 
tection with  a  minimum  utilization  of  structural  material.  The  deeper 
the  organ,  the  more  complete  is  its  protection  from  cold  or  transpira- 
tion, while  the  shallower  the  organ,  the  less  is  the  amount  of  material 
consumed  in  reaching  the  surface  at  the  inception  of  the  growing 
season.  Rhizome  and  runner  plants  surpass  all  other  land  plants  from 
the  standpoint  of  vegetative  propagation,  and  bulbs,  tubers,  and  corms 
are  especially  advantageous  by  reason  of  their  abundant  food  supply, 
which  facilitates  the  rapid  development  of  aerial  organs.  Rosettes  and 
multicipital  stems  have  similar  advantages,  if  the  roots  contain  abun- 
dant food  (as  in  the  dandelion  and  the  dock),  though  such  habits  are 
poorly  suited  for  vegetative  reproduction.  Because  the  new  shoots 
each  year  arise  from  the  soil  level  or  below,  ground  perennials,  like 
annuals,  are  unable  to  display  a  maximum  amount  of  foliage.  How- 
ever, rhizome  perennials  have  about  the  most  advantageous  of  plant 
habits,  since  they  exhibit  the  combination  of  an  enduring  horizontal 
stem  with  a  periodic  erect  stem;  such  a  combination  results  in  maxi- 
mum protection  with  a  minimum  utilization  of  structual  material,  in 
maximum  reproduction,  and  in  adequate,  though  minor,  foliage  dis- 
play. Plants  with  bulbs,  tubers,  or  corms  are  suited  especially  for 
districts  with  short  vegetative  periods,  as  the  Mediterranean  region, 
where  the  winter  is  too  cold  and  the  summer  too  dry  for  optimum 
growth  activity,  the  most  favorable  seasons  being  periods  of  short 
duration  in  the  spring  and  autumn ;  in  these  plants  much  of  the 
food  utilized  in  a  season's  growth  is  accumulated  the  year  previous, 


STEMS  717 

and  is  ready  for  almost  instant    use   upon  the  arrival  of  favorable 
conditions. 

Trees  and  shrubs.  —  Trees  and  shrubs  utilize  a  large  amount  of 
energy  and  constructive  material  in  the  development  of  a  protective 
bark  and  of  a  mechanical  skeleton,  and  they  are  at  a  disadvantage  in 
the  matter  of  vegetative  reproduction.  However,  growth  resumption  at 
the  point  of  growth  cessation  the  year  previous  makes  possible  an  opti- 
mum display  of  foliage.  Trees  and  shrubs  are  fitted  for  all  climates 
where  there  is  an  adequate  supply  of  available  water.  In  deserts  tree 
development  is  slight,  because  of  constant  and  extreme  exposure  to  ex- 
cessive transpiration  together  with  limited  absorption ;  shrubs,  however, 
often  are  abundant  in  arid  climates.  In  alpine  and  arctic  regions  there 
is  sufficient  moisture,  but  its  unavailability  through  the  long  winter 
makes  life  conditions  severe  for  trees,  on  account  of  continued  transpi- 
ration; shrubs  are  more  fully  developed,  because  they  are  better  pro- 
tected during  the  winter.  There  are  no  places  too  cold  for  trees,  if 
sufficiently  protected  from  transpiration.  Trees  are  absent  from  many 
alpine  and  arctic  habitats  where  snow  lies  on  the  ground  for  most  of 
the  summer,  but  the  most  extensive  treeless  tracts  are  the  prairies, 
where  it  is  probable  that  a  combination  of  inadequate  rainfall  and  ex- 
cessive winter  transpiration  best  accounts  for  the  absence  of  trees. 

The  duration  of  steins.  —  At  one  extreme  as  to  duration  are  ephemeral 
annuals  that  live  but  a  few  weeks  or  even  days,  and  at  the  other  extreme 
are  trees  whose  life  may  be  measured  by  centuries.  Annual  aerial  stems 
occur  not  alone  in  annuals,  but  also  in  most  biennials  and  in  most 
herbaceous  perennials  of  periodic  climates.  Most  bulbs  and  tubers 
live  but  a  year  or  two,  the  old  organ  dying  upon  the  development  of 
new  bulbs  or  tubers.  Of  somewhat  longer  life,  but  still  relatively 
short-lived,  are  various  rhizomes,  which  advance  anteriorly  each  year, 
while  dying  posteriorly,  a  given  portion  commonly  enduring  for  a  few 
years.  Trees  and  shrubs  remain  alive  much  longer,  appearing  to  have 
a  more  or  less  definite  period  of  life,  varying  with  the  species.  While 
in  some  cases  a  trunk  may  endure  for  a  number  of  centuries,  any  given 
part  lives  but  a  few  years,  namely,  for  the  length  of  time  elapsing  before 
the  sap-wood  becomes  transformed  into  heart-wood;  in  many  trees 
the  dead  heart-wood  resists  decay  for  centuries. 

Roots  commonly  equal  or  surpass  stems  in  the  matter  of  duration.  The  stems 
of  annuals  and  of  most  trees  and  shrubs  and  the  underground  stems  of  perennial 
herbs  commonly  are  as  long-lived  as  are  the  roots,  but  the  stems  of  biennials, 


7i8  ECOLOGY 

the  aerial  stems  of  perennial  herbs,  and  the  trunks  of  shrubs  and  trees  that  de- 
velop basal  suckers  usually  are  shorter  lived  than  are  the  roots;  while  the  trunks  of 
the  redwood  live  for  centuries,  the  roots  might  live  indefinitely.  Little  is  known 
concerning  the  causes  of  varying  duration.  In  trees  the  continued  decay  of  the 
heart-wood  may  be  a  factor,  and  perhaps  the  increasing  distance  year  by  year  be- 
tween the  root  tips  and  the  upper  branches  may  involve  a  decreasing  water  supply. 


6.    THE    ACCUMULATION    IN    STEMS    OF   WATER,    FOOD,    AND 
WASTE  PRODUCTS 

Introductory  remarks.  —  Plants  often  are  supposed  to  store  food  or 
water,  which  they  or  their  progeny  utilize  later,  much  as  men  and  ani- 
mals store  food  for  winter  use.  Such  a  conception  appears  to  involve 
forethought,  and  should  be  discarded.1  A  better  conception  is  that 
unused  material  accumulates.  If  a  plant  manufactures  more  carbo- 
hydrate or  protein  than  it  utilizes,  or  if  it  takes  in  more  water  than  it 
utilizes  or  gives  off,  the  residue  necessarily  accumulates.  Sometimes 
such  surplus  food  and  water  are  subsequently  utilized,  but  at  other 
times  they  remain  unused.  In  the  latter  event  they  differ  from  ordi- 
nary waste  products  only  in  that  they  are  capable  of  use  in  constructive 
metabolism. 

The  accumulation  of  air  and  water.  —  Stems,  as  well  as  leaves, 
are  characterized  by  air  spaces  and  air  passages  and  sometimes  by 
capacious  air  chambers,  especially  in  such  hydrophytes  as  Myriophyl- 
lum  (fig.  791),  Hippuris,  and  Hottonia;  the  oxygen  and  carbon  dioxid 
contained  herein  may  be  of  especial  advantage  to  submersed  hydro- 
phytes, if  their  gas  supply  is  otherwise  deficient.  In  some  cases  stem 
air  spaces  assist  in  flotation  (as  in  Hottonia).  Water  accumulation, 
which  has  been  discussed  in  connection  with  leaves,  is  a  conspicuous  phe- 
nomenon also  in  stems,  notably  in  deserts.  The  most  remarkable 
cases  of  water  accumulation  are  in  the  cacti  and  in  plants  of  similar 
form  (as  Euphorbia,  Stapelia,  and  Cavanillesia,  the  latter  having  barrel- 
shaped  trunks);  such  habits  are  advantageous,  because  of  the  small 
transpiring  surface  in  proportion  to  the  stem  volume  (figs.  1035,  1040- 
1042).  During  rainy  periods  the  stems  accumulate  large  quantities  of 
water,  which  become  depleted  during  subsequent  drought.  The  fluted 
stem  of  the  giant  cactus  (fig.  1035)  undergoes  accordion-like  expan- 
sions and  contractions  during  wet  and  dry  periods  respectively,  the  maxi- 

1  The  term,  reserve  food,  is  similarly  objectionable,  since  it  directly  expresses  fore- 
thought ;  a  preferable  expression  is  surplus  food. 


STEMS  719 

mum  circumferential  difference  being  considerable.  Many  plants  out- 
side of  arid  regions  have  fleshy  stems,  as  Portulaca  and  Begonia,  and 
succulence  is  a  notable  feature  of  many  halophytes,  as  Salicornia. 
Many  underground  stems  accumulate  water,  as  well  as  food,  in  large 
amount;  this  is  true  especially  of  tubers  and  corms. 

The  accumulation  of  foods.  —  General  remarks.  —  In  sunshine  most 
leaves  manufacture  more  food  than  is  disposed  of  during  the  day,  this 
excess  commonly  accumulating  as  starch.  During  the  night,  when 
carbohydrate  manufacture  ceases,  this  starch  is  transformed  into  sugar 
and  migrates  to  other  parts  of  the  plant.  Food  accumulations  of 
much  greater  permanence  occur  in  stems,  roots,  and  seeds,  because  the 
manufacture  of  food  is  more  rapid  than  is  its  use  in  growth  or  otherwise. 

Food  accumulation  in  aerial  and  aquatic  stems.  —  In  trees  and  shrubs, 
food  accumulates,  especially  in  the  cortex,  medullary  rays,  and  wood 
parenchyma,  and  in  some  instances  even  in  the  central  pith  region 
(medulla).  The  carbohydrates  that  accumulate  in  the  trunk  may  as- 
sume various  forms,  starch  on  the  whole  being  the  most  representative. 
In  most  trees,  shrubs,  and  evergreen  herbs  of  temperate  and  cold  cli- 
mates, there  is  in  autumn  a  maximum  of  starch,  which  accumulates 
chiefly  in  the  parenchyma.  Since  low  temperatures  favor  the  conver- 
sion of  starch  into  sugar,  the  latter  then  increases  at  the  expense  of  the 
former;  it  is  believed  that  this  sugar  is  of  protective  value  (p.  587). 
In  early  spring  the  sugar  moves  toward  the  buds,  and  again  is  trans- 
formed into  starch,  which  accumulates  especially  in  the  embryonic  leaves. 

In  some  trees  carbohydrates  are  transformed  into  fats  at  the  inception  of  winter, 
while  the  reverse  transformation  takes  place  in  spring;  among  the  trees  that  ac- 
cumulate fats  are  such  northern  trees  as  birches  and  conifers,  and  it  has  been  sup- 
posed that  these  fats  are  of  protective  significance  during  periods  of  low  tempera- 
ture. Sometimes  there  is  developed  in  autumn  in  living  leptome  and  wood  cells 
a  hemicellulose  layer  which  is  dissolved  the  following  spring.  In  herbaceous  stems 
the  cortex  and  bundle  sheath  frequently  are  regions  of  starch  accumulation,  especially 
in  water  plants  (fig.  1017). 

Food  accumulation  in  subterranean  stems.  —  The  chief  subterranean 
stem  organs  of  food  accumulation  are  tubers  and  corms  (figs.  989,  990, 
993),  in  which  the  food  commonly  is  starch  that  is  formed  by  leuco- 
plasts  in  the  cells  in  which  it  accumulates.  Sometimes  the  starch 
grains  of  underground  stems  are  relatively  large,  as  in  the  potato  tuber 
(figs.  1206,  1207),  and  in  the  rhizome  of  Canna,  where  individual  grains 
may  be  0.17  mm.  in  length.  The  chief  advantage  associated  with 


720  ECOLOGY 

corms  and  tubers  is  that  their  food  supply  enables  them  to  renew 
activity  speedily  at  the  inception  of  a  favorable  period,  a  matter  of 
much  significance  in  climates  like  that  of  Italy,  where  short  favorable 
periods  are  intercalated  between  long  periods  of  inactivity. 

The  role  of  accumulated  foods.  —  The  plant  that  accumulates  the 
food  may  itself  utilize  it;  for  example,  during  the  vegetative  season  the 
trunks  of  deciduous  trees  gradually  accumulate  quantities  of  starch 
and  other  foods,  which  are  utilized  in  the  development  of  shoots  the 
following  spring;  or  the  accumulated  food  may  be  utilized  by  vege- 
tative offshoots,  as  in  plants  that  give  rise  to  bulbs  and  tubers.  Again, 
the  food  accumulated  in  seeds  may  be  used  by  the  progeny  of  the  ac- 
cumulating plant.  Finally,  accumulated  food  may  be  used  by  a 
foreign  organism  or  even  remain  unused.  Non-utilization  is  more  fre- 
quent than  usually  has  been  supposed.  Most  tubers  and  bulbs  and 
some  seeds  contain  more  food  than  is  used  under  ordinary  conditions 
by  the  germinating  plant,  though  it  is  probable  that  an  excess  beyond 
the  amount  necessary  to  support  the  plant  until  it  reaches  the  light  is 
beneficial  in  the  way  of  giving  it  a  "  running  start." 

In  the  potato  tuber  there  is  a  large  excess  of  food  beyond  the  amount  utilized  in 

germination  (fig.  1037).  1°  various  orchids 
(as  in  Neottia)  the  amount  of  accumulated 
food  which  is  utilized  is  small  compared  with 
that  which  is  left;  after  a  time  such  food  de- 
cays and  contributes  to  the  humus.  The  large 
quantities  of  food  accumulated  in  galls  are  value- 
less to  the  accumulating  plants.  Latex  contains 
starch  and  other  foods  that  never  are  utilized,  so 
far  as  is  known.  During  the  summer  and  autumn 
there  is  a  well-marked  migration  of  food  from 
FIG.  1037. -A  tuber  of  the  the  leaves  to  the  trunk  in  deciduous  trees;  how- 
potato  (bolanum  tuberosum).  .  ,  ,  ,  r  ,  .  . 

which  has  germinated  in  a  dark,       ever'  co™derable  food  remains  m  the  leaves  at 
moist  cellar;  note  the  wrinkling  of       Ieaf  fal1  and  hence  never  is  utilized, 
the  tuber,  evidencing    the  with-  Tne  detailed  consideration    of   the    structure 

drawal  therefrom  of  food  and  and  arrangement  of  plant  foods  is  deferred  to 
water  by  the  developing  shoots.  the  chapter  dealing  with  seeds  (p.  911). 

Latex.  —  The  structural  features  of  latex  elements.  —  A  few  plant  fami- 
lies are  characterized  by  the  presence  of  milky  juice,  or  latex.  Geneti- 
cally, latex  elements  are  of  three  sorts,  the  simplest  being  the  latex  sac, 
where  the  milky  juice  is  contained  in  uninucleate  cells,  usually  arranged 
in  longitudinal  rows,  as  in  Sanguinaria,  and  also  in  the  Convolvulaceae 


STEMS 


721 


and  Sapotaceae,  the  latter  family  including  Palaquium,  which  furnishes 
the  gutta  percha  of  commerce.  The  second  and  commonest  kind  of 
latex  tissue  is  that  in  which  the  cells  fuse  (as  in  tracheae)  by  the  resorp- 
tion  of  the  connecting  walls,  thus  forming  syncytes  known  as  laticiferous 
vessels,  which  form  a  connected  cortical  system  throughout  the  plant 
(fig.  1038).  Sometimes  the  cells  fuse  in  rows,  as  in  the  celandine  poppy, 
thus  forming  an  easy  transition 
to  rows  of  latex  sacs.  More 
commonly  there  is  lateral  as 
well  as  terminal  fusion,  result- 
ing in  an  anastomosing  net- 
work, as  in  the  milky-juiced 
composites  (Cichorieae) ;  simi- 
lar laticiferous  vessels  charac- 
terize the  fungus  Lactarius. 
The  third  and  most  extraor- 
dinary kind  of  laticiferous  tissue 
is  that  characterizing  Euphorbia 
and  the  milkweeds  (Asclepia- 
daceae),  where  the  laticiferous 
element  is  a  coenocyte,  arising 
in  the  embryo  from  a  single 
ordinary  cell.  Later  this  de- 
velops at"  an  equal  rate  with 
the  plant,  penetrating  among 
the  cells  as  do  the  hyphae  of 
a  parasitic  fungus,  and  sometimes  attaining  a  length  of  several  meters; 
branching  occurs  freely,  but  the  branches  rarely  anastomose.  In  all 
cases  latex  tissues  occupy  definite  regions  and  traverse  the  entire  plant, 
as  do  vascular  tissues. 

The  contents  of  latex  tubes.  —  Latex  elements,  whether  they  are  cells, 
syncytes,  or  coenocytes,  contain  an  extraordinary  assemblage  of  sub- 
stances. Latex  consists  of  a  watery  fluid,  which  holds  in  suspension 
gums,  resins,  caoutchouc,  fats,  and  waxes,  and  therefore  is  an  emulsion; 
in  addition  there  are  held  in  solution  tannins,  soluble  gums,  sugars, 
alkaloids,  salts,  and  occasionally  proteolytic  ferments  (as  papain  in 
Carica  Papaya),  Leucoplasts,  elaioplasts,  and  proteinoplasts  occur  in 
the  latex,  organizing  respectively  starch  grains,  oil  bodies,  and  protein 
granules;  in  Euphorbia  there  are  starch  grains  of  unusual  shape,  re- 


FIG.  1038.  —  A  longitudinal  section  through 
a  portion  of  a  root  of  the  prickly  lettuce 
(Lactuca  scariola),  showing  the  anastomosing 
latex  tubes  (/) ;  note  the  absence  of  cross 
walls;  highly  magnified. 


722  ECOLOGY 

sembling  rods,  dumb-bells,  etc.  As  a  rule  latex  is  white,  as  suggested 
by  the  common  name  of  milky  juice,  but  in  the  poppy  family  it  may  be 
yellow,  orange,  or  red.  Latex  elements  are  living,  having  thin  plas- 
matic  layers  along  the  walls,  which  commonly  are  thin  and  readily  per- 
meable to  water  and  solutes,  though  in  Euphorbia  the  walls  are  thicker 
and  pitted. 

In  many  plants  (as  in  the  common  milkweed,  Asclepias  syriaca}  latex  flows  copi- 
ously from  a  wounded  surface,  indicating  its  great  abundance  and  also  the  high 
pressure  at  which  it  exists  in  the  tubes;  it  soon  coagulates  upon  exposure  to  air. 
Latex  is  of  great  commercial  value,  as  it  is  the  chief  source  of  rubber;  the  leading 
rubber-producing  plants  are  tropical  trees  of  the  nettle,  dogbane,  and  spurge  fami- 
lies. Opium  is  a  mixture  of  alkaloids  from  the  latex  of  the  poppy. 

The  role  of  latex.  —  The  presence  in  latex  of  leucoplasts,  proteino- 
plasts,  and  elaioplasts  (together  with  the  starch,  protein,  and  oil  which 
they  form),  as  well  as  of  proteolytic  ferments,  has  led  to  the  theory  that 
latex  tubes  are  food  reservoirs;  this  theory  finds  further  support  in 
the  fact  that  in  rapidly  growing  young  plants  of  Euphorbia  Lathyris 
the  latex  is  very  milky  and  is  rich  in  starch,  fats,  and  albuminous  sub- 
stances, while  in  old  plants  the  latex  is  translucent  and  watery  and  is 
poor  in  these  substances;  furthermore,  in  winter,  albumin  is  scarce  in 
the  stem  latex  and  abundant  in  the  root  latex.  Again,  plants  grown  in 
the  dark  or  in  air  that  is  deprived  of  carbon  dioxid  have  weak  watery 
latex  without  starch.  However,  the  presence  of  such  waste  products  as 
caoutchouc,  gums,  resins,  waxes,  tannins,  and  alkaloids  has  led  equally 
to  the  theory  that  latex  tubes  are  excretory  reservoirs.  The  presence 
of  starch  does  not  necessarily  favor  the  food  reservoir  theory,  since  there 
is  little  evidence  that  the  starch  is  ever  used;  in  starved  plants,  for 
example,  it  suffers  no  appreciable  decrease.  There  is  some  evidence 
that  the  fats  and  proteins  of  latex  are  utilized  as  food.  Very  probably 
latex  tubes  are  general  catch-alls,  containing  both  surplus  foods  and 
waste  products;  the  latter  generally  are  greater  in  amount,  and  it  is 
likely  that  the  latex  tubes  are  of  significance  chiefly  as  excretory  reser- 
voirs. 

Related  to  the  food  reservoir  theory  is  the  hypothesis  that  latex  tubes 
represent  a  conductive  system,  a  hypothesis  favored  supposedly  by  the 
continuity  of  the  tubes  and  by  the  paucity  of  cross  walls,  as  well  as 
by  the  intimate  connection  sometimes  existing  between  the  ends  of  the 
tubes  and  the  palisade  cells.  The  adherents  of  the  conduction  theory 
regard  the  latex  tubes  as  paths  of  movement  of  carbohydrates  and  pro- 


STEMS  723 

teins;  even  starch  grains  are  thought  capable  of  movement  herein, 
since  they  sometimes  accumulate  behind  a  wall  or  other  obstruction,  ap- 
parently as  logs  pile  up  in  a  jam.  The  conduction  theory  rests  more 
on  analogy  than  on  experiment.  Also  related  to  the  conduction  and 
food  reservoir  theories  is  the  hypothesis  that  latex  tissue  represents 
merely  water  tissue.  Laticiferous  plants  usually  are  very  succulent, 
and  it  is  possible  that  some  of  the  substances  associated  with 
water  in  latex  have  an  effect  similar  to  that  of  salts  in  halophytes  in 
retarding  evaporation.  While  many  laticiferous  plants  grow  in  the  rain 
forest,  the  majority,  perhaps,  are  xerophytic,  Euphorbia  furnishing 
many  notable  examples;  however,  milky -juiced  plants  are  not  as  strik- 
ingly xerophytic  in  distribution  as  are  ordinary  succulents. 

Another  theory  as  to  latex  is  that  its  coagulability  is  advantageous  in  healing 
wounds,  but  this  is  no  more  a  role  of  primary  importance  here  than  it  is  in  blood, 
which  behaves  similarly.  A  final  theory  is  that  latex,  because  of  its  poisonous  or  at 
least  unpalatable  nature,  protects  plants  from  animals;  mere  contact  with  the  milky 
juice  of  Lactarius  is  said  to  be  fatal  to  snails,  and  it  is  conceivable  that  its  general 
alkaloid  content  may  make  latex  prejudicial  to  other  animals.  Many  laticiferous 
plants,  however,  are  favorite  food  plants  for  man  and  for  grazing  animals.  Further 
experimentation  is  needed  before  the  latex  problem  can  be  solved,  and  particularly 
experimentation  bearing  on  the  causes  underlying  the  formation  of  latex  and  latex 
tubes.  The  presence  of  various  kinds  of  plastids  in  latex  tubes  may  indicate  a  high 
degree  of  independent  nutritive  activity  in  spite  of  the  absence  of  chlorophyll. 

The  accumulation  in  stems  of  mucilage,  oils,  resins,  crystals,  tannins, 
and  dyes.  —  Ducts.  —  Stems,  as  well  as  leaves,  may  contain  crystals  or 
be  clothed  with  glandular  hairs.  In  many  stems  there  are  ducts  that 
secrete  and  accumulate  resins,  oils,  or  mucilage,  all  gradations  exist- 
ing, especially  in  conifers,  between  these  structures  and  internal 
glands.  Ducts  originate  as  do  internal  glands,  and  their  structural  fea- 
tures are  similar,  the  chief  difference  being  that  they  are  elongated  in  an 
axial  direction,  and  are  more  or  less  continuous.  In  most  cases  there  is 
a  branched  and  anastomosing  system  of  continuous  ducts  throughout 
the  plant,  though  in  pine  needles  the  ducts  end  blindly.  Often  (as  in 
the  pine,  fig.  1039)  the  secreting  cells  are  enclosed  by  a  protective  sheath 
of  thick- walled  cells,  interrupted  here  and  there  by  permeable  transfusion 
cells.  Mucilage  ducts  are  characteristic  of  cycads  and  of  many  ferns, 
and  oil  or  resin  ducts  characterize  most  conifers  and  many  composites,  as 
the  rosin-weeds  (Silphium).  The  mucilage  tubes  of  certain  liliaceous 
plants  resemble  latex  tubes,  not  alone  in  structure,  but  also  in  diversity 
of  contents  since  they  contain  protein  crystals,  starch,  tannin,  and 


724 


ECOLOGY 


FIG.  1039.  —  A  cross  section  of  an  edge  of  the  needle- 
like  leaf  of  the  Austrian  pine  (Pinus  Laricio),  showing 
the  epidermis  (e)  with  its  much  thickened  walls,  the 
outer  part  (c)  being  cutinized  and  the  inner  part  (?) 
not;  x,  hypodermal  sclerenchymatous  tissue;  /,  chloren- 
chyma  with  infolded  cell  walls,  the  outermost  cells  (/>) 
having  these  walls  perpendicular  to  the  surface,  suggest- 
ing palisades;  s,  stoma  with  guard  cells  (g),  subsidiary 
cells  (6),  stomatal  cavity  (i\  and  pit  (0);  r,  resin  duct, 
the  secretory  cells  (y)  being  surrounded  by  a  scleren- 
chymatous cylinder  (xf);  highly  magnified. 


glucose;  in  addition  they 
have  remarkable  fila- 
mentous nuclei.  In 
some  conifers,  reservoirs 
filled  with  resin  occur 
in  the  bark,  as  in  the 
balsam  blisters  of  the  fir 
(Abies  balsamea). 

The  role  of  duct  se- 
cretions. —  Probably  the 
contents  of  resin  and  mu- 
cilage ducts  are  chiefly 
waste  products.  Since 
such  substances  usually 
cannot  be  excreted  ex- 
ternally, it  is  presumably 
advantageous  that  they 
accumulate  in  reservoirs 
outside  the  regions  of 
nutritive  activity.  Even 
if  resins  and  similar  ex- 
cretions are  waste  prod- 


ucts, they  may  have  subsidiary  advantages;  for  example,  they  may 
preserve  the  wood  from  decay,  as  in  the  conifers,  thus  facilitating 
longevity.  Perhaps  resins  and  gums  are  of  advantage  in  healing  wounds 
and  in  checking  loss  of  water,  as  in  the  pines  and  cherries,  where  they 
exude  copiously  at  the  injured  places.  Incisions  cause  not  only  the  flow 
of  resin,  but  also  in  some  cases  the  development  of  accessory  ducts. 

Tannins  and  other  bark  excretions.  —  Among  the  commoner  excretions  found  in 
bark  (as  in  the  oak)  are  tannins,  which  are  astringent  glucosids.  In  Sambucus  the 
tannin  is  contained  in  special  sacs,  twenty  millimeters  long  or  thereabouts,  but 
usually  the  tannin-containing  cells  are  in  rows  and  often  near  the  vascular  tract. 
Brown  and  red  colors  in  the  bark  interior  often  are  due  to  tannins.  Similar  to  the 
tannins  is  salicin,  which  occasions  the  bitter  taste  of  willow  bark.  Most  tannins 
doubtless  are  waste  products  and  eventually  they  are  removed  through  the  exfolia- 
tion of  the  bark  ;  similarly,  exfoliation  rids  trees  of  many  other  waste  products  that 
accumulate  in  the  bark,  such  as  alkaloids,  gums,  resins,  and  calcium  oxalate. 
Tannins,  because  of  their  bitterness,  may  be  useful  incidentally  in  protecting  from 
animal  depredations  ;  some  tannins,  known  as  plastic  tannins,  probably  are  of 
value  in  nutrition.  Tannin  production  appears  to  be  especially  characteristic  of 


STEMS 


725 


xerophytes  ;  desert  plants  growing  in  mesophytic  conditions  have  much  less  tannin 
than  in  their  natural  habitat.  In  Jussiaea,  tannin  formation  has  been  shown  to 
be  favored  by  exposure  to  dry  air  and  to  light. 

The  accumulation  of  waste  in  wood.  —  In  many  trees  the  heart-wood  serves  as  a 
reservoir  of  various  excreta  which  may  give  it  a  color  different  from  that  of  the  sap- 
wood,  as  in  the  red  cedar  (Juniperus  virginiana)  and  in  the  black  walnut.  The 
colored  heart-wood  usually  is  much  harder  than  the  white  sap-wood  (as  in  ma- 
hogany and  ebony),  whence  the  significance  of  the  name  duramen.  Occasionally 
the  medulla  is  a  reservoir  of  excreta,  as  in  the  sumac,  where  it  is  colored  yellow. 
Doubtless  the  chief  advantage  of  the  accumulation  of  such  substances  in  the  dura- 
men is  that  thus  they  are  removed  from  the  active  tissues,  though  it  is  an  important 
subsidiary  advantage  that  they  increase  the  durability  of  the  heart-wood  and  thus 
promote  longevity. 


7.    VARIATION  IN  STEM  FORM 

Elongation  in  aerial  stems.  —  Variation  in  tree  form.  —  Forest  in- 
dividuals of  most  trees  differ  widely  in  form  from  individuals  grown  in 
the  open,  the  trunks  of  the 
former  being  tall  and  slender, 
while  those  of  the  latter  are 
short  and  stout.  Further- 
more, trees  in  the  open  are 
profusely  branched,  even 
near  the  base,  whereas  in 
forest  individuals  the 
branches  at  the  lower  levels 
soon  die  and  fall  to  the 
ground,  leaving  the  tree  rel- 
atively unbranched  except 
near  the  top.  Many  herbs 
show  comparable  phenom- 
ena, isolated  individuals 
being  relatively  short,  stout, 
and  branched,  and  crowded 
individuals  relatively  tall, 
slender,  and  unbranched; 
in  dense  cultures  the  lower 
leaves  die  much  sooner  than 
on  isolated  individuals.  The 
death  of  the  lower  leaves  in 


FIGS.  1040-1042.  Young  plants  of  a  prickly 
pear  cactus  (Opuntia  Rafinesquii),  showing  the  on- 
togenetic  differentiation  of  a  flattened  stem  from  a 
cylindrical  "juvenile"  stern:  1040,  a  very  young 
plant  with  a  cylindrical  stem,  showing  the  prod- 
uct of  two  growth  periods;  1041,  an  older  plant 
in  which  the  second  segment,  although  cylindrical, 
is  much  broader  than  the  first;  1042,  a  still  older 
plant  in  which  the  third  segment  has  the  flattened 
form  characteristic  of  "adult"  individuals  of  the 
genus;  the  small  and  soon  deciduous  leaves  (/) 
bear  spines  (s)  in  their  axils. 


726 


ECOLOGY 


crowded  cultures,  and  probably  the  death  of  the  lower  branches  of  trees 
in  the  forest,  is  due  to  insufficient  light;  if  a  forest  tree  is  left  standing 
when  its  neighbors  are  cut,  the  trunk  often  develops  adventitious  leafy 
shoots  in  great  abundance,  probably  because  it  is  more  exposed  to  light. 
In  various  trees  (e.g.  willows  and  poplars)  there  occurs  "  self-pruning," 
or  branch  fall,  that  is  not  readily  referable  to  definite  factors. 

Light  as  a  factor  in  elongation.  —  The  causes  of  elongation  in  crowded 
cultures  are  not  certainly  established,  though  they  are  clearly  external. 
Sometimes  the  differences  to  be  accounted  for  are  very  great,  as  in  the 
palmetto,  which  in  dry  open  habitats  often  is  stemless  above  ground, 
while  in  moist  woods,  plants  of  equal  age  have  long,  slender  trunks 
several  meters  in  height.  When  the  prickly  pear  (Opuntia)  grows  in 
the  light,  the  stems  become  much  flattened  (fig.  1042),  while  in  dark- 
ness they  become  slender,  elongated  cylinders,  somewhat  comparable 
to  their  "juvenile"  stems  (figs.  1040,  1041).  The  stems  of  Genista 
develop  flattened  wings  in  the  light  but  not  in  darkness.  In  germi- 


1043 


1045 


FIGS.  1043-1045.  —  Stem  and  leaf  variation  in  Sempervivum  assimile:  1043,  a  stem- 
less  rosette,  as  seen  in  nature,  having  imbricated  leaves  in  many  ranks;  1044,  an  individual 
that  has  been  grown  in  a  moist  chamber;  note  the  conspicuous  erect  stem  with  its  terminal 
rosette;  1045,  an  individual  that  has  been  grown  in  a  moist  dark  chamber  ;  note  the  small 
and  scattered  leaves.  —  After  BRENNER. 

nating  potato  tubers,  slender  elongated  stems  issue  from  buds  located 
below  the  soil  level,  while  short  and  very  stout  shoots  may  issue  from 
buds  above  ground  (fig.  1046).  The  stipes  of  Mucor  and  Coprinus 
elongate  in  the  darkness  and  are  relatively  short  and  stout  in  the  light. 
These  and  similar  facts  have  led  to  the  view  that  light  retards,  and 
that  darkness  favors  elongation. 


STEMS  727 

Moisture  as  a  factor  in  elongation,  —  Certain  of  the  Crassulaceae 
(notably  Sempervivum  assimile,  fig.  1043)  usually  are  stemless  in  their 
natural  xerophytic  habitats,  while  growth  in  moist,  light  chambers  results 
in  the  conspicuous  development  of  erect  leafy  stems  (fig.  1044) ;  growth 
in  moist,  dark  chambers  results  similarly,  except  that  leaf  formation  is 
greatly  reduced  (fig.  1045).  These  and  many  similar  experiments  show 
conclusively  that  moisture  favors  elongation,  while  desiccation  results  in 
shortened  and  often  in  laterally  enlarged  stems;  as  might  be  expected, 


FIG.  1046.  —  A  potato  plant  (Solanum  tuberosum),  grown  from  a  tuber  planted  with 
one  end  in  the  soil,  but  with  the  larger  portion  in  the  air;  note  the  short  and  stout  aerial 
shoots,  which  contrast  strikingly  with  the  long  and  slender  shoot  which  originated  in  the 
soil;  note  also  the  much  larger  leaves  on  the  latter  shoot.  —  Photograph  by  FULLER. 

the  presence  of  soluble  salts  in  the  culture  media  produces  the  same  effect 
as  desiccation.  There  are  few  if  any  similarly  conclusive  experiments 
as  to  light,  and  it  seems  probable  that  moisture  differences  constitute  the 
chief  factors  in  determining  the  phenomena  noted  in  the  preceding  para- 
graphs. Indeed,  in  some  cases  (as  in  Jussiaea  and  in  the  seedlings  of 
Vicia  Faba)  there  is  greater  elongation  in  illuminated  than  in  dark  cul- 
tures, if  the  moisture  conditions  are  equal  and  favorable.  Stem  inter- 
nodes  become  shorter  and  thicker  and  the  plant  more  bushy,  if  the  ordi- 
nary supply  of  carbon  dioxid  is  doubled  or  quadrupled,  but  in  nature  this 
factor  is  not  likely  to  be  important.  Variations  in  the  length  of  petioles, 


728  ECOLOGY 

as  in  the  maple  (fig.  779),  doubtless  are  due  to  the  same  cause  as  are 
variations  in  the  length  of  stems,  that  is,  to  differential  shading  or  more 
probably  to  moisture  differences  due  to  differential  shading.  For  a  con- 
sideration of  the  striking  elongation  of  pendulous  stems,  see  p.  657. 

Elongation  in  aquatic  stems  and  petioles.  —  The  phenomena.  —  In 
water  plants  with  floating  leaves  (as  in  Polygonum  amphibium)  or  with 
emersed  leaves  (as  in  Hippuris),  the  length  of  the  stem  may  vary  widely, 
depending  upon  the  depth  of  the  water.  In  swamps  that  have  been 
flooded,  stem  elongation  may  be  extraordinary,  ten  times  the  usual 
length  having  been  reported;  for  example,  Eleocharis  stems,  usually  but 
a  few  centimeters  in  length,  have  been  known  to  grow  to  a  length  of  a 
meter  and  a  half.  Differences  in  the  length  of  aquatic  petioles  are  more 
common,  and  if  anything  more  striking.  The  petioles  of  the  floating 
leaves  of  the  water  lilies,  Castalia  and  Nymphaea,  vary  with  the  depth  of 
the  water  in  which  they  grow,  their  length  commonly  being  somewhat 
greater  than  the  depth  of  the  water,  a  phenomenon  that  results  in  oblique 
orientation.  In  emersed  leaves  (as  in  Sagittaria)  the  petiole  length  varies 
similarly,  and  under  experiment  the  leaves  continue  to  emerge,  even  if 
the  depth  of  the  water  is  greatly  increased.  Elongated  water  stems  and 
petioles,  like  elongated  aerial  organs,  are  very  slender,  so  that  an  in- 
creased amount  of  structural  material  is  not  necessarily  involved  in  elon- 
gation. As  might  be  expected,  it  is  possible  experimentally  to  find  a 
depth  too  great  for  the  elongating  petiole  to  raise  its  blade  into  the  air. 

The  probable  factors .  —  The  striking  fact  in  hydrophytes  with  floating 
leaves  is  not  so  much. the  varying  length  of  stems  and  petioles  as  the 
cessation  of  growth  at  the  water  line,  as  though  at  that  level  some  new 
factor  came  suddenly  into  activity.  Such  a  factor,  of  course,  is  transpi- 
ration, and  it  seems  probable  that  this  is  the  chief  factor  involved  in  such 
cases,  since  growth  in  covered  aquaria,  where  transpiration  is  greatly 
reduced,  results  in  the  development  of  emersed  leaves  in  place  of  floating 
leaves  in  Hydrocharis,  Nymphaea,  and  Potamogeton  natans.  Short 
aerial  stems  and  petioles  as  compared  with  the  corresponding  elongated 
aquatic  organs  also  would  seem  adequately  accounted  for  by  differences 
in  transpiration.  Variations  in  oxygen  content  sometimes  have  been 
thought  to  explain  differential  elongation  in  aquatic  stems,  though  it  is 
not  easy  to  see  why  a  stem  should  grow  longest  where  the  oxygen  supply 
is  smallest,  nor  why  there  should  be  a  sudden  arrest  of  growth  at  the 
water  line.  Similarly,  differences  in  light  intensity  often  are  cited  as 
factors  in  the  elongation  of  aquatic  stems  as  well  as  of  aerial  sterns.^ 


STEMS 


729 


While  diminished  light  might  cause  elongation  in  deep  water,  the  in- 
crease of  light  in  passing  from  water  to  air  is  not  sufficiently  sudden  to 
account  for  the  arrest  of  growth  at  the  water  line.  The  failure  of  deeply 
submersed  leaves  to  reach  the  surface  may  be  due  to  diminished  synthe- 
sis, and  thus  indirectly  to  light.  The  mechanical  support  given  by 
water  occasionally  has  been  thought  to  have  some  connection  with 
the  great  elongation  in  that  medium. 

Elongation  in  stems  submerged  by  sand.  —  The  phenomena.  —  Still   more  re- 
markable than  the  elongation  of  aquatic  stems  is  the  elongation  exhibited  by  certain 


1047 


1048 


1  049 

FIGS.  1047-1049.  — •  Diagrams  showing  the  stem  elongation  of  certain  plants  (as  wil- 
lows or  dogwoods)  when  submerged  by  advancing  sand  dunes:  1047,  a  dune  in  whose 
path  of  advance  is  a  swamp  with  such  shrubs  of  ordinary  height;  1048,  the  same  place  a 
few  years  later,  the  dune  having  advanced;  note  that  the  stems  of  the  shrubs  have  twice 
their  usual  length;  1049,  the  same  place,  after  the  lapse  of  a  few  more  years;  the  stems 
of  the  shrubs  have  four  times  their  usual  length. 

aerial  stems  that  are  partially  buried  by  the  sand  of  moving  dunes.  While  some 
trees  (as  oaks  and  pines)  soon  succumb  to  the  sand,  others  (as  the  white  elm  and  the 
red  maple)  remain  alive  for  years,  unless  completely  covered.  A  number  of  trees 
and  shrubs,  notably  willows  (as  Salix  syrticola  and  S.  glaucophylla),  dogwoods 
(as  Cornus  stolonifera),  and  poplars  (as  Populus  deltoides),  not  only  remain  alive, 
but  are  stimulated  to  a  growth  far  exceeding  that  of  ordinary  aerial  stems.  Dog- 
woods and  willows,  which  in  ordinary  habitats  rarely  attain  a  height  of  more  than 
two  or  three  meters,  may  have  stems  twelve,  fifteen,  or  even  eighteen  meters  above 
the  original  ground  level,  if  partially  submerged  by  sand  (figs.  1047-1049).  The 
growing  stems  usually  keep  pace  with  the  rising  sand,  so  that  the  height  above  the 
ground  remains  about  the  same  year  after  year,  approximately  equalling  the  usual 
height  of  the  shrubs. 


73° 


ECOLOGY 


TItc  possible  factors.  —  While  the  exact  factors  determining  elongation  in  par- 
tially buried  stems  are  not  clearly  known,  it  is  significant  that  only  those  stems 

exhibit  elongation  which  develop  adventi- 
tious roots  in  the  moist  sand  (fig.  1050). 
Some  trees  (as  the  red  cedar)  ordinarily  are 
of  low  stature,  while  other  trees  (as  the  euca- 
lyptus) are  very  tall;  the  factors  determining 
the  potential  height  of  the  stem  in  various 
species  are  quite  unknown.  Increasing  height 
probably  is  accompanied  by  accelerated  tran- 
spiration, and  an  increasing  root  system  makes 
possible  increased  absorption.  It  is  likely, 
however,  that  the  transpiration  increase 
gradually  exceeds  the  increase  of  available 
water,  owing  to  the  conjunction  of  increased 
transpiring  surface,  increased  exposure  to 
transpiration,  and  increased  length  of  the 
conductive  tract;  ultimately  the  available 
water  may  be  sufficient  merely  to  make  good 
the  loss  by  transpiration,  leaving  no  surplus 
for  elongation.  This  condition,  ultimately 
reached  in  all  trees,  may  be  reached  much 
sooner  in  some  species  than  in  others,  owing 
to  peculiarities  of  structure  and  behavior. 
If  this  hypothesis  is  valid,  stem  elongation  in 
dune  sand  may  be  explained  by  the  fact  that 
the  absorptive  system  increases  as  fast  as  the 
transpiration  is  accelerated ;  the  distance 
traversed  by  water  in  reaching  the  topmost 
organs  remains  essentially  the  same,  since 
the  adventitious  roots  keep  pace  in  their  de- 
.velopment  with  the  increasing  elongation  of 
the  stem. 


FIG.  1050.  — The  apical  portion  of 
a  plant  of  the  red-osier  dogwood 
(Cornus  stolonifcra)  that  has  been 
almost  buried  by  dune  sand;  note  the 
adventitious  roots  (r)  that  have  issued 
from  the  stem;  the  main  shoot  (m) 
has  been  killed,  but  one  of  the  lateral 
shoots  (ri)  still  keeps  above  the  sand; 
note  that  the  scars  (w)  left  by  the  fall- 
ing of  the  scales  of  the  previous  winter 
bud  are  at  the  sand  line  (s),  showing 
that  the  plant  was  almost  completely 
buried. 


Stem  dwarfing.  —  Alpine  and  low- 
land cultures.  —  Alpine  stems  com- 
monly are  dwarf,  and,  if  much-branched, 
they  are  compact  and  bushy,  often 
forming  dense  mats  or  cushions.  In 
some  very  careful  experiments  both  al- 
pine and  lowland  individuals  of  many 


species  were  split  into  two  parts,  one 
of  which  was  grown  in  an  alpine  garden,  and  the  other  in  the  lowlands; 
the  portions  taken  from  alpine  districts  to  low  altitudes  developed  slender 
elongated  stems,  while  the  portions  taken  from  the  lowlands  to  the 


STEMS 


731 


mountains  developed  short  and  stout  stems,  the  whole  aerial  system 

assuming  a  bushy  and  compact  habit.     The  leaves  and  flower  stalks, 

as  well  as  the  stems, 

underwent  reduction 

(figs.  1051,1052,869, 

870).     Hence  alpine 

conditions    are 

thought    to    account 

for     the     prevalent 

habits  of  alpine 

plants. 

Krummholz. — 
The  mountain  pine 
of  Europe  (Pinus 
montana)  in  the 
Alps  is  a  gnarled 
and  sprawling,much- 
branched  shrub,  but 
when  grown  in  the 
lowlands  it  is  a  tree 
much  likeotherpines. 
German  botanists 
use  their  cetnmon 
name  for  this  plant, 
Krummholz  (gnarled 
wood),  as  a  general 
term  for  the  scrubby 
growth  of  woody 
plants  above  the  tim- 
ber on  mountains, 
and  for  lack  of  a  suit- 
able English  term, 
the  word  may  be  em- 
ployed here.  On 
most  mountains  the 
"timber  line"  trees 
pass  gradually  up- 


ward into  a  scraggy 
Krummholz;     good 


FIGS.  1051,  1052.  —  Leaf  and  stem  variation  in  Heli- 
anthemum  vulgar e:  1051,  an  individual  grown  in  a  lowland 
garden,  showing  elongated  stems  and  relatively  large  leaves ; 
1052,  an  individual  grown  in  an  alpine  garden  from  a  part 
,of  the  same  plant  from  which  1051  was  taken;  note  the 
numerous  dwarfed  branches  with  their  much  smaller  leaves 
and  but  slightly  smaller  flowers;  b,  bracts;  x,  calyx;  c, 
corolla;  both  figures  equally  reduced. — After  BONNIER. 


732  ECOLOGY 

illustrations  of  this  habit  are  afforded  by  the  spruce  (Picea)  and  fir 
(Abies  balsamea)  on  the  New  England  mountains,  the  white-barked 
pine  (Pityts  albicaulis)  on  the  Rocky  Mountains,  the  foxtail  pine 
(Pinus  aristata)  on  the  San  Francisco  Mountains  of  Arizona,  and 
the  mountain  hemlock  (Tsuga  Mertensiand)  on  many  mountains 
from  Oregon  to  Alaska  (fig.  1053).  In  many  alpine  regions  dwarfed 
alders,  willows,  and  birches  mingle  with  the  conifers.  The  impene- 
trability of  the  Krummholz  is  due  in  part  to  the  multiplicity  of  scraggy 
lateral  branches,  but  more  to  the  fact  that  these  branches  bend  down 
close  to  the  ground  and  twist  and  turn  in  all  directions. 


FIG.  1053.  —  Alpine  Krummholz,  made  up  of  gnarled  and  weather-beaten  trees  of 
the  mountain  hemlock  (Tsuga  Mertensiana) ;  note  that  the  living  branches  bend  down  and 
trail  over  the  ground,  forming  a  dense  tangle ;  Mount  Hood,  Ore.  —  Photograph  by 
MEYERS. 

The  determining  factors  of  the  Krummholz.  —  The  factors  determin- 
ing the  Krummholz  probably  are  complex,  though  not  as  yet  ade- 
quately tested  by  experiment.  Probably  the  short,  thick  growth  of 
the  stems  is  due  chiefly  to  relatively  high  transpiration  in  proportion 
to  absorption,  absorption  being  reduced  because  of  the  dryness  and 
the  low  temperature  of  the  soil,  while  the  transpiration  often  is  ac- 
celerated because  of  strong  winds,  atmospheric  rarity,  intense  sun- 
light, and  low  humidity.  The  multiplicity  of  branching  is  due  to  the 
replacement  of  the  terminal  shoots  by  numerous  lateral  shoots,  as 
soon  as  the  former  reach  a  height  where  the  excessive  transpiration 
makes  further  growth  impossible;  a  prominent  factor  here  is  the  depth  of 
winter  snow,  since  branches  above  the  snow  level  are  exposed  to  trans- 
piration for  many  months  during  which  there  is  no  absorption.  Here, 


STEMS 


733 


as  elsewhere,  the  destruction  of  the  terminal  shoot  is  followed  by  the 
development  of  many  lateral  branches,  whose  subsequent  destruction 
results  in  a  still  greater  number  of  new  laterals,  and  so  on,  until  there  is 
at  last  an  inextricable  tangle  of  branches.  The  tortuous  descending 
branches  so  characteristic  of  alpine  conifers  probably  are  associated 
with  severe  mechanical  factors,  such  as  strong  winds,  the  weight  of  the 
winter  snow,  and  snowslides.  Even  below  the  "  timber  line,"  tree 
trunks  sometimes  bend  down-hill  at  the  base,  owing  to  the  weight  of 
snow  borne  by  the  plant  when  it  was  a 
young  and  flexible  sapling. 

Arctic  dwarfs.  —  The  polar  regions,  like  the 
mountain  tops,  are  characterized  by  dwarf  vege- 
tation, composed  largely  of  cushion  herbs, 
rosette  herbs,  mat-forming  plants,  and  Krumm- 
holz.  The  Krummholz  is  made  up  largely  of 
dwarfed  specimens  of  woody  plants  (e.g.  larch, 
spruce,  and  birch)  which  in  more  genial  climates 
develop  into  trees.  While  not  experimentally 
attested,  it  is  likely  that  the  factors  involved  are 
similar  to  those  that  are  supposed  to  obtain 
on  mountains;  transpiration,  perhaps,  is  less 
than  in  alpine  regions,  on  account  of  the  less 
intense  light  and  the  greater  atmospheric  pres- 
sure, but  absorption  also  is  less,  because  the 
soil  is  more  constantly  cold. 

Dwarfing  in  arid  situations.  —  Xerophytic 
vegetation,  such  as  that  of  dry  rocks  and  sand, 
often  is  dwarf,  though  much  less  so  than  that  of 
alpine  and  arctic  regions,  and  the  dwarfness  is 
less  clearly  due  to  the  surrounding  conditions. 
The  stem  of  Equisetum  hyemale,  which  com- 
monly is  unbranched  in  mesophytic  and  swampy 
habitats,  often  is  much  branched  in  dry,  exposed 
situations,  the  destruction  of  the  terminal  shoot 
by  excessive  transpiration  or  otherwise  being 
followed  by  a  strong  development  of  lateral 
branches,  as  in  the  Krummholz  (figs.  1054, 
1055).  The  mesquit  (Prosopis  juliflora},  which 
is  a  tree  in  the  moist  river  bottoms  of  the  arid 
Southwest,  becomes  a  sprawling  shrub  in  dry, 
exposed  soil  (see  also  fig.  725).  The  lower  slopes 
of  a  mountain  in  a  prairie  or  desert  region  often 
have  Krummholz  which  is  quite  comparable  to 
that  toward  its  summit,  and  it  is  made  up  of 


FIGS.  1054,  1055.  —  Stem  varia- 
tion in  the  scouring  rush  (Equisetum 
hyemale):  1054,  an  erect,  un- 
branched stem,  terminated  by  a 
strobilus  (s),  being  the  form  com- 
monly seen  in  protected  situations ; 
1055,  a  much-branched  stem  from 
an  exposed  sand  dune;  following 
an  injury  to  the  terminal  shoot  (m) 
there  has  been  conspicuous  regener- 
ation, a  number  of  latent  buds  hav- 
ing given  rise  to  lateral  branches  (I) ; 
note  the  absence  of  foliage  leaves; 
c,  scale  leaves. 


734  ECOLOGY 

species  which  develop  into  trees  on  intermediate  slopes,  where  the  rainfall  is  greater. 
These  and  other  dwarfed  forms  of  arid  soils  and  climates  probably  are  due  to  the 
excessive  transpiration  in  proportion  to  the  limited  absorption. 

Dwarfing  in  bogs.  —  When  the  bald  cypress  (Taxodium)  is  grown  in  upland 
parks,  the  trunk  is  excurrent  and  the  branches  symmetrical,  but  in  its  natural  swamp 
habitat  the  main  shoot  dies  after  a  number  of  years,  and  the  subsequent  vigorous 
development  of  lateral  shoots  results  in  a  spreading  crown.  Possibly  the  imperfect 
absorption  which  is  characteristic  of  swamps  makes  it  impossible  for  the  water  column 
to  rise  as  high  as  in  other  habitats.  In  western  bogs,  Pinus  contorta,  elsewhere  a 
slender  tree,  frequently  becomes  a  gnarled  and  sprawling  shrub,  doubtless  because 
of  the  unfavorable  conditions  for  absorption. 

Nanism.  —  Among  the  mutants  of  Oenothera  Lamarckiana  (p.  288) 
there  appeared  some  forms  that  were  much  smaller  than  the  parents 
or  the  other  mutants,  and  as  these  dwarfs  reproduce  true  from  seed,  they 
were  given  the  specific  name,  Oenothera  nanella.  Plants  of  this  sort, 
whose  dwarfness  appears  to  be  inherent,  rather  than  caused  by  external 
conditions,  are  said  to  exhibit  nanism.  Similarly,  species  whose  in- 
dividuals are  inherently  large  are  said  to  exhibit  gigantism.  Experi- 
mental cultures  have  shown  that  of  the  many  dwarf  xerophytes  of  eastern 
Sweden,  some  are  inherently  dwarf  (obligate  dwarfs) ,  illustrating  nanism, 
while  others  (facultative  dwarfs)  develop  readily  into  tall  plants  when 
grown  in  favorable  conditions.  In  obligate  dwarfs  all  of  the  organs 
commonly  are  reduced,  but  in  facultative  dwarfs  the  roots  and  often 
the  flowers  are  as  large  as  in  full-sized  individuals;  however,  experimen- 
tation is  necessary  in  order  to  determine  adequately  whether  any  given 
dwarf  is  facultative  or  obligate. 

Asymmetric  stems.  —  Most  aerial  stems  tend  toward  symmetry  in 
their  branch  development,  but  asymmetry  often  is  seen,  as  in  trees  at 
the  edge  of  a  forest,  which  branch  profusely  toward  the  open,  while 
they  are  nearly  branchless  toward  the  forest  interior,  probably  because 
of  insufficient  light.  The  most  striking  cases  of  one-sided  trees,  how- 
ever, occur  along  seacoasts,  branch  development  often  being  almost 
entirely  inhibited  on  the  seaward  side,  so  that  most  of  the  branches 
point  landward.  Usually  the  crown  of  the  tree  presents  an  even  slope 
upward  from  the  seaward  side,  giving  a  peculiarly  wind-swept  aspect, 
and  indicating  that  the  inhibition  of  branch  development  is  diminished 
landward  (fig.  1056).  Two  theories  have  been  put  forward  to  account 
for  such  asymmetry,  neither  having  adequate  experimental  evidence; 
the  one  maintains  that  the  stronger  transpiration  on  the  seaward  or 
windward  side  causes  the  branches  to  die  earliest  there,  while  the  other 


STEMS 


735 


theory  maintains  that  salt  particles  carried  in  the  spray  account  for 
branch  destruction.  It  is  possible  and  even  probable  that  both  theories 
are  valid,  the  two  factors  supplementing  one  another.  Apparently 
favoring  the  salt  theory  is  the  relative  absence  of  one-sided  trees  about 
the  Great  Lakes,  but  apparently  favoring  the  transpiration  theory  is 
the  frequent  one-sidedness  of  trees  near  the  "  timber  line  "  of  mountains. 


FIG.  1056.  —  Trees  of  the  Douglas  spruce  (Pseudotsuga  mucronata)  growing  near  the 
sea ;  note  the  relative  absence  of  branches  on  the  exposed  (seaward)  side,  thus  showing  the 
destructive  influence  of  the  sea  winds;  San  Juan  Island,  Wash.  —  Photograph  by 

O'BRIEN. 

Periodicity  in  stem  development.  —  Periodicity  in  tree  branches.  — 
The  twigs  of  deciduous  trees  are  made  up  of  alternating  dwarfed  and 
elongated  portions,  the  leaf  scars  of  the  former  being  closely  grouped, 
while  those  of  the  latter  are  more  widely  separated  (fig.  1057).  A  branch 
begins  as  a  lateral  bud  closely  enveloped  by  scale  leaves,  and  during  the 
first  season  growth  commonly  is  slight;  the  following  spring  there  is  a 
period  of  elongation  accompanied  by  the  fall  of  the  scales,  whose  posi- 
tion is  marked  by  the  closely  grouped  scars.  Later,  elongation  ceases 


736 


ECOLOGY 


and  there  is  formed  a  dwarfed  portion,  the  terminal  bud,  with  closely 
imbricated  scales;    in  the  autumn  the  leaves  fall  from  the  elongated 

portion,  leaving  the 
widely  separated  scars. 
Thus  the  age  of  a 
branch  may  be  deter- 
mined by  noting  the 
number  either  of  the 
dwarfed  or  of  the  elon- 
gated regions.  Some- 
times the  dwarfed 
portions  are  thicker 
than  the  elongated 
portions. 

The  possible  factors. 
—The  factors  involved 
in  stem  periodicity 
probably  are  external, 
but  they  have  not 
been  determined  ex- 
perimentally. If  the 
terminal  bud  is  re- 
moved early  in  the 
first  season,  the  lat- 
eral shoots  elongate, 
indicating  that  ordi- 
narily the  main  shoot 
for  a  time  inhibits 
elongation  in  the  lat- 
eral shoot,  possibly 
because  it  utilizes  the 
food  necessary  for  the 
development  of  the 
latter  (see  p.  749). 
However,  when  once 
the  lateral  shoot  be- 
gins to  develop,  its 
growth  phenomena 
appear  to  be  con- 


1059 


FIGS.  1057-1059.  —  Twigs,  illustrating  growth  period- 
icity and  the  characteristics  of  winter  buds:  1057,  a  twig 
of  an  ash  (Fraxinus),  showing  alternating  regions  charac- 
terized by  slight  stem  elongation  (viz.  at  b,  the  present 
winter  bud;  at  b',  the  position  of  the  previous  winter  bud; 
at  b",  the  position  of  the  winter  bud  of  two  years  previous, 
etc.)  and  by  considerable  stem  elongation  (viz.  at  a,  repre- 
senting the  growth  of  the  previous  summer ;  at  a',  represent- 
ing the  growth  of  the  summer  previous  to  that,  etc.)  ;  note 
the  large  size  of  the  terminal  bud  (b)  in  proportion  to  that  of 
the  lateral  buds  (c} ;  /,  leaf  scar ;  d,  lenticels ;  at  cf  are  scars 
left  upon  the  fall  of  lateral  buds  of  previous  years  ;  1058,  a 
twig  of  the  cottonwood  (Populus  deltoides) ;  note  the  large 
buds  with  imbricated  scale  leaves  (e);  note  also  that  the 
lateral  buds  (c)  are  about  as  well  developed  as  the  terminal 
bud  (/) ;  other  lettering  as  in  1057 ;  1059,  a  twig  of  Catalpa  ; 
note  the  small  winter  buds,  both  terminal  (/)  and  lateral 
(c) ;  note  also  the  prominent  circular  leaf  scars  (/)  with  an 
inner  circle  (v\  representing  the  position  of  the  vascular 
bundles  severed  upon  leaf  fall. 


STEMS  737 

trolled  in  the  main  by  definite  external  factors  rather  than  by  an 
influence  residing  in  the  main  shoot.  Elongation  in  spring  may  be 
associated  with  the  vigorous  movement  of  structural  materials  at  that 
season,  and  subsequent  dwarfing  may  be  associated  with  a  reduced 
movement,  perhaps  supplemented  by  increased  desiccation.  The 
amount  of  branch  elongation  varies  with  the  season  and  is  reciprocal  to 
the  width  of  the  annual  ring  (p.  691),  maximum  elongation  occurring  in 
moist  and  minimum  elongation  in  dry  seasons.  Differences  between 
twigs  of  different  species  (e.g.  the  slender  elongated  twigs  of  willows,  as 
compared  with  the  stout  twigs  of  the  sumacs)  appear  inherent  rather  than 
related  to  external  factors. 

Additional  periodic  phenomena.  —  Periodicity  is  exhibited  in  the  daily  growth  of 
stems,  elongation  being  greater  by  night  than  by  day,  as  is  illustrated  by  the  bam- 
boo, of  whose  extraordinary  growth  about  two  thirds  occurs  by  night.  The  factors 
here  involved  are  complex,  but  may  be  associated  in  part  with  the  absence  of  light 
(directly  or  indirectly  or  both)  and  with  lessened  transpiration.  Notable  periodicity 
is  exhibited  by  biennial  rosette  plants  (fig.  1036),  in  which,  possibly  because  of  insuffi- 
cient structural  material,  there  is  no  stem  development  during  the  first  season,  though 
a  vigorous  erect  stem  rises  in  the  second  season.  Alternating  elongated  and  dwarfed 
shoots  also  are  illustrated  by  the  summer  stems  and  winter  buds  of  Utricularia 
(p.  678),  the  phenomena  and  perhaps  the  causative  factors  recalling  the  situation  in 
tree  branches. 

Inherent  rhythm.  —  While  the  plants  of  uniform  climates  are  evergreen,  they 
are  not  necessarily  continuous  growers,  though  there  rarely  occurs  such  definite 
periodicity  as  in  periodic  climates.  Some  trees,  as  the  coconut  and  the  papaw 
(Carica),  are  essentially  ever  growers,  developing  both  vegetative  and  reproductive 
organs  on  the  same  shoot  in  unbroken  continuity.  Probably  most  trees  of  the  rain 
forest  exhibit  what  might  be  termed  spasmodicity  in  contrast  to  periodicity,  the  vary- 
ing branches  appearing  independent  of  one  another  and  of  external  conditions;  one 
branch  elongates  and  puts  forth  new  leaves,  while  another  is  blooming,  another  fruit- 
ing, and  still  another  is  quiescent.  When  the  grape  is  taken  from  a  temperate  region 
to  the  rain  forest,  it  develops  this  spasmodic  habit.  Such  plants,  then,  are  neither 
uniform  evergrowers  nor  are  they  uniformly  rhythmic,  but  each  branch  appears  to 
exhibit  a  rhythm  of  its  own.  It  is  doubtful,  however,  if  this  or  any  other  plant 
rhythm  is  entirely  unrelated  to  rhythmic  external  conditions. 

The  origin  of  trees.  —  The  most  distinctive  features  of  trees  are  per- 
ennial elongation,  perennial  diametral  increase,  perennial  lignification, 
and  permanence  of  tissues.  Elongation  has  been  seen  to  be  stimu- 
lated by  conditions  that  favor  absorption  or  impede  desiccation,  while 
lignification  and  diametral  increase  are  favored  more  by  xerophytic 
conditions.  Maximum  elongation,  on  the  whole,  occurs  in  the  tropical 
rain  forest,  where  trees  are  predominantly  tall  and  slender?  though  the 


738 


ECOLOGY 


tallest  individual  forms,  Sequoia  and  Eucalyptus,  occur  in  much  drier 
regions.  Maximum  diametral  increase  occurs,  on  the  whole,  in  rela- 
tively arid  climates.  It  is  sometimes  stated  that  the  baobab,  a  xero- 
phytic  African  tree,  has  the  greatest  diametral  growth  of  all  known 
trees;  other  xerophy tic  trees  with  great  diametral  enlargement  are  the 
dragon  tree  (Dracaena  Draco}  and  Cavanillesia.  However,  some 
trees  of  great  diameter  (as  the  redwood  and  the  hemlock)  are  meso- 
phytic. 

Probably  trees  are  a  relatively  recent   product   of   evolution,  and 
doubtless  the  tree  habit  has  originated  many  times  and  in  many  habi- 


FIG.  1060.  —  The  mountain  sheep  (Raoulia  eximia),  a  remarkable  alpine  composite, 
illustrating  the  culmination  of  the  cushion  habit  among  seed  plants ;  each  coral-like  colony 
is  composed  of  thousands  of  separate  but  closely  compacted  shoots,  the  whole  mass  being 
attached  to  solid  rock  by  a  single  root  system ;  New  Zealand.  —  From  COCKAYNE. 

tats.  Sometimes  it  has  been  assumed  that  the  tree  habit  is  a  result 
of  the  "  struggle  for  existence  "  in  mesophytic  climates;  this,  per- 
haps, is  a  tenable  view,  since  growth  in  dense  cultures  increases  stem 
elongation.  However,  there  is  equal  reason  for  believing  that  trees 
have  arisen  also  in  dry  regions,  since  lignification  and  diametral  en- 
largement are  best  developed  there.  Perhaps  the  most  essential  step 
in  the  evolution  of  the  tree  habit  is  in  the  passage  from  an  herb  to  a 
shrub;  if  so,  experiments  on  such  plants  as  Spiraea  salicifolia  and 


STEMS 


739 


Amorpha  canescens,  which  are  sometimes  one  and  sometimes  the  other, 
might  throw  light  upon  the  problem. 

The  advantages  of  variation  in  stem  form.  —  The  capacity  for  dif- 
ferential elongation  possessed  by  aquatic  stems  and  by  many  aerial 
stems  when  submerged  by  sand  is  of  obvious  advantage  in  that  the  lift- 
ing of  the  leaves  into  the  light  and  the  air  is  thus  made  possible.  Of 
unusual  significance  is  stem  dwarfness,  as  illustrated  by  cushion  plants 
and  by  the  Krummholz,  since  such  habits  are  admirably  suited  for  pro- 
tection, especially  from  excessive  transpiration.  Such  protection  is  due 
in  large  part  to  the  reduced  surface  exposure  resulting  from  the  compact 


1062 


FIGS.  1061,  1062.  —  Spinose  branches:  1061  A,  a  spinose  branch  of  the  wild  crab 
apple  (Pyrus  coronaria)  in  its  first  year;  note  that  the  terminal  bud  soon  ceased  to  develop ; 
1061  B,  a  similar  branch  in  its  second  year,  showing  a  lateral  bud  that  continued  to  develop  ; 
5,  leaf  scar  indicating  the  position  of  a  leaf  of  the  first  season;  1062,  a  compound  spi- 
nose branch  of  a  hawthorn  (Crataegus  punctata),  showing  that  such  spines  are  branches 
whose  terminal  buds  soon  cease  development;  note  that  leaves  occur  on  the  lateral 
branches  (f),  as  well  as  on  the  main  axis  (a). 


740 


ECOLOGY 


arrangement  of  the  parts.  For  example,  in  Raoulia  (fig.  1060)  and  in 
other  alpine  or  arctic  cushion  plants,  the  short  stems  are  so  closely  packed 
together  that  only  the  tips  are  exposed.  Cushions  are  formed  by  various 
mosses  (as  Leucobryum,  Bartramia,  and  Dicranum)  and  by  some  lichens 
(as  Cladonia  rangiferina,  fig.  898),  but  among  seed  plants  the  habit  is 
rare  outside  of  inclement  climates.  An  additional  advantage  from 


FIG.  1063. — A  spherical  cactus  (Echinocactus  Wislizeni),  representing  an  extreme  con- 
trast to  a  thin-leaved  tropical  evergreen,  since  it  has  a  minimum  transpiring  surface  in  pro- 
portion to  its  volume;  note  the  numerous  recurved  spines.  —  Photograph  by  FULLER. 

dwarf  ness  is  increased  ground  protection;  as  a  consequence,  transpira- 
tion is  still  further  reduced  and  temperature  changes  are  less  rapid. 
Perhaps  the  advantages  of  proximity  to  the  ground  are  best  realized  in 
rosette  herbs,  whose  leaves  often  are  closely  appressed  to  the  soil  (fig. 
1036).  A  third  and  perhaps  the  greatest  advantage  in  dwarf  ness  is 
seen  where  the  latter  is  most  in  evidence,  namely,  in  alpine  and  arctic 
regions.  There  it  makes  possible  protection  by  snow,  and  con- 
sequently there  is  a  suspension  of  transpiration  during  the  very 
months  when  it  would  be  most  dangerous  because  of  the  cessation 
of  absorption;  furthermore,  snow  covers  protect  greatly  from  cold, 


STEMS 


741 


So  severe  are  the   conditions  at  times  that  only  those  parts  covered  / 
by  the  snow  are  able  to  survive. 

Spinescence. —  The  structural  features  of  spines. — Stem  spines  are 
of  two  fundamentally  different  sorts,  namely,  reduced  branches,  as  in 
the  honey-locust  (Gleditsia) ,  wild  crab  (Pyrus  coronaria,  fig.  1061), 
Prunus,  and  Crataegus  (fig.  1062),  and  stem  emergences,  as  in  the  roses 
(figs.  1066,  1068)  and  the  gooseberries.  In  the  spinescent  branches, 
which  often  are  compound,  the  branch  character  usually  is  easily  recog- 
nized in  youth  through  the  presence  of  leaves  and  axillary  buds, 
and  scars  may  be  found  even  on  old  spinescent  branches.  Spinescent 
emergences,  however,  commonly  are  simple,  and  may  be  either  stout 
thorns  or  delicate  prickles;  both  kinds  occur  in  the  roses,  the  former 
near  the  leaves,  and  the  latter  scattered  along  the  stem.  Perhaps  the 
culmination  of  spinescence  is  seen  in  cacti  (figs.  1063,  1040-1042), 
where  all  gradations  occur  between  the  stoutest  thorns  and  the  most 
delicate  prickles. 

The  factors  determining  spinescent  branches.  —  Experiments  on 
Ulex,  Berberis  (figs.  885,  886),  and  other  plants  show  that  spinescence 
may  be  induced  by  intense 
light  and  especially  by  desic- 
cation ;  in  Ulex  the  shoots  de- 
veloped in  moist  air  bear 
foliage  leaves,  while  in  dry 
air  the  branches  and  even 
the  leaves  become  spinescent 
(figs.  1064,  1065).  Pyrus 
coronaria  and  Prunus  ameri- 
cana  are  much  thornier  in 
xerophytic  than  in  mesophytic 
situations;  Celtis  occidentalis 
(the  hackberry)  is  a  spineless 
mesophytic  tree,  while  Celtis 
occidentalis  pumila  is  a  thorny 
xerophytic  shrub.  Thus  such 
spinescence  seems  to  be  a  re- 
sult of  hard  conditions,  the 
branches  remaining  reduced 

because  of  pronounced  desiccation,  supplemented,  perhaps,  by  other  fac- 
tors.    Individuals  of  Ulex  grown  in  concentrated  glucose  solutions  de- 


FIGS.  1064,  1065. — •  Spinescence  in  Ulex  euro- 
paeus :  1064,  an  individual  grown  in  saturated  air; 
note  the  conspicuous  leaves;  1065,  an  individual 
grown  in  dry  air;  note  that  the  branches  are  re- 
duced to  spines.  —  From  LOTHELIER. 


742 


ECOLOGY 


velop  spines  even  in  moist  air.     A  recent  worker  claims  that  the  leafy 
shoots  of  Ulex  usually  developed  in  moist  air  are  merely  juvenile  shoots. 

Spinescent  branches,  as  appears  from  the  preceding,  are  dwarfed  shoots,  and  are 
produced  much  as  are  other  dwarfed  stems.  However,  there  is  no  tendency  toward 
lateral  enlargement,  as  in  most  dwarfed  stems,  but  rather  the  reverse,  since  one  of 
the  chief  characteristics  of  spines  is  attenuation.  If  elongation  occurs  when  the 
growth  conditions  are  very  favorable,  and  lateral  enlargement  accompanied  by 
dwarfing  when  they  are  less  favorable,  then  attenuation  with  dwarfing  (i.e.  spines- 
cence)  may  be  a  result  of  very  unfavorable  conditions.  Attenuation  seems  to  imply 
progressive  severity  in  the  developmental  conditions,  the  supply  of  structural  ma- 
terial becoming  less  and  less  as  the  branch  develops.  Furthermore,  there  is  an  in- 
creasing development  of  mechanical  tissue  in  proportion  to  the  other  tissues,  the 
tip  being  composed  chiefly  of  thick-walled  elements,  which  account  for  its  extreme 
stiffness;  this  fact  also  fits  in  with  the  desiccation  theory  of  spinescence.  Desic- 
cation does  not  account  readily  for  all  spinescent  branches,  since  Crataegus  and 
Gleditsia  develop  them  profusely  in  moist  and  fertile  flood  plains;  furthermore, 
thorny  lianas  are  extremely  abundant  in  humid  tropical  forests. 

The  factors  determining  spinescent  emergences.  —  The  cause  of  spines- 
cent  emergences  is  much  more  difficult  to  determine  than  that  of  spines- 
cent  branches.  Their  variation  is  considerably  less  and  their  develop- 
ment is  not  so  obviously  related  to  severe  conditions.  The  stem  of  Rosa 

blanda  frequently  is  smooth,  but  in 
xerophytic  conditions,  prickles  appear 
in  abundance  (figs.  1066-1068);  oddly 
enough,  abundant  prickles  also  char- 
acterize vigorous  rose  and  gooseberry 
suckers  in  mesophytic  habitats.  So 
far  as  desiccation  or  other  hard  con- 
ditions favor  the  development  of  spi- 
nescent emergences,  as  in  the  roses, 
there  might  seem  to  be  agreement  with 
Ulex,  but  there  is  the  fundamental 
difference  that  here  desiccation  causes 
the  appearance  of  a  new  organ  rather 
than  the  reduction  of  an  organ  com- 
monly better  developed;  in  other  words, 
the  influence  of  an  increased  supply  of 
water  is  directly  inhibitory.  Thus  desic- 
cation in  the  case  of  prickles  and  of 
other  spiny  emergences,  much  as  with 


1066  1067  1068 

FIGS.  1066-1068.  —  Variation  in 
prickle  development  on  the  stem  of 
a  wild  rose  (Rosa  blanda) :  1066,  a 
portion  of  a  young  root  sucker  of 
a  mesophytic  individual;  note  the 
abundance  of  prickles;  1067,  a 
portion  of  a  branch  from  the  same 
stem,  showing  an  entire  absence  of 
prickles;  1068,  a  portion  of  a 
branch  from  a  xerophyticindividual, 
corresponding  to  that  figured  in 
1067 ;  note  the  numerous  prickles. 


STEMS  743 

the  hairs  of  Polygonum  amphibium  (p.  574),  has  a  determinative  rather 
than  a  formative  influence,  as  in  the  case  of  spinescent  branches. 
Possibly  many  spines  and  prickles  are  obligate  rather  than  facultative. 

Spines  and  "  natural  selection"  —  A  curious  though  widely  accepted  view  is 
that  spines  have  been  evolved  by  "  natural  selection  "  through  age-long  "  compe- 
tition "  between  plants  and  grazing  animals.  It  has  been  supposed  that  plants 
happening  to  have  spines  have  survived  and  had  progeny,  while  many  others  with- 
out such  "  weapons  of  defense  "  have  failed.  While  the  xerophytic  theory  of  spines- 
cence  has  good  experimental  proof,  the  selection  hypothesis  has  almost  none,  the 
sole  argument  in  its  favor  being  derived  from  overgrazed  pastures,  where  thistles, 
brambles,  and  hawthorns  tend  sometimes  to  increase  their  area  in  place  of  more 
palatable  plants.  Many  considerations  weaken  the  force  of  this  argument.  For 
example,  spinescent  plants  are  not  necessarily  unpalatable  ;  hawthorns  are  eaten 
by  cows,  until  they  can  no  longer  reach  the  foliage,  the  lower  parts  of  the  trees  often 
being  cropped  into  fantastic  shapes;  it  is  reported  also  that  in  Arizona  grazing 
animals  avoid  a  relatively  thornless  cactus  (probably  because  of  its  flavor),  while 
greedily  eating  a  thorny  variety.  Furthermore,  most  thorns  are  tender  and  more 
or  less  edible  when  young,  that  is,  when  the  plants  most  need  protection. 

As  a  whole,  the  plants  most  eaten  by  grazing  animals  are  grasses,  and  it  is  well 
known  that  grasses  flourish  as  a  result  of  grazing  ;  indeed,  one  of  the  prominent  theo- 
ries attempting  to  account  for  the  prairie  is  that  grazing  animals  keep  down  shrubs  and 
trees.  Grasses  increase  and  trees  decrease  in  pastures  in  the  Central  States,  because 
the  former  have  underground  propagative  organs  and  the  latter  not,  spinescence 
being  a  negligible  factor.  In  the  Eastern  States  shrubs  and  trees,  whether  spines- 
cent  or  not,  rapidly  invade  pastures  because  of  the  favorable  climate,  in  spite  of  the 
combined  influence  of  rhizomatous  herbs,  farmers,  and  grazing  animals.  How- 
ever, there  are  still  stronger  arguments  against  the  selection  theory.  In  the  first 
place,  the  close  confinement  of  grazing  animals  is  a  recent  and  insignificant  thing  in 
biological  history  ;  the  overplus  of  vegetation  always  has  been  so  enormous  that  the 
influence  of  herbivorous  animals  on  plant  evolution  must  have  been  infinitesimal. 
In  the  second  place,  the  culmination  of  spinescence  is  in  deserts,  where  grazing  ani- 
mals are  scarce,  and  where  "  competition,"  if  such  exists,  is  least  important.  Finally, 
in  so  far  as  "  natural  selection  "  is  applicable  to  the  problem  of  spinescence,  its  in- 
fluence has  to  do  with  the  survival  of  spinescent  plants  rather  than  with  the  origin  of 
their  spines. 

The  advantages  of  spines.  —  Probably  spines  and  prickles  are  of  some 
advantage  as  a  means  of  protection  from  herbivorous  animals,  though 
this  advantage  has  been  greatly  overestimated.  The  perpetuation  of 
thistles  in  sheep  pastures  doubtless  is  due  in  part  to  leaf  spinescence,  and 
the  thorniest  cacti  (fig.  1063)  certainly  are  amply  protected  from  the  in- 
cursions of  ordinary  grazing  animals.  In  the  holly  the  upper  leaves, 
which  are  out  of  the  reach  of  grazing  animals,  are  less  spinescent  than 
are  the  lower  leaves.  In  the  cacti,  spines  often  are  abundant  enough  to 


744  ECOLOGY 

lessen  the  intensity  of  the  incident  light,  thus,  perhaps,  reducing  trans- 
piration. Thorns  are  of  undoubted  advantage  in  the  climbing  of  many 
lianas  (p.  654).  Probably  many  and  perhaps  most  spines  subserve  no 
role  of  importance;  in  GledUsia  they  occur  chiefly  on  old  and  inedible 
branches,  while  the  tender  young  shoots  commonly  are  spineless;  the 
increased  spinescence  of  xerophytic  forms  (as  in  Ulex  and  Rosa) 
appears  to  have  no  advantage. 

Tuberization.  —  Early  experiments.  —  Tubers  represent  the  cul- 
mination of  stem  shortening  accompanied  by  diametral  enlargement 
(figs.  1069,  983,  989,  990),  and  commonly  they  accumulate  quantities 
of  food  and  water  which  are  utilized  when  their  buds  develop  into  shoots. 
Experiments  show  that  tuberization  is  directly  favored  by  darkness  and 


FIG.  1069.  —  A  rhizome  of  Scutellaria  parvula,  in  which  there  has  taken  place  alter- 
nately stem  elongation  and  tuberization;   r,  adventitious  roots;   t,  tuberized  portions. 

checked  by  light,  although  indirectly  light  favors  tuberization  in  that 
it  stimulates  food  formation  in-  the  foliage  leaves.  The  removal  of  all 
rhizomes  from  a  developing  potato  plant  results  in  tuber  formation  on 
aerial  shoots,  regardless  of  illumination.  Moderate  desiccation  favors 
tuberization,  while  moisture  often  inhibits  tuber  formation  (as  in  the 
potato  and  in  species  of  Juncus).  Low  temperatures  appear  to  favor 
tuberization,  only  tuber-bearing  shoots  developing  in  the  Marjolin  potato 
below  7°  C.,  while  only  leafy  shoots  develop  above  20°  C.;  horizontal 
tuber-bearing  shoots  may  be  transformed  into  erect  leafy  shoots  by  rais- 
ing the  temperature,  and  the  reverse  transformation  may  be  effected  by 
lowering  the  temperature.  More  heat  is  necessary  to  transform  dex- 
trose into  cellulose  than  into  starch ;  this  may  explain  the  abundant  for- 
mation of  starch  accompanying  tuberization  at  relatively  low  temperatures, 
and  the  great  formation  of  cellulose  associated  with  elongation  at  high 
temperatures.  There  appears  to  be  a  reciprocal  relation  between  leafy 
shoots  and  tuberized  shoots,  any  factor  tending  to  suppress  the  former 
stimulating  the  development  of  the  latter  ;  for  example,  tuberization  is 


STEMS  745 

induced  by  various  substances  which  check  the  growth  of  shoots.  An 
instance  of  tuberized  aerial  stems  is  seen  in  the  kohl-rabi;  even  the  leaves 
of  this  plant  become  tuberized  if  flowering  is  suppressed.  Similar  tuber- 
ous swellings  occur  on  the  stems  of  Eucalyptus;  neither  in  the  kohl-rabi 
nor  in  Eucalyptus  are  the  stimulating  factors  known  except  that  in  the 
former,  light  is  necessary,  and  that  in  the  latter,  the  size  increases  with 
the  food  supply.  In  Nephrolepis  similar  primordia  may,  as  the  con- 
ditions vary,  give  rise  to  rhizomes,  runners,  or  tubers. 

The  fungal  theory  of  tuberization.  —  It  has  long  been  known  that  tubers 
almost  universally  are  infected  with  fungi,  potato  tubers,  for  example, 
containing  Fusarium  Solani  and  other  fungal  forms.  The  experimental 
study  of  the  tuber  problem  as  related  to  fungi  is  attended  with  difficulty, 
owing  to  the  fact  that  the  isolation  and  the  subsequent  cultivation  of 
tuber  fungi,  as  of  mycorhiza  fungi  in  general,  is  far  from  easy.  As  yet, 
inoculation  experiments  with  potato  rhizomes  are  uncertain  in  their 
results,  but  in  many  orchids,  where  fungal  symbiosis  is  more  obligate 
than  elsewhere,  some  exceedingly  suggestive  studies  have  been  made. 
As  a  class,  orchids  have  two  growth  periods,  one  of  relatively  active  dif- 
ferentiation and  elongation,  and  another  of  lessened  activity  and  dia- 
metral increase,  that  is,  of  tuber  formation.  Tuberization  may  involve 
the  roots,  as  in  Habenaria,  the  underground  stem,  as  in  A  plectrum,  or 
the  aerial  stem,  as  in  most  epiphytic  forms.  It  has  been  shown  by  inoc- 
ulation experiments  and  otherwise  that  tuberization  is  initiated  when  the 
stem  or  root  is  infected  with  the  proper  fungus,  the  latter  organism  ap- 
pearing to  arrest  the  growth  of  the  terminal  bud  and  to  cause  the  develop- 
ment of  hypertrophied  cells.  Shoots  arising  from  infected  tubers  are  at 
first  free  from  fungi,  and  they  differentiate  rapidly  until  they  in  turn  are 
infected,  whereupon  tuberization  sets  in,  so  that  usually  there  is  one 
period  of  infection  and  tuberization  each  year  in  a  given  rhizome  or  root. 
In  the  shoots  of  Neottia,  however,  fungal  infection  and  tuberization  are 
present  from  the  start.  Simultaneously  with  tuberization  starch  accumu- 
lation is  greatly  accelerated. 

Circumstantial  evidence  favoring  the  fungus  theory.  — When  the  potato  was  first 
introduced  into  France,  gardens  sown  to  tubers  produced  crops  of  tubers,  while 
gardens  sown  to  seed  produced  no  tubers,  the  inference  being  that  the  fungi  from 
the  old  tubers  infected  the  new  tubers,  whereas  the  seeds  were  free  from  fungi. 
Nowadays  seed  cultures  produce  crops  of  tubers,  presumably  because  the  soil  has 
by  this  time  become  infected  with  the  necessary  fungus.  The  gametophytes  of 
Lycopodium  and  Botrychium  are  tuberous  and  are  infected  by  fungi  when  subter- 


746 


ECOLOGY 


ranean,  but  not  when  aerial  (figs.  1070,  1108).  The  root  tubercles  of  legumes  (see 
p.  787),  which  are  known  to  be  the  direct  result  of  bacterial  infection,  resemble  tubers 
not  alone  in  the  replacement  of  elongation  by  lateral  enlargement,  but  also  in  the 
great  accumulation  of  food  and  water  in  the  hypertrophied  cells.  Again,  in  many 
fungus  and  insect  galls  (as  in  the  cedar  apple  and  in  cynipid  oak  galls),  elongation 
is  checked,  while  diametral  increase  and  the  accumu- 
lation of  food  and  water  are  greatly  stimulated;  the 
food  may  accumulate  even  in  definite  carbohydrate  and 
protein  layers.  Thus  root  tubercles,  galls,  and  tubers 
agree  in  all  essential  structural  features;  root  tubercles 
and  galls  admittedly  are  due  to  the  influence  of  out- 
side organisms,  and  it  seems  fairly  certain  that  the 
same  is  true  of  tubers. 

If  a  gall  is  defined  as  a  structural  modification  due 
to  a  foreign  organism,  then  potato  tubers  and  the 
gametophytes  of  Lycopodium  and  Botrychium  may  be 
classed  as  galls,  if  the  fungus  theory  is  confirmed.  The 
fact  that  tubers  and  root  tubercles  are  advantageous, 
as  is  not  true  of  many  fungus  galls  and  of  most  insect 
galls,  is  of  no  significance  from  the  standpoint  of  cau- 
sation, and  would  seem  unimportant  in  classification; 
the  harmful  root  tubercles  caused  by  nematode  worms 
and  the  beneficial  root  tubercles  caused  by  bacteria  are 
remarkably  alike  in  form  and  origin,  and  may  well  be 
classed  as  similar  structures. 

Bulbs,  as  well  as  tubers,  commonly  are  infected 
with  fungi,  which,  therefore,  may  have  formative  sig- 
nificance, though  the  case  is  much  more  doubtful  than 
with  tubers.  While  bulbs  agree  with  tubers  in  possess- 
ing shortened  stems,  they  differ  in  that  lateral  stem  en- 
largement is  replaced  by  hypertrophied  growth  and  by 
food  accumulation  in  the  scale  leaves.  Arrhenatherum 
bulbosum  has  been  shown  to  be  merely  a  bulbous 
form  of  A.  elatius,  resulting  from  bacterial  infection. 


FIG.  1070. — The  sub- 
terranean tuberous  game- 
to  phyte  (g)  of  Lycopodium 
annotium,  bearing  a  young 
sporophyte  (s),  whose 
aerial  portion  has  numer- 
ous awl-shaped  foliage 
leaves  (/)  in  many  orthos- 
tichies. 
—  From  BRUCHMANN. 


The  nature  of  the  fungal  influence.  —  In  orchids  and  in  the  potato, 
tuberization  has  been  induced  in  concentrated  solutions  (as  of  sac- 
charose or  glycerin)  without  the  agency  of  fungi;  similarly  in  the  onion, 
bulb  formation  has  been  induced  in  sterilized  cultures.  When  radishes 
are  grown  in  concentrated  glucose  solutions,  the  root  becomes  suberized, 
somewhat  resembling  a  potato  tuber,  and  starch  accumulates  instead 
of  sugar.  Apparently,  then,  tuberization  may  result  when  the  osmotic 
pressure  in  the  culture  medium  is  high,  though  this  does  not  appear  to  be 
the  sole  factor,  since  solutions  of  glucose  and  glycerin  of  equal  pressure 
give  different  results.  Certainly  for  starch  formation,  and  perhaps  for 


STEMS  747 

tuberization,  the  presence  of  abundant  sugar  also  is  necessary.  Since 
fungi  commonly  have  a  much  more  concentrated  cell  sap  than  do  other 
plants  (p.  755),  it  seems  very  probable  that  the  role  of  fungi  in  tuberiza- 
tion is  in  raising  the  concentration  of  the  media  which  they  enter ;  in- 
deed, it  has  been  demonstrated  that  Fusarium  cultures  in  macerated 
preparations  of  potato  tubers  raise  the  concentration.  Very  probably 
tuberization  resulting  from  low  temperatures  or  from  soil  dryness  may  be 
similarly  explained  by  increased  cell  sap  concentration.  In  some  cases 
(as  in  the  radish)  the  increasing  concentration  of  the  sugar  manufactured 
by  the  leaves  may  alone  be  sufficient  to  stimulate  tuberization. 

Stem  succulence.  —  As  in  leaves,  so  in  stems,  succulence  appears  to  be  of  two  fun- 
damentally different  sorts,  namely,  halophytic  and  similar  succulence,  in  which 
water  accumulation  appears  to  be  associated  with  a  concentrated  cell  sap,  and 
the  sort  of  succulence  found  in  the  cacti,  which  rarely  occur  in  alkaline  situations 
and  whose  cell  sap  is  not  known  to  be  particularly  concentrated.  In  Salicornia 
and  in  other  fleshy  halophytes,  succulence  has  been  shown  to  vary  with  the  salt 
content  of  the  medium,  and  the  retention  of  water  would  appear  to  be  due  largely 
to  the  presence  of  salts.  In  the  cacti  and  in  similar  forms  the  retention  of  water 
probably  is  due  in  large  part  to  a  thick  cuticle  and  to  other  relatively  impermeable 
tissues,  supplemented  by  a  small  transpiring  surface.  As  stated  elsewhere  (p.  632), 
the  cells  of  succulent  plants  do  not  necessarily  contain  more  water  than  do  those 
of  other  plants,  but  they  are  more  compactly  placed.  Axial  shortening  an,d  dia- 
metral enlargement  are  significant  features  of  succulent  stems,  and  in  these  re- 
spects such  stems  agree  with  tubers  and  with  xerophytic  stems  generally; 
furthermore,  these  features  appear  to  be  due  to  similar  causes. 

Correlation ;  regeneration ;  polarity.  —  Correlation.  —  In  the  preced- 
ing chapters  much  has  been  said  concerning  the  influence  of  explicit 
external  factors,  such  as  light,  temperature,  and  water;  allusion  has  been 
made  also  to  internal  or  inherent  factors,  which  are  supposed  to  represent 
hereditary  as  opposed  to  environmental  influences.  Intermediate  be- 
tween these  are  influences  residing  within  the  organism,  but  not  heredi- 
tary, such  as  the  influence  of  one  cell  or  organ  upon  the  development  of 
another.  Phenomena  due  to  such  influences  are  known  as  correlations. 
If  an  individual  of  Sempervivum  assimile,  ordinarily  a  stemless  plant 
with  succulent  leaves,  is  removed  from  its  natural  dry  habitat  to  a  moist 
chamber,  there  soon  develops  as  a  reaction  to  the  new  conditions  an  erect 
stem,  on  which  the  first  leaves  also  are  succulent  (figs.  1043, 1044).  After 
a  time  thin  leaves  develop,  whereupon  the  stem  ceases  to  elongate.  Here 
axial  elongation  is  correlated  with  thick  leaves,  and  axial  shortening  with 
thin  leaves.  Many  plants  (as  Penthorum  and  Satureja,  figs.  981,  985) 


74$ 


ECOLOGY 


possess  erect  or  orthotropic  and  horizontal  or  plagiotropic  shoots,  which 
obviously  are  correlated,  since  the  removal  of  the  former  causes  the 
latter  to  become  erect;  the  presence  of  the 
erect  shoot  appears  to  inhibit  the  horizontal 
shoot  from  becoming  erect.  The  reference 
of  phenomena  to  correlation,  or  for  that 
matter  to  inherent  factors,  does  not  explain 
them.  The  use  of  such  indefinite  terms  is 
mystifying  rather  than  illuminating,  and  ex- 
planations must  be  sought  in  actual  under- 
lying causes.  However,  terms  like  correla- 
tion and  inherent  causes  may  be  useful 
temporarily,  as  serving  to  denominate  these 
particular  regions  of  our  ignorance. 

Regeneration.  —  In  most  plants  the  termi- 
nal buds  are  stronger  than  the  others,  and 
they  develop  into  vigorous  shoots  (figs.  952, 
953);  the  upper  lateral  buds  develop  into 
less  vigorous  shoots,  and  the  lower  lateral 
buds  usually  remain  undeveloped.  Such 
plants  are  characterized  by  excurrent  branch- 
ing. In  some  cases  (as  in  the  lilac)  the  lateral 
buds  are  the  stronger,  and  their  continued 
development  results  in  deliquescent  branch- 
ing. If  a  terminal  bud  of  an  excurrent  shoot 
is  injured  or  removed  during  development, 
one  or  more  lateral  buds,  which  other- 
wise might  have  remained  latent,  grow  out 
into  shoots  (figs.  873,  1055).  Such  a  re- 
placement by  similar  organs  of  an  organ 
that  has  been  removed,  or  whose  growth 
has  been  checked,  is  known  as  regeneration. 
In  animals  a  lost  part  commonly  is  regener- 
ated at  the  place  of  severance.  This  rarely 
occurs  in  plants,  possibly  because  of  the 
presence  of  latent  buds.  In  roots  and  in 
some  leaves  (as  in  Cyclamen),  where  there  are  no  such  latent  buds,  the 
lost  part  may  be  restored  at  the  cut  surface,  as  in  animals  (p.  503); 
the  regeneration  of  a  lost  part  at  the  cut  surface  may  be  termed  restitution. 


FIG.  1071.  —  Regeneration 
in  the  scarlet  runner  bean 
(Phaseolus  multiflorus)-,  the 
epicotyl  of  a  seedling  was  cut 
away,  whereupon  the  minute 
primordia  in  the  axils  of  the 
cotyledons  developed  into 
vigorous  shoots.  —  After 
McCALLUM  (drawn  from  a 
photographic  reproduction). 


STEMS 


749 


When  a  growing  tip  is  removed,  the  latent  buds  that  develop  and 
replace  it  commonly  are  those  nearest  the  apex.  If  these  are  removed, 
those  next  below  develop,  and  so  on  until  no  more  buds  remain.  A 
remarkable  instance  of  this  is  seen  in  Phaseolus  (fig.  1071),  where  buds 
in  the  axils  of  the  cotyledons  may  thus  be  induced  to  develop  into  shoots. 
If  all  buds  are  removed,  new 
buds  may  organize,  as  from  the 
exposed  part  of  the  cambium 
ring  in  a  beech  stump.  The 
factors  operative  in  regeneration 
are  unknown,  the  current  theory 
relating  the  phenomena  to  cor- 
relation; for  example,  a  main 
shoot  is  supposed  to  inhibit  the 
development  of  lateral  shoots, 
possibly,  because  it  utilizes  the 
available  food  through  having  a 
better  position  or  through  making 
an  earlier  start.  The  suppression 
of  the  main  shoot  removes  the 
inhibition,  and  permits  the  lateral 
shoots  to  develop.  Such  a  con- 
ception is  of  value  chiefly  in 
stating  the  problem. 

Polarity.  —  If  a  willow  shoot 
is  placed  in  water,  shoots  de- 
velop from  the  uppermost  buds, 
and  roots  originate  near  the  base. 
If  the  shoot  is  inverted,  roots  de- 
velop above  and  shoots  below 


FIGS.  1072,  1073.  —  Polarity  in  a  willow 
(Salix):  1072,  an  ordinary  erect  cutting; 
1073,  a  cutting  that  has  been  grown  in  an 
inverted  position;  note  that  in  each  case  the 


roots  (r)  arise  from  the  basal  portion  of  the 
stem  (root  pole)  and  the  new  shoots  (s)  from 
the  apical  portion  of  the  stem  (shoot  pole). 
—  After  PFEFFER. 


(figs.  1072,  1073).  Such  a  phe- 
nomenon illustrates  polarity,  and 
appears  to  indicate  that  plants 
and  portions  of  plants  possess 

root  poles  and  shoot  poles.  Among  other  instances  of  polarity,  there 
may  be  cited  the  appearance  of  shoots  at  the  uppermost  parts  of  exposed 
roots  of  Crataegus  (fig.  722),  of  buds  at  the  basal  portions  of  severed 
leaves  in  Sansevieria  (fig.  934),  and  of  tubers  on  the  upper  ends  of 
reversed  shoots  of  the  potato. 


750 


ECOLOGY 


Attempts  have  been  made  to  explain  polarity  by  postulating  the  ex- 
istence in  plants  of  shoot-forming  substances  that  migrate  upwards  and 
of  root-forming  substances  that  migrate  downwards.  In  a  willow  branch 
new  shoots  develop  toward  the  apex,  where  the  shoot-forming  sub- 
stances are  supposed  to  congregate  in  greatest  abundance,  and  new  roots 
develop  toward  the  base,  where  the  root-forming  .substances  are  thought 
to  be  most  abundant.  Apparently  favoring  this  theory  is  the  fact  that 

severed  old  leaves  of 
Begonia  or  of  Achi- 
menes  regenerate  shoots 
that  soon  produce 
flowers,  whereas  shoots 
developing  from  young 
leaves  do  not  develop 
flowers  for  some  time, 
as  though  old  leaves 
were  much  fuller  of 
flower-forming  sub- 
stances than  are  young 
leaves.  However,  un- 
til something  is  known 
concerning  these  pos- 
tulated substances,  the 
theory  must  be  re- 
garded as  mystical  if 
not  actually  erroneous. 
To  some  extent  tend- 
encies toward  polarity 


1074 


FIGS.  1074,  1075.  —  Reversal  of  polarity  in  an  alga, 
Bryopsis  muscosa:  1074,  an  ordinary  plant  (somewhat 
schematic) ;  1075,  the  apex  of  a  plant  that  has  been  grown 
in  an  inverted  position;  note  that  rhizoids  (r)  have  de- 
veloped from  the  apex  of  the  shoot  (a),  and  that  shoot 


branches  (6)  nearer  the  original  rhizoid  pole  have  con- 
tinued to  develop  as  branches  but  have  taken  a  new 
direction  (&') ;  the  dotted  portions  represent  the  original 
part  of  the  plant  before  inversion,  while  the  undotted 
portions  represent  portions  growing  after  inversion;  note 
the  intimate  contact  between  the  rhizoids  and  the  soil 
particles  (/>) ;  this  alga  is  a  coenocyte,  being  from  the 
outset  without  internal  cell  walls;  considerably  magnified. 
—  After  NOLL. 


may  be  counterbal- 
anced by  external  fac- 
tors; for  example,  if  a 
willow  shoot  is  laid 
horizontally  in  the 
water  or  on  the  soil, 
shoots  and  roots  often 


develop    more   or  less 

equally  along  the  whole  length  of  the  shoot,  the  former  chiefly  above  and 
the  latter  below;  gravity,  light,  and  water  probably  enter  here  as  factors 
of  importance.  In  Zamia,  shoots  may  appear  at  both  ends  of  a  stem 


STEMS  751 

fragment.  The  gemmae  of  Marchantia  develop  chlorenchyma  on  which- 
ever side  is  illuminated,  rhizoids  appearing  on  the  other  side.  Bryopsis 
and  some  other  algae,  when  reversed,  develop  shoots  from  the  former 
rhizoids,  and  rhizoids  from  the  former  shoots  (figs.  1074,  1075),  light 
being  regarded  as  the  chief  factor  determining  shoot  formation;  the 
roots  of  Neottia  and  of  Platycerium  under  similar  conditions  develop 
into  shoots.  An  inverted  piece  of  a  dandelion  root  develops  shoots 
from  the  end  toward  the  root  tip,  if  the  other  end  is  in  water.  Such 
phenomena  illustrate  what  has  been  termed  a  reversal  of  polarity. 


CHAPTER  IV  — SAPROPHYTISM  AND  SYMBIOSIS 
i.  COMMENSALISM  AND  SAPROPHYTISM 

Symbiosis  and  related  phenomena.  —  Autophytes  and  heterophytes.  — 
The  preceding  sections  have  been  devoted  to  a  detailed  consideration  of 
the  ecological  aspects  of  nutrition  in  green  plants.  Such  plants  may  be 
termed  autophytes,  autotrophic  plants,  or  independent  plants,  because 
they  are  able  to  obtain  all  necessary  food  materials  directly  from  inorganic 
sources,  subsequently  converting  them  into  foods.  In  striking  contrast 
thereto  are  the  heterophytes,  heterotrophic  plants,  or  dependent  plants, 
whose  existence  depends  upon  antecedent  or  coexistent  organic  forms, 
because  they  derive  at  least  a  part  of  their  food  from  organic  sources. 
Since  the  nutritive  relations  of  the  heterophytes  are  so  different  from 
those  of  the  autophytes,  they  are  made  the  subject  of  a  separate  chapter. 
Heterophytes  may  be  subdivided  into  saprophytes,  which  obtain  food 
from  dead  organic  matter,  and  parasites,  which  obtain  food  or  food 
materials  from  living  organisms. 

Symbiosis.  —  When  two  or  more  diverse  organisms  live  together  in 
more  or  less  intimate  relationship,  the  phenomenon  is  termed  symbiosis, 
and  the  individual  organisms  are  termed  symbionts.  The  phenomena 
included  in  symbiosis  may  be  conveniently  grouped  under  the  subheads, 
parasitism  and  commensalism.1  Parasitism  is  that  form  of  symbiosis 
in  which  one  organism,  the  parasite,  derives  food  or  food  materials  from 
another,  the  host,  manifestly  to  the  detriment  of  the  latter;  sometimes 
each  symbiont  derives  food  from  the  other  (as  in  the  case  of  clover  and 
bacteria),  a  relationship  that  may  be  termed  reciprocal  parasitism.  Com- 
mensalism includes  those  cases  of  symbiosis  in  which  two  or  more  organ- 
isms live  together  with  possible  benefit  to  some  or  all  of  the  symbionts, 
but  with  injury  to  none.  The  individuals  in  commensalistic  symbiosis 
are  termed  commensals.  Dependent  and  interrelated  plants  thus  may  be 
divided  into  three  classes:  commensals,  which  are  symbiotic  but  not 
inecessarily  heterotrophic;  saprophytes,  which  are  heterotrophic  but 

1  Saprophytism,  of  course,  is  not  included  under  symbiosis,  since  it  does  not  involve 
a  relationship  between  living  organisms. 

752 


SAPROPHYTISM   AND   SYMBIOSIS  753 

not  symbiotic;   and  parasites,  which  are  both  symbiotic  and  hetero- 
trophic. 

Commensalism.  —  Commensalism  between  plants.  —  The  liverworts, 
Anthoceros  and  Blasia,  and  the  water  fern,  Azolla,  contain  colonies  of 
the  alga,  Nostoc,  in  certain  rather  definite  regions  of  the  plant  body;  it 
is  not  known  that  either  of  the  commensals  derives  any  particular  benefit 
from  the  association,  though  it  may  be  supposed  that  the  alga  has  a 
somewhat  better  protected  and  more  uniformly  moist  habitat  than  when 
it  lives  independently.  Palmetto  trees  are  inhabited  by  certain  charac- 
teristic epiphytes,  probably  because  the  soft  bark  provides  an  easy  place 
for  them  to  become  established;  similarly  certain  trees  have  charac- 
teristic lichens  that  doubtless  are  associated  with  some  physical  or  chemi- 
cal peculiarities  of  the  bark.  Such  associations  may  be  regarded  as 
illustrating  a  sort  of  loose  Commensalism,  and  the  same  may  be  said  of 
the  characteristic  associations  of  microorganisms  which  live  in  the  slimy 
coats  of  water  lily  leaves.  The  relationship  of  a  liana  to  its  support  is 
still  less  intimate,  since  one  tree  serves  as  well  as  another.  There  exists 
an  easy  transition  from  such  a  loose  commensalism  as  that  of  the  water 
lily  leaf  to  a  representative  plant  association  like  a  forest,  where  the 
shade  benefits  the  mosses  underneath  the  trees,  and  where  the  moisture 
conserved  by  the  mosses  benefits  the  trees. 

Myrmecophytes.  —  The  best  examples  of  commensalism  are  found  among  ani- 
mals or  in  such  associations  of  plants  and  animals  as  those  between  plants  and  ants. 
In  the  tropics,  many  ants  establish  domatia  (i.e.  abodes)  in  the  internal  cavities  which 
occur  in  various  plants;  in  Cecropia  the  domatia  consist  of  large  chambers,  and  in 
Myrmecodia  they  consist  of  chambers  connected  by  labyrinthine  passages.  In  Acacia 
sphaerocephala,  hollow  thorns  serve  as  domatia  (fig.  1076),  and  albuminous  food 
bodies  at  the  ends  of  the  leaf  pinnules  are  eaten  by  the  ants  (fig.  1077);  in  other 
plants  nectar  secretions  are  utilized  similarly.  Such  plants  are  known  as  myrmeco- 
phytes  (i.e.  ant  plants.) 1  The  ants  that  live  in  myrmecophytes  have  been  said  to  be 
very  warlike  and  to  defend  their  domatia  with  great  vigor;  such  commensalism  has 
been  regarded  as  mutualistic,  the  plants  which  serve  as  an  abode  and  as  a  source 
of  food  for  these  ants,  supposedly  being  protected  by  them  from  leaf-cutting 
ants.  Recent  studies  afford  this  theory  little  support,  demonstrating  that  the  inter- 
relation is  comparatively  incidental  and  that  the  plants  gain  little  or  nothing  from  the 
symbiosis;  leaf-cutting  ants  are  not  so  destructive  as  has  been  supposed,  and  they 
are  absent  in  many  regions  where  myrmecophytes  are  abundant  (as  in  Malaysia); 

1  The  commensalism  between  plants  and  ants  often  is  known  as  myrmecophily,  and  the 
plants  are  called  myrmecophilous  (i.e.  ant-loving);  such  terms  are  very  objectionable, 
since  they  imply  the  existence  of  emotions  in  plants.  The  term  myrmecophily  is  espe- 
cially objectionable,  because  the  ants  are  of  no  particular  advantage  to  the  plants. 


754 


ECOLOGY 


furthermore,  the  commensalistic  ants  (as  Azteca)  usually  are  without  effective  weap- 
ons of  offense  and  are  not  disposed  to  attack  intruders.  The  domatial  chambers 
develop  quite  independently  of  ant  stimulation. 

A  remarkable  case  of  symbiosis  is  found  among  certain  South  American  leaf- 
cutting  ants.     The  fungus,  Rozites  gongylophdra,  is  said  to  furnish  the  sole  food  of 

certain  ants,  which  cultivate  it  in  their 
"  fungus  gardens."  The  ants  cut  off 
leaves  and  take  them  to  the  "gardens," 
where  they  serve  as  food  for  the  fungi. 
Similarly,  the  termites  or  white  ants 
have  "  fungus  gardens,"  and  even  are 
supposed  to  weed  out  undesirable  fungi. 


Saprophytism.  —  General  con- 
siderations, —  Saprophytes  are 
defined  as  plants  that  obtain  their 
food  from  dead  organic  matter, 
appearing  to  contrast  sharply  on 
the  one  hand  with  autophytes  or 
independent  plants,  and  on  the 
other  hand  with  parasites,  which 
derive  their  food  from  living  or- 
ganisms. However,  careful  study 
has  shown  that  all  gradations 
occur  between  saprophytes  and 
autophytes  and  between  sapro- 
phytes and  parasites,  making  it 
often  a  matter  of  extreme  dim- 


FIGS.  1076,  1077.  —  A  myrmecophyte, 
Acacia  sphaerocepliala:  1076,  a  portion  of  a 
shoot,  showing  part  of  a  doubly  pinnate 
leaf  (/),  whose  pinnae  terminate  in  "food- 
bodies";  note  the  hollow  paired  thorns  (£) 
which  are  punctured  by  ants  that  live  within 
the  thorns;  1077,  a  single  pinna  with  its 
terminal  "food -body"  (/),  somewhat  en- 
larged. 


culty  to  determine  how  certain 
plants  should  be  classified;  indeed,  in  many  cases  a  particular  plant 
may  vary  in  its  nutritive  relations,  belonging  sometimes  to  one  group 
and  sometimes  to  another.  Those  plants  which  obtain  all  their  food 
from  dead  organic  matter  may  be  termed  holosaprophytes,  while  those 
plants  that  are  partially  saprophytic  and  partially  autophytic  may 
be  termed  partial  saprophytes.  The  more  facultative  or  plastic  forms, 
which  may  live  as  autophytes,  as  saprophytes,  or  as  partial  saprophytes, 
may  be  termed  mixophytes. 

Saprophytism  in  the  fungi  and  bacteria.  —  The  most  representative 
holosaprophytes  occur  among  the  fungi  and  bacteria.  Among  the  com- 
mon saprophytic  bacteria  are  the  nitrifying  organisms  of  soil  and  water, 
the  organisms  causing  the  putrefaction  of  meat  and  the  decomposition  of 


SAPROPHYTISM    AND    SYMBIOSIS 


755 


milk,  and  the  bacteria  of  hay  infusions  (figs.  14-17).  Among  the  com- 
moner saprophytic  fungi  are  the  molds  (e.g.  Penicillium  and  Mucor, 
fig.  1078),  the  yeasts  (figs.  168-173),  an(^  most  fleshy  fungi  (figs.  197-198). 
Saprophytic  fungi  and  bacteria  occur  wherever  there  is  dead  organic 
matter,  particularly  in  humus,  the  processes  of  decay  being  associated 
with  these  organisms. 

The  vegetative  body  of  fungi,  the  mycelium,  is  composed  of  delicate 
threads,  the  hyphae  (fig.  1078),  which  penetrate  the  substratum  in  all 
directions,  often  giving  it  a  characteristic  color,  as  in  "white  or  yellow 
dead  wood.  In  Ly coper  don,  the  hyphae  form  rootlike  strands  of  different 


FIG.  1078.  —  A  diagrammatic  representation  of  a  common  mold  (Mucor),  showing 
branching  rhizoid-like  hyphae  which  get  food  from  the  substratum  ;  note  also  the  globular 
sporangia  borne  on  stalks,  the  sporophores;  considerably  magnified. — From  COULTER 
(Parti).  \.^i: 

-sizes.  Hyphae,  like  root  hairs,  are  filamentous  plasmatic  structures  with 
permeable  thin  walls,  but  as  might  be  expected  from  their  heterotrophic 
relations,  they  appear  to  surpass  root  hairs  as  organs  of  absorption,  prob- 
ably because  of  their  greater  power  of  penetration  and  because  of  the 
higher  concentration  of  their  cell  sap.  Some  hyphae  penetrate  dead  bark 
and  wood  mechanically,  chiefly  through  cracks  and  wounds,  while  others 
secrete  enzyms,  which  increase  the  solubility  and  digestibility  of  many 
of  the  materials  in  the  substratum.  Fungi  secrete  organic  acids  much 
more  actively  than  do  root  hairs,  and  thus  are  more  potent  factors  in 
disintegration.  But  it  is  probably  the  concentrated  cell  sap  that  is  chiefly 
responsible  for  the  absorptive  efficiency  of  fungi,  since  this  makes  pos- 
sible not  only  a  wider  range  of  habitats  (as  jelly  and  sirup,  in  which  root 
hairs  cannot  develop),  but  also  the  absorption  of  a  greater  number  of 


756  ECOLOGY 

substances  such  as  various  organic  compounds  (qualitative  efficiency), 
and  the  greater  absorption  of  various  substances,  such  as  water  (quanti- 
tative efficiency).  Perhaps  another  advantageous  feature  in  hyphae  is 
the  fact  that  cross  walls  are  absent  for  long  distances.  Thus  it  seems 
very  probable  that  hyphae  flourish  much  better  than  do  root  hairs,  when 
the  two  develop  in  a  common  medium,  such  as  the  forest  humus;  this 
appears  to  be  a  consideration  of  great  importance  in  connection  with  the 
mycorhiza  plants  (p.  791). 

Saprophytism  in  the  algae. —  Algae  as  a  class  possess  synthetic  pigments 
and  are  autophytic,  these  being  the  chief  characters  that  distinguish 
them  from  fungi;  however,  the  saprophytic  potentiality  of  many  algal 
forms  has  been  established.  Some  plants  that  are  classed  as  algae  are 
habitually  saprophytic  and  colorless,  particularly  some  of  the  Peridineae 
and  diatoms  (as  Nitzschia  putrida} ;  in  Euglena  and  in  various  diatoms, 
the  members  of  a  given  species  sometimes  are  pigmented  and  presum- 
ably autophytic,  and  sometimes  they  are  colorless  and  presumably  sapro- 
phytic. In  one  alga,  supposed  to  be  Carteria  or  a  closely  allied  form, 
there  appears  to  be  nutritive  dimorphism,  approximately  half  of  the  in- 
dividuals in  each  colony  being  pigmented  and  autophytic,  while  half 
are  colorless  and  saprophytic.  A  cave-inhabiting  form  of  Gloeothece 
rupestris,  namely,  the  variety  cavernarum,  habitually  utilizes  organic 
food  and  is  colorless.  Comprehensive  experiments  show  that  many 
species  of  the  lower  algae  may  flourish  in  darkness,  even  when  organic 
acids  (as  acetic  acid)  are  the  only  source  of  carbon.  If  organic  sub- 
stances occur  in  abundance,  many  algae  grow  about  as  well  in  darkness 
as  in  light;  indeed,  there  are  some  forms  that  grow  more  luxuriantly  as 
saprophytes  in  darkness  than  as  autophytes  in  the  light.  The  abundant 
growth  of  algae  in  water  pipes  and  in  drains  doubtless  is  due  to  their 
capacity  for  saprophytism.  Some  algae,  especially  those  occurring  in 
lichens,  almost  equal  the  fungi  in  their  saprophytic  potentiality,  being 
able  to  utilize  such  proteins  as  peptones  for  food. 

In  addition  to  the  colorless  algae  noted  above  there  are  now  known  a 
number  of  forms  which  lose  their  pigment  when  grown  in  media  rich  in 
organic  food  (as  in  strong  glycerine  solutions),  the  colorless  condition 
ensuing  either  in  darkness  or  in  sunlight.  Thus  many  green  algae,  blue- 
green  algae,  and  diatoms  are  representative  mixophytes,  having  on  the 
one  hand  a  capacity  for  independent  existence,  and  on  the  other  a  possi- 
bility of  saprophytism  comparable  to  that  habitually  exhibited  by  fungi. 
Such  phenomena  show  that  the  presence  of  chlorophyll  indicates  rather 


SAPROPHYTISM   AND    SYMBIOSIS  757 

the  possibility  of  complete  autophytism  than  its  actual  existence.  The 
autophytic  bacteria  (p.  526)  also  are  capable  of  living  saprophytically. 
and  hence  are  representative  mixophytes. 

Saprophytism  in  the  seed  plants.  —  The  views  of  botanists  as  to  sapro- 
phytism  in  the  seed  plants  often  have  been  altered,  and  even  now  the 
paucity  of  experimental  data  makes  anything  like  a  definite  statement 
quite  impossible.  The  first  view  was  that  all  plants  without  chlorophyll 
are  parasites.  When  it  was  discovered  that  the  roots  of  Monotropa  (fig. 
1104)  are  not  connected  with  the  roots  of  green  plants,  the  term  sapro- 
phyte was  used  to  distinguish  such  plants  from  true  parasites,  which  always 
are  attached  to  other  plants.  Afterward  it  was  discovered  that  the  roots 
of  Monotropa  usually  are  completely  enveloped  by  fungi,  which  act  in  an 
intermediary  capacity  between  the  seed  plant  and  the  humus,  so  that 
once  more  Monotropa  is,  perhaps,  to  be  regarded  as  parasitic,  though 
on  fungi  rather  than  on  roots  (see  also  p.  792).  Similarly,  all  of  the 
higher  plants  (except  possibly  the  West  Indian  orchid,  Wullschlaegelia) 
that  commonly  have  been  regarded  as  saprophytic,  such  as  Corallorhiza 
and  many  other  orchids,  and  such  as  the  gametophytes  of  Lycopodium 
and  Botrychium  (fig.  1108),  have  symbiotic  relations  with  fungi  and 
probably  are  parasitic  rather  than  saprophytic;  however,  in  such 
orchids  as  Corallorhiza  partial  saprophytism  still  seems  a  possibility, 
especially  since  the  fungi  occur  within  the  root.  The  term  symbiotic 
saprophytism  has  been  applied  to  this  phenomenon  to  express  the 
double  relationship,  namely,  the  symbiotic  relation  between  the  fungus 
and  the  higher  plant,  and  the  saprophytic  relation  between  the  fungus 
and  the  soil  (see  further,  p.  798). 

While  the  supposedly  saprophytic  seed  plants  and  pteridophytes  ap- 
pear, in  reality,  to  be  parasites,  a  capacity  for  saprophytism  has  been 
found  to  exist  in  various  ordinary  green  plants.  Long  ago  it  was  sup- 
posed that  the  luxuriant  development  of  plants  in  humus  is  due  to  the 
absorption  of  organic  food,  but  later  this  view  was  abandoned,  the  luxu- 
riance being  referred  to  various  factors,  such  as  the  high  water  content, 
the  abundance  of  nitrates,  and  the  beneficial  activities  of  earthworms, 
fungi,  and  bacteria.  However,  it  has  been  discovered  that  when  maize 
is  grown  in  glucose  or  in  invert  sugar  with  its  leaves  in  air  which  is  de- 
void of  carbon  dioxid,  sugar  is  absorbed  by  the  root  hairs  and  is  utilized 
directly  in  the  manufacture  of  starch.  In  such  an  experiment  the  maize 
behaves  as  a  holosaprophyte,  and  it  may  be  a  partial  saprophyte  in  ordi- 
nary cultivation,  although  the  roots  appear  to  absorb  very  little  organic 


758  ECOLOGY 

food  when  the  conditions  are  favorable  for  autophytism.  It  has  been 
claimed,  but  scarcely  proven,  that  root  hairs  secrete  enzyms,  as  do  fungi. 
Results  similar  to  those  obtained  in  maize  have  been  reported  in  the  vetch, 
the  cress,  and  the  radish,  and  it  is  very  probable  that  many  if  not  most 
green  plants  have  a  greater  or  less  capacity  for  partial  saprophytism. 

Some  seed  plants  (principally  among  the  Scrophulariaceae)  that  commonly  are 
partial  parasites  (p.  772)  are  said  to  be  saprophytic  in  some  cases;  for  example, 
Melampyrum  develops  haustoria  among  dead  leaves  and  becomes  attached  indiffer- 
ently to  dead  or  to  living  roots.  Even  those  species  which  become  attached  only  to 
living  hosts  continue  to  derive  nourishment  therefrom  after  the  latter  are  dead. 
Lathraea,  which  commonly  is  a  holoparasite  (p.  772),  sometimes  lives  as  a  sapro- 
phyte. It  has  been  claimed  that  certain  mosses  with  few  leaves  (as  Buxbaumia)  are 
partially  saprophytic;  apart  from  the  sparse  foliage,  the  only  argument  for  this 
view  is  the  fact  that  the  rhizoids  penetrate  among  dead  leaves  and  sometimes  have 
knoblike  processes  resembling  haustoria. 

While  much  is  yet  to  be  learned  about  saprophytism  in  the  seed  plants, 
it  certainly  is  an  odd  circumstance  that  the  plants  least  likely  to  exhibit 
saprophytism  are  forms  like  Monotropa,  which  once  were  regarded 
as  holosaprophytes,  while  saprophytism  is  best  evidenced  in  such  plants 
as  maize,  which  has  been  regarded  as  a  representative  autophyte. 
However,  the  nutritive  evolution  of  heterotrophic  seed  plants  as  a  class 
seems  rather  to  have  been  in  the  direction  of  symbiotic  saprophytism 
(p.  798). 

The  origin  of  saprophytism.  —  The  prevalent  conception  of  the  origin 
.of  saprophytism  is  that  from  time  to  time  saprophytic  branches  have 
diverged  at  various  levels  from  the  main  autophytic  trunks.  The  sapro- 
phytic fungi  and  bacteria  do  not  appear  to  form  an  independent  and 
connected  genetic  line,  but  occur  as  disconnected  groups,  some  of  which 
resemble  algae  of  widely  different  families.  For  example,  in  form  and 
structure  and  in  method  of  reproduction  the  bacteria  resemble  the  blue- 
green  algae,  differing  from  them  chiefly  in  the  general  absence  of  food- 
making  pigments  and  in  their  much  smaller  size;  the  close  relationship 
between  these  groups  is  emphasized  further  by  the  great  saprophytic 
capacity  of  the  blue-green  algae,  by  the  pigmentation  of  certain  bacteria 
and  fungi,1  and  by  the  fact  that  various  bacteria  are  able  to  manufacture 
carbohydrates.  Both  among  the  bacteria  and  the  blue-green  algae  there 

1  Notable  among  the  pigmented  forms  are  the  purple  bacteria,  which  contain,  in  addi- 
tion to  the  purple  pigment,  a  green  pigment  that  resembles  chlorophyll.  Manilla  silophila, 
a  fungal  form,  is  colorless  when  grown  in  the  dark,  but  when  grown  in  the  light,  it  ex- 
hibits an  orange  color  due  to  carotin. 


SAPROPHYTISM    AND    SYMBIOSIS  759 

are  unicellular,  filamentous,  and  colony-forming  species.  The  bacterial 
form,  Leuconostoc,  resembles  the  algal  form,  Nostoc  (fig.  8),  both  in  gen- 
eral appearance  and  in  behavior.  Similarly,  the  alga  groups,  Siphonales 
and  Conjugales,  show  many  resemblances  to  the  fungus  groups,  Oomy- 
cetes  and  Zygomycetes,  respectively  (see  Part  I). 

It  is  not  difficult  to  picture  the  probable  stages  in  the  development  of 
holosaprophytism  and  to  imagine  some  of  the  possible  underlying  causes. 
Since  the  lower  algae  are  mixophytes,  saprophytism  does  not  seem  to  in- 
volve the  introduction  of  a  new  character,  but  chiefly  the  elimination  of 
autophytism;  probably  there  develops  also  an  increased  capacity  for  the 
utilization  of  organic  food.  The  experiments  previously  cited  appear  to 
suggest  that  food-making  by  chlorophyll  may  be  diminished  or  even 
checked  in  the  presence  of  an  excess  of  soluble  carbohydrates.  A  sec- 
ond stage  appears  to  be  the  gradual  disappearance  of  chlorophyll,  which 
also  is  associated  with  an  abundant  external  food  supply;  this  stage 
indicates  the  passage  from  partial  saprophytism  to  temporary  holo- 
saprophytism. The  final  stage,  the  elimination  of  the  possibility  of 
chlorophyll  formation  and  hence  of  autophytism,  may  be  occasioned  by 
the  disappearance  of  plastids,  but  the  factors  involved  in  this  are  at 
present  quite  unknown. 

Progressive  variability  in  saprophytes.  —  While  most  saprophytic 
fungi  and  bacteria  can  be  grown  in  various  culture  media,  a  sudden 
transfer  to  an  unaccustomed  medium  results  frequently  in  impaired 
activity  or  in  death.  However,  if  the  change  is  made  gradually  through 
a  series  of  intermediate  solutions,  successful  cultures  may  result.  For 
example,  Bacillus  fluorescens  pulridus  ordinarily  has  its  optimum  growth 
conditions  at  22°  C,  and  can  scarcely  grow  above  35°  C;  however,  if 
media  of  gradually  increasing  temperature  are  used,  a  race  is  developed 
that  grows  vigorously  at  41°  C.  Thermal  bacteria,  with  an  optimum  of 
37°  C,  can  be  treated  similarly  until  they  thrive  at  66°  C.  Likewise,  many 
saprophytes  can  be  grown  in  toxic  media  if  the  poisons  are  increased  grad- 
ually in  successive  cultures;  in  this  way  Penicillium  can  be  grown  ul- 
timately in  concentrated  solutions  of  copper  sulfate.  Many  forms,  as 
Aspergillus  and  Penicillium,  can  be  cultivated  utlimately  in  salt  solu- 
tions that  are  so  highly  concentrated  as  to  produce  plasmolysis  or  death 
if  the  transfer  from  ordinary  media  is  made  suddenly.  Some  forms 
secrete  enzyms  whose  character  differs  with  the  nature  of  the  substratum, 
Penicillium  even  secreting  a  wood-destroying  enzym,  hadromase,  when 
grown  on  wood.  Anaerobic  bacteria  may  be  accustomed  "gradually  to 


760  ECOLOGY 

the  presence  of  oxgyen,  finally  even  thriving  when  it  is  present  in  consider- 
able amount.  Such  phenomena,  as  a  whole,  have  been  called  accom- 
modation, the  implication  being  that  plants  have  the  power  of  adapting 
or  adjusting  themselves  to  new  situations.  One  of  the  most  striking 
features  of  "  accommodation  "  is  the  inability  of  saprophytes  that  have 
been  grown  for  a  long  time  under  unusual  conditions  (e.g.  at  high  tem- 
peratures, at  high  concentrations,  or  in  toxic  media)  to  thrive  when  they 
are  suddenly  transferred  back  to  the  ordinary  media;  in  order  to 
flourish  in  the  latter,  they  must  again  be  grown  in  a  series  of  inter- 
mediate solutions  of  gradually  decreasing  temperature,  concentration, 
or  toxicity. 

While  the  phenomena  of  "  accommodation  "  are  not  well  understood, 
no  explanation  is  given  by  referring  them  to  adaptation  or  adjustment 
(see  p.  947).  If  a  fungus  is  transferred  to  a  salt  solution  of  slightly 
higher  concentration,  the  solute  enters  the  plant  and  water  passes  out, 
tending  to  establish  an  equilibrium.  If  a  series  of  like  transfers  is  made 
to  media  of  progressively  greater  concentration,  similar  processes  take 
place,  until  finally  the  concentration  within  the  fungus  becomes  so  great 
that  it  can  thrive  in  a  medium  which  would  have  brought  about  plas- 
molysis  through  sudden  water  withdrawal,  if  it  had  been  placed  therein 
at  the  outset.  Similarly,  when  a  transfer  is  made  suddenly  from  a  strong 
to  a  weak  solution,  water  may  enter  so  rapidly  as  to  burst  the  fungus, 
although  a  series  of  transfers  to  progressively  weaker  solutions  makes 
possible  the  gradual  entry  of  water  and  the  gradual  exit  of  salts.  Per- 
haps the  behavior  of  saprophytes  in  solutions  that  are  subjected  to  pro- 
gressive changes  in  temperature  or  in  toxicity  also  may  be  capable  of 
a  comparable  explanation.  In  any  event,  it  seems  advisable  to  substi- 
tute the  term  progressive  variability  for  accommodation  and  similar 
vitalistic  expressions. 

Phosphorescence.  —  A  few  plants,  chiefly  saprophytes,  exhibit  phosphorescence, 
the  mycelium  of  Agaricus  melleus  and  of  a  few  other  fleshy  fungi  often  being  lumi- 
nous in  the  dark,  especially  in  wet  weather,  if  there  is  an  abundant  supply  of  oxygen. 
Certain  bacteria  associated  with  meat  decay  also  are  phosphorescent.  The  phos- 
phorescence of  decaying  wood  probably  is  due  to  fungi  or  to  bacteria.  Luminosity 
is  thought  to  be  of  no  value  to  plants,  although  it  is  of  possible  use  to  certain  animals. 

The  distribution  of  saprophytes.  —  While  light  is  a  large  factor  in 
determining  the  distribution  of  green  plants,  it  plays  almost  no  part  in 
the  holosaprophytes,  whose  abundance  is  determined  chiefly  by  the 
amount  of  dead  organic  matter  and  by  the  degree  of  freedom  from 


SAPROPHYTISM   AND   SYMBIOSIS  761 

transpiration.  Probably  saprophytes  culminate  in  mesophytic  woods, 
where  soil  bacteria  are  abundant,  and  where  numerous  saprophytic 
fungi  permeate  the  soil  and  the  rotting  trunks.  There  is  also  a  char- 
acteristic saprophytic  flora  in  the  humus  underlying  ponds  and  swamps, 
and  even  in  the  dark  abysses  of  the  ocean.  Both  in  the  forest  and  in 
the  ocean  depths  the  saprophytes  play  a  part  of  great  import  to  all 
organic  life;  as  the  organisms  of  decay  and  as  agents  of  nitrification 
they  make  much  material  available  for  other  organisms,  and  their  sym- 
biotic relations  with  green  plants  also  are  most  important. 

Probably  saprophytic  fungi  display  their  greatest  luxuriance  in  old 
mines,  where  the  decaying  timbers  furnish  a  rich  supply  of  food.  The 
mycelia  spread  abundantly  over  the  surface  of  the  wood  as  well  as  within, 
and  even  far  out  on  the  adjoining  rocks,  the  hyphal  strands  remaining 
connected  with  the  source  of  food.  Undoubtedly  the  uniformly  high 
humidity  is  the  chief  factor  in  determining  this  astonishing  luxuriance; 
that  this  factor  is  not  darkness  is  shown  by  the  fact  that  similar  rich 
cultures  may  be  obtained  in  the  light  on  bread  or  on  cheese  enclosed 
in  moist  chambers.  In  caves,  where  there  are  no  timbers,  saprophytes 
often  luxuriate,  the  necessary  organic  matter  being  supplied  by  streams 
entering  from  the  outer  world,  by  the  excrements  of  cave  animals  (as 
the  blindfish),  and  of  cave-frequenting  animals  (such  as  bats),  and  even 
by  tallow  dropped  from  the  candles  of  human  visitors.  Were  it  not  for 
the  saprophytes,  ocean  depths  and  caves  would  be  essentially  devoid  of 
plant  life. 

2.    PARASITISM 

General  considerations.  —  Definitions.  —  Parasites  are  plants  or 
animals  that  derive  foods  or  food  materials  from  other  plants  or  animals 
to  which  they  are  attached.  A  holoparasite  is  one  that  is  entirely  de- 
pendent for  its  food  upon  the  organism  to  which  it  is  attached,  com- 
mon illustrations  being  parasitic  fungi,  broom  rapes,  and  parasitic  ani- 
mals. A  partial  parasite  is  one  that  is  only  partially  dependent  upon 
the  organism  to  which  it  is  attached,  being  capable  of  manufacturing  a 
part  or  even  all  of  its  food;  in  the  former  case  it  derives  food  from  its 
host,  in  the  latter  case  only  food  materials.  Representative  partial 
parasites  are  the  mistletoe  and  many  of  the  Scrophulariaceae. 

Inter  gradations.  —  All  possible  gradations  connect  partial  parasites 
with  holoparasites;  the  former  also  grade  similarly  into  autophytes  and 
partial  saprophytes,  and  the  latter  into  holosaprophytes.  The  mistle- 


762  ECOLOGY 

toe,  like  the  autophytes,  probably  makes  its  own  carbohydrates,  differ- 
ing from  them  chiefly  in  taking  its  water  and  salts  from  a  host  plant 
instead  of  from  the  soil;  such  a  plant  is  termed  a  water  parasite.  Color- 
less parasites  take  organic  foods  from  their  host  plants;  even  among 
these  there  probably  are  degrees  of  parasitism,  some  manufacturing 
proteins,  while  others  absorb  them  along  with  carbohydrates.  Grada- 
tions between  autophytes  and  partial  parasites  are  best  illustrated  by 
the  Scrophulariaceae,  and  are  described  below.  There  are  many  fungi 
and  bacteria  that  can  exist  either  as  holosaprophytes  or  as  holoparasites; 
such  forms  are  called  facultative  saprophytes  or  facultative  parasites,  in 
contrast  with  obligate  saprophytes  (as  most  molds)  and  obligate  para- 
sites (as  the  more  familiar  phases  of  rusts  and  smuts). 

Various  bacteria  that  ordinarily  are  saprophytic  become  parasitic  when  several 
successive  generations  are  grown  on  carrots  or  turnips.  The  fungus,  Empusa, 
infects  living  flies,  causing  their  death,  but  it  grows  with  undiminished  vigor  after 
the  death  of  the  host.  Similarly,  a  species  of  Saprolegnia  causes  the  death  of  fishes, 
continuing  to  live  afterward  as  a  saprophyte.  Various  parasitic  bacteria  are  ca- 
pable of  saprophytic  existence,  and  a  number  of  the  fungi  causing  plant  cankers, 
stem  rots,  and  leaf  spot  diseases  also  may  live  as  saprophytes  ;  some  of  the  latter  are 
regularly  saprophylic  in  their  later  stages.  The  common  saprophytic  mold,  Mucor 
Mucedo,  causes  a  destructive  rot  in  sweet  potatoes.  Similarly,  no  sharp  lines  of 
demarcation  separate  saprophytic  and  parasitic  timber  fungi.  Most  smuts  and 
rusts  have  alternating  stages  of  parasitism  and  saprophytism.  Even  among  the 
seed  plants  there  are  a  few  forms  which  usually  are  parasitic,  but  which  seem  ca- 
pable of  partial  saprophytism,  notably  Lathraea  and  Melampyrum,  the  latter  also 
having  a  capacity  for  autophytism.  In  many  cases  parasites,  while  attached  to 
living  plants,  do  not  come  into  contact  with  living  cells,  secretions  from  the  invad- 
ing fungus  killing  the  cells  before  any  of  the  contents  are  absorbed.  In  water 
parasitism  (e.g.  in  the  mistletoe)  the  dead  hadrome  of  the  parasite  is  in  contact  with 
the  dead  hadrome  of  the  host.  Yet  the  latter  instances  properly  belong  to  the 
phenomena  of  parasitism,  since  the  host  plants  are  injured  and  are  deprived  of 
materials  which  otherwise  they  might  have  used. 

Parasitic  fungi  and  bacteria.  —  General  characteristics.  —  The  best- 
known  parasitic  bacteria  are  the  pathogenic  forms,  which  occasion  many 
diseases  in  man  and  in  other  animals,  and  also  in  plants  (for  example, 
the  organisms  causing  tuberculosis,  cholera,  and  typhoid  fever;  figs. 
13,  18,  19).  These  forms  generally  are  less  differentiated  than  are  the 
saprophytic  bacteria,  perhaps  giving  an  illustration  of  reduction  from 
a  more  specialized  ancestry.  Some  bacteria  that  apparently  are  para- 
sitic are  not  pathogenic,  such  as  Leptothrix  and  Sarcina,  which  live 
respectively  in  the  mouth  and  the  stomach.  Bacillus  radicicola,  the 


SAPROPHYTISM    AND    SYMBIOSIS 


763 


species  that  forms  galls  on  legume  roots  (p.  787),  also  lives  saprophyti- 
cally  in  the  soil,  and  thus  is  a  facultative  form. 

The  mildews,  rusts,  and  smuts  are  representative  parasitic  fungi, 
most  of  which  are  deleterious  to  their  host  plants  (fig.  180),  some  species 
producing  conspicuous  galls  in  various  organs.  Some  of  the  Poly- 
poraceae  (as  the  bracket  fungi)  are  harmful  parasites  on  trees.  The 


1079 


hyphae  of  parasitic  fungi 
are  thought  to  be  more 
specialized  than  are  those 
of  saprophytic  forms,  hav- 
ing greater  power  of  pene- 
tration into  woody  and 
mechanical  tissues,  and  it 
is  likely  also  that  their 
absorptive  efficiency  is 
greater.  The  penetrative 
power  of  the  hyphae  is 
due  in  large  part  to  the 
substances  which  they  se- 
crete, particularly  various 
enzyms,  such  as  the  wood- 
destroying  enzyms  of  the 
tree-inhabiting  fungi. 
Wounds  of  various  kinds 
greatly  facilitate  the  inva- 

J  calotneca   from   which  there  originate  richly  brancn- 

sion    of    plant     organs     by  ,-ng  haustoria  that  penetrate  all  parts  of  the  paren- 

parasitCS.      While  ordinary  chyma  cells  of  the  host  plant,  Asperula  odorata;   both 

hyphae    often    invade    the  figures  highly  magnified.  -  From  DEBARY. 

living  host  cells,  in  many  cases  they  creep  along  the  outer  surface  (as  in 
various  mildews)  or  penetrate  between  the  cells,  special  branches  known 
as  haustoria  piercing  the  walls  (especially  through  the  pits)  and  absorbing 
the  contents  of  the  lumina;  surface  forms  are  known  as  ectoparasites, 
while  internal  forms  are  termed  endo parasites.  In  Albugo  the  haustoria 
are  knoblike  processes  (figs.  1079,  158)  and  in  Peronospora  the  hausto- 
rial  branch  may  divide  into  a  number  of  finger-like  absorptive  organs 
(fig.  1080).  The  spores  of  most  parasitic  fungi,  even  of  such  obligate 
forms  as  the  rusts,  germinate  somewhat  readily  in  a  number  of  media, 
thus  differing  from  the  seeds  of  the  higher  holoparasites;  oddly  enough, 
the  spores  of  many  saprophytic  fungi  germinate  much  less  readily. 


FIGS.  1079,  1080.  —  Haustoria  of  parasitic  fungi: 
1079,  a  hyphal  filament  of  Albugo  Candida  from  which 
there  originate  small  spherical  haustoria  that  pene- 
trate the  parenchyma  cells  of  the  host  plant,  Lepi- 


764  ECOLOGY 

Heteroecious  and  autoecious  parasites.  —  If  a  parasitic  fungus  has  but  a  single 
host  in  its  life  history,  it  is  called  autoecious,  while  if  different  stages  occur  habitually 
on  different  hosts,  it  is  called  heteroecious.  In  the  smuts  there  is  an  alternation  of 
saprophytic  and  parasitic  phases  (see  p.  81),  but  in  the  heteroecious  rusts,  in  addi- 
tion to  a  similar  alternation,  the  parasitic  phase  is  partly  on  one  host  and  partly  on 
another.  Representative  heteroecism  is  illustrated  by  the  wheat  rust,  Puccinia 
graminis,  whose  saprophytic  phase  occurs  in  spring,  when  the  resting  spores,  known 
as  teleutospores  (fig.  1127),  germinate  on  the  ground,  and  give  rise  to  a  mycelium, 
which  produces  basidiospores  (fig.  194).  If  any  basidiospores,  scattered  by  the 
wind,  chance  to  fall  on  a  barberry  leaf,  a  parasitic  mycelium  is  developed,  which 
in  turn  produces  spores  known  as  aecidios pores  (fig.  196).  If  these  spores  are  carried 
by  wind  to  wheat  leaves,  they  develop  into  a  mycelium  which  in  summer  gives  rise 
to  reddish  spores,  the  uredospores  (fig.  1125),  which  also  germinate  on  wheat  leaves. 
Later  in  the  season  the  same  mycelium  gives  rise  to  black,  thick-walled  teleutospores 
(figs.  1126,  1127),  which  fall  to  the  ground  and  remain  dormant  until  spring.  An- 
other well-known  heteroecious  fungus  is  Gymnosporangium,  whose  aecidial  stage 
occurs  on  Pyrus  or  on  other  Rosaceae,  while  the  teleutospore  stage  occurs  on  Ju- 
niperus,  causing  the  galls  known  as  cedar  apples. 

One  of  the  most  remarkable  features  of  the  heteroecious  rusts  is  their  diverse 
behavior.  In  some  species  of  rusts  certain  stages  are  habitually  lacking;  for  ex- 
ample, the  uredo  stage  may  be  absent,  and  sometimes  the  teleuto  stage  also,  the  life 
history  then  consisting  solely  of  aecidial  and  saprophytic  stages.  Again,  the  aecidial 
stage  may  be  absent,  as  in  Puccinia  Malvacearum,  resulting  in  an  alternation  of 
the  saprophytic  and  the  uredo-teleuto  stages  ;  or  both  the  aecidial  and  uredo  stages 
may  be  wanting,  the  basidiospores  giving  rise  directly  to  a  teleutospore  mycelium. 
Moreover,  there  are  strictly  autoecious  forms,  such  as  Puccinia  Asparagi,  in  which 
the  aecidial  stages  and  the  uredo  stages  develop  from  the  same  mycelium.  Still 
more  interesting  are  the  variations  within  a  single  species.  The  wheat  rust,  for 
example,  thrives  in  regions  where  the  barberry  does  not  exist,  partly  because  the 
uredo  mycelium  even  in  very  cold  climates  is  able  to  hibernate  in  winter  wheat,  thus 
eliminating  the  necessity  of  either  saprophytic  or  aecidial  stages,  and  partly  because 
uredospores  that  have  survived  the  winter  may  infect  young  wheat  the  following 
spring;  even  the  teleutospores  or  basidiospores  may  directly  infect  young  wheat 
in  certain  instances.  In  tropical  climates  Uromyces  Fabae,  a  parasite  of  Vicia 
Faba,  spreads  solely  by  uredospores.  Thus  heteroecism  is  seen  to  be  in  part  facul- 
tative; it  is  obligate,  however,  in  certain  cases,  as  in  Coleosporium  Melampyri, 
whose  host  is  an  annual. 

Sometimes  it  has  been  thought  that  the  ease  with  which  rusts  make  new  infec- 
tions cannot  be  explained  merely  by  their  remarkable  variability.  Consequently 
it  has  been  suggested  that  under  certain  conditions  the  protoplasm  of  the  fungus 
and  the  host  may  merge  into  a  common  mycoplasm,  and  in  this  invisible  or  imper- 
fectly evident  form,  the  fungus  is  thought  to  exist  in  the  seed.  When  the  seed 
germinates,  the  fungus  is  supposed  once  more  to  become  differentiated  into  an 
obvious  mycelium.  This  strange  theory  of  the  mycoplasm  was  formulated  to 
account  for  cases  of  parasitism,  in  which  all  evident  external  sources  of  infection 
are  lacking.  Such  a  theory,  appearing  to  defy  the  possibility  of  experimental 
analysis,  like  the  theories  of  vitalism  and  of  adaptation,  should  be  resorted  to,  if 


SAPROPHYTISM   AND    SYMBIOSIS  765 

at  all,  only  after  all  other  tenable  hypotheses  have  been  overthrown.  Infection 
through  seeds  is  much  more  likely  to  result  from  spores  intimately  associated  with 
them  than  from  mycoplasm;  indeed,  mycelia  with  uredospores  and  teleutospores 
have  been  observed  just  beneath  the  bran  layer  in  wheat  grains  that  were  produced 
by  plants  badly  infected  by  rust,  and  nests  of  hyphae  have  been  seen  even  in 
the  embryo. 

Physiological  species  and  progressive  variability.  —  Some  species  of 
parasitic  fungi  (e.g.  Botrytis  cinerea)  may  live  on  two  or  more  hosts, 
and  hence  are  known  as  plurivores,  while  those  that  are  confined  to  a 
single  host  plant  are  termed  univores;  a  familiar  instance  of  the  latter 
is  seen  in  the  corn  smut,  Ustilago  Maydis.  In  many  rusts,  in  some 
mildews,  and  in  the  ergot,  species  that  appear  plurivorous  have,  in  a 
sense,  been  found  to  be  univorous.  For  example,  Puccinia  graminis 
is  in  reality  a  complex  of  forms  morphologically  alike  but  physiologically 
dissimilar;  common  forms  referred  to  this  species  occur  on  rye,  oats, 
and  various  other  grasses,  as  well  as  on  wheat.  However,  uredospores 
from  the  oat  rust  will  not  infect  rye,  nor  vice  versa,  though  their  aecidial 
stages,  as  well  as  that  of  the  wheat  rust  proper,  occur  on  the  barberry. 
Even  aecidiospores  which  have  developed  from  an  ancestry  that  has 
grown  on  oats  will  not  grow  on  rye,  nor  vice  versa.  Forms  that  thus  are 
alike  morphologically  but  unlike  physiologically  are  known  as  physi- 
ological species,  and  the  phenomenon  is  called  specialization.  The 
true  wheat  rust  (Puccinia  graminis  Tritici)  is  much  more  generalized, 
growing  readily  on  barley,  oats,  and  rye,  as  well  as  on  wheat.  As  a 
rule  the  uredo  and  teleuto  stages  of  a  rust  are  much  more  specialized 
than  are  the  aecidial  stages. 

The  common  view  regarding  physiological  species  is  that  the  generalized  forms 
are  the  more  ancestral;  for  example,  the  specialized  oat  rust  (Puccinia  graminis 
Avenae)  and  the  rye  rust  (P.  g.  Secalis)  are  supposed  to  have  been  derived  from 
the  more  generalized  wheat  rust  (P.  g.  Tritici}  by  progressive  variability  on  a 
special  host.  There  is  experimental  evidence  for  this  view.  For  example,  Puccinia 
Smilacearum  Digraphidis  is  a  plurivorous  species,  whose  uredo  and  teleuto  stages 
occur  on  Phalaris  arundinacea,  while  the  aecidial  stage  occurs  indifferently  on 
various  Liliaceae;  however,  after  the  aecidial  stages  were  cultivated  for  ten  years 
solely  on  Polygonatum  multiflorum,  a  univorous  form  was  produced. 

It  is  possible  to  induce  variations  leading  to  greater  generalization  as  well  as  to 
greater  specialization.  Comparable  to  the  rusts  in  many  respects  is  Erysiphe 
graminis,  one  of  the  mildews  which  has  many  physiological  species.  While  the 
spores  from  one  physiological  species  ordinarily  do  not  infect  the  host  of  another, 
they  may  do  so,  if  the  host  is  wounded;  for  example,  rye,  which  usually  is  immune 
to  wheat  mildew,  is  susceptible  if  cut  or  bruised.  Much  more  significant  is  the  role 
of  the  bridging  hosts.  Spores  from  that  form  of  Erysiphe  graminis  that  is  parasitic 


766  ECOLOGY 

on  Bromus  commutatus  do  not  directly  infect  B.  mollis,  but  if  B.  hordeaceus  is  used 
as  a  bridge,  that  is,  if  spores  from  the  parasite  of  B.  commutatus  are  used  to  infect 
B.  hordeaceus,  the  spores  developing  on  the  latter  readily  infect  B.  mollis,  which 
otherwise  is  immune  to  this  particular  parasite.  Similarly,  by  the  use  of  certain 
hosts  as  bridges,  the  ordinary  host  range  of  various  forms  of  Puccinia  graminis  may 
be  extended;  for  example,  barley  serves  as  a  bridge  from  oats  to  rye  or  to  wheat,  or 
from  rye  or  wheat  to  oats.  While  Erysiphe  graminis  is  made  up  of  highly  specialized 
races,  a  related  species,  Erysiphe  Cichoracearum,  is  a  generalized  form;  spores  of 
this  species  that  develop  on  one  of  the  Cucurbitaceae  readily  infect  not  only  other 
Cucurbitaceae,  but  even  plantains  and  sunflowers.  Interesting  tendencies  away 
from  specialization  are  furnished  also  by  certain  parasites  that  have  attacked  new 
hosts,  when  one  or  the  other  is  introduced  from  foreign  countries.  For  example, 
Puccinia  Malvacearum  which  was  introduced  from  Chile  to  Europe  with  a  culti- 
vated Althaea,  has  spread  to  species  of  Malva.  The  aecidial  stage  of  Cronartium 
ribicola,  a  parasite  of  the  European  Pinus  Cembra,  has  become  a  destructive  para- 
site on  P.  Strobus,  which  was  introduced  into  Europe  from  America.  One  of  the 
most  remarkable  of  plurivorous  rusts,  in  view  of  the  generally  close  relationship 
of  the  hosts  of  any  given  parasite,  is  Cronartium  asclepiadeum,  a  common  parasite 
on  Vincetoxicum,  one  of  the  Asclepiadaceae,  which  is  equally  at  home  on  the  peony, 
one  of  the  Ranunculaceae;  recently  it  has  become  parasitic  also  on  Nemesia  versi- 
color,  one  of  the  Scrophulariaceae;  the  systematic  relationship  of  host  plants  is  of 
significance  to  parasites  only  as  it  happens  to  involve  similarity  in  the  physical  or 
chemical  character  of  the  substratum. 

The  factors  involved  in  progressive  variability  among  the  parasitic  fungi  are 
quite  unknown,  and  doubtless  are  much  more  complicated  than  among  saprophytes, 
where  the  culture  media  may  be  relatively  simple,  and  capable  of  analysis.  How- 
ever, it  is  to  be  believed  that  the  explanation  will  be  along  similar  lines,  and  that  it 
need  not  involve  the  mysticism  which  is  inherent  in  the  accommodation  theory. 

The  origin  of  parasitism  in  fungi  and  bacteria.  —  Parasites  appear 
to  be  more  highly  specialized  than  saprophytes,  and  most  parasitic  fungi 
and  bacteria  probably  have  arisen  from  algae  by  way  of  saprophytism; 
among  the  thallophytes  there  are  many  transitions  between  mixophytes 
and  holosaprophytes,  and  between  the  latter  and  holoparasites,  but  few, 
if  any,  between  mixophytes  and  holoparasites.1  The  probable  stages  in 
the  origin  of  saprophytism  have  been  mentioned.  The  next  step, 
namely,  the  development  of  facultative  parasitism,  seems  to  follow 
easily,  and  is  illustrated  by  many  existing  species  (p.  762).  The  succeed- 

1  There  are  a  few  parasitic  algae  that  may  have  developed  directly  from  mixophytes 
into  parasites  rather  than  by  way  of  saprophytism.  One  of  the  most  interesting  of  these 
parasitic  algae  is  the  destructive  tea  rust,  Cephaleuros  virescens.  Another  parasitic 
alga  is  Rhodochytrium  Spilanthidis,  which  grows  on  Ambrosia  and  on  other  hosts.  There 
are  also  a  number  of  marine  red  algae  that  are  parasitic;  among  these  is  Choreocolax 
Polysiphoniae,  which  is  completely  colorless  and  has  plasmatic  connections  with  its 
algal  host. 


SAPROPHYTISM   AND    SYMBIOSIS  767 

ing  stage  probably  is  that  of  obligate  parasitism,  involving  the  loss  of  a 
capacity  for  saprophytism.  Doubtless  there  are  degrees  of  parasitism 
even  among  the  obligate  parasites,  since  some  species  (as  Pythium  De 
Baryanum)  soon  cause  the  death  of  the  host  and  hence  their  own  death. 
The  rusts,  which  rarely  kill  their  host  plants,  appear  on  this  account  to 
exhibit  a  higher  stage  of  parasitic  evolution.  Another  possible  stage, 
still  further  removed  from  autophytism,  is  illustrated  in  the  univorous 
habit;  it  has  been  noted  that  univores  may  arise  from  plurivores  through 
progressive  variability,  and  that  a  return  to  a  plurivorous  condition  is 
equally  possible. 

Probably  the  most  complicated  situation  of  all  is  afforded  by  heteroecism,  and 
no  species  has  been  so  much  discussed  in  this  connection  as  the  wheat  rust.  This 
species  may  once  have  had  all  stages  both  on  the  barberry  and  on  wheat;  if  the 
aecidial  stage  developed  poorly  on  the  wheat  and  the  uredo  stage  poorly  on  the 
barberry,  each  ultimately  may  have  become  eliminated  on  those  hosts,  resulting 
in  the  present  heteroecious  state.  Another  conception  regarding  the  wheat  rust  is 
that  the  barberry  was  the  original  host,  and  that  by  mutation  or  otherwise  the  uredo 
and  teleuto  stages  came  to  develop  on  various  grasses;  this  view  is  favored  by  the 
greater  generalization  of  the  aecidial  stage.  A  possible  mode  of  evolution  of  physi- 
ological species  is  suggested  by  Puccinia  Hieracii,  a  rust  that  infects  a  great  many 
species  of  Hieracium.  It  appears  that  each  infected  species  of  the  latter  is  char- 
acterized by  a  physiological  species  of  the  former.  There  is  evidence  that  the 
evolution  of  the  species  of  Hieracium  is  very  recent,  and  it  is  believed  that  the 
physiological  species  of  the  rust  have  developed  with  the  species  of  the  host. 

Chemotropism  is  thought  to  have  some  connection  with  the  origin 
of  parasitism.  All  fungi  are  prochemotropic  with  reference  to  certain 
substances,  and  apochemotropic  with  reference  to  others.  For  example, 
most  saprophytes  and  facultative  parasites  grow  toward  saccharose. 
The  notably  plurivorous  parasite,  Botrytis  cinerea,  though  prochemo- 
tropic with  reference  to  saccharose,  does  not  penetrate  apples,  because 
it  is  apochemotropic  with  reference  to  malic  acid.  The  fact  that  para- 
sitic hyphae  grow  toward  decoctions  of  their  host  plants  suggests  the 
likelihood  of  chemotropic  relations.  Penicillium,  a  representative 
saprophyte,  grows  in  living  tissues  if  the  latter  are  injected  with  sub- 
stances to  which  Penicillium  reacts  prochemotropically.  Trichothe- 
cium,  another  saprophyte,  when  grown  for  twelve  or  fifteen  generations 
on  begonias  that  have  been  injected  with  sugar,  is  able  thenceforth  to 
continue  as  a  parasite  without  such  injection  and  to  bear  fruit  in  the 
usual  manner.  Similarly,  certain  parasites  may  be  made  to  infect  new 
hosts,  if  the  latter  are  thus  injected. 


768 


ECOLOGY 


The  chemotropic  theory  of  parasitism  would  regard  obligate  parasites  as  forms 
that  react  only  to  substances  that  are  found  in  living  plants,  and  univorous  parasites 
as  forms  that  react  only  to  substances  that  are  found  in  a  single  host  species.  This 
theory  is  not  universally  accepted,  but  no  more  tenable  hypothesis  has  been  sug- 
gested. In  all  theories  of  parasitism,  especially  in  those  that  attempt  to  account 
for  the  phenomena  of  heteroecism  and  specialization,  it  is  difficult  to  explain  the 
fixation  of  new  habits,  inasmuch  as  there  is  no  evidence  that  variability  is  ever  lost. 
Hence  appeal  often  is  made  to  mutation  as  the  decisive  element  in  determining  the 
origin  of  the  forms. 

Immunity  and  related  phenomena.  —  Most  cultivated  plants  are  sub- 
ject to  bacterial  or  fungal  diseases.  In  many  species  certain  varieties 
are  particularly  subject  or  predisposed  to  infection,  whereas  other 
varieties  are  disease-resistant  or  immune.  Few  phenomena  are 

more  puzzling  than  are  those 
of  immunity;  in  many  cases 
it  may  be  attributed  to  the  ab- 
sence of  substances  that  induce 
prochemotropic  reactions,  or 
to  the  presence  of  substances 
which  either  induce  apochemo- 
tropic  reactions  or  neutralize 
the  enzyms  secreted  by  the 
fungus.  In  addition  to  factors 
of  a  chemotropic  nature,  im- 
munity has  been  referred  to 
the  presence  of  an  impene- 
trable epidermis,  to  the  se- 
cretion of  antitoxins,  and  to 
the  absence  of  the  proper 
food  for  the  parasite.  Some 
forms,  as  Penicillium  italicum 
and  Pseudomonas  destmctans, 
secrete  toxins  poisonous  to 
themselves,  thus  making  the 
host  plant  immune  through 
their  own  activity.  Immunity 
FIG.  1081.  —  A  portion  of  a  shoot  of  a  dodder  of  this  character  is  experi- 
(Cuscuta  Gronovii),  a  sinistrorse  twining  para-  enced  commonly  by  many 
site;  note  the  haustoria  (h)  which  penetrate  the  .  .  ,  , 

tissues  of  the  host  plant  (»);    the  leaves  (,)  are      P^ntS  and  animals  that  have 

minute  scales ;/,  flowers.  been  subject   to  certain  bac- 


SAPROPHYTISM   AND    SYMBIOSIS 


terial  diseases.  In  some  cases  "  disease- resistance  "  involves  very 
complicated  phenomena;  for  example,  when  the  olive  is  infected  by 
Bacterium  Oleae,  there  are  developed  in  the  host  plant,  layers  of  scleren- 
chyma  and  cork  that  isolate  the  bacteria,  and  antibodies  are  formed 
which  are  specifically  toxic  to  the  bacteria.  In  the  lack  of  all  obvious 
determining  factors,  appeal  sometimes  has  been  made  to  inherent 
hidden  differences  between  the  various  members  of  a  given  species,  but 
this  "  explanation  "  fails  to  explain.  In  any  case,  immunity  is  not  always 
or  even  usually  due  to 
the  failure  of  spores  to 
germinate,  since  para- 
sitic fungi  often  are 
found  in  various  early 
stages  of  development 
on  plants  which  are 
immune  to  effective 
parasitism. 

Parasitic  seed  plants. 
—  General  features.  — 
Parasitic  seed  plants, 
while  relatively  few  in 
number,  are  of  remark- 
able interest.  A  familiar 
holoparasite  is  the  dod- 
der (Cuscuta) ,  whose 
yellowish  stem  twines 
closely  about  that  of  the 
host  plant  (fig.  1081), 
into  which  many  haus- 
toria  penetrate  (fig.  1082).  Among  the  important  holoparasites  are  the 
Orobanchaceae,  a  family  of  root  parasites,  one  of  the  most  interesting 
of  which  is  Orobanche  fasciculata,  a  frequent  parasite  on  Artemisia 
(fig.  1083) ;  in  this  species  there  is  but  a  single  point  of  attachment,  which 
may  be  on  a  small  lateral  root  at  some  distance  from  the  main  stem  axis 
of  the  host.  Perhaps  the  extreme  of  holoparasitism  is  found  in  the 
tropical  families,  Rafflesiaceae  and  Balanophoraceae,  in  many  of  which 
the  vegetative  body  of  the  parasite  is  entirely  within  the  host,  so  that  the 
plant  is  in  evidence  only  when  in  blossom;  the  largest  known  flower, 
which  sometimes  is  a  meter  in  diameter,  belongs  to  one  of  these,  Raffle- 


FIG.  1082.  —  A  longitudinal  section  through  two 
haustoria  of  a  dodder  (Cuscuta  Gronovii)  and  a  cross 
section  of  the  stem  of  the  host  plant  which  they  are 
penetrating;  note  that  the  vascular  tract  (v)  of  the  para- 
site with  its  hadrome  comes  into  contact  with  the  same 
region  (vf)  in  the  host;  p,  central  pith  of  the  host;  c, 
cortex  of  the  parasite ;  cf,  cortex  of  the  host ;  et  epidermis 
of  the  parasite;  e',  epidermis  of  the  host;  r,  cambium  of 
the  host;  highly  magnified. 


770 


ECOLOGY 


sia  Arnoldii.  Among  the  commoner  partial  parasites  are  various 
mistletoes  (as  Viscum  and  Phoradendron,  fig.  1084)  and  a  large  number 
of  the  Scrophulariaceae,  especially  in  the  tribe  Euphrasieae  (as  Melam- 
pyrum,  Rhinanthus,  and  Euphrasia) ;  all  such  plants  differ  from  the 
holoparasites  in  containing  chlorophyll. 

Holoparasitic  seed  plants  differ  from  their  autophytic  relatives  not 
only  in  the  presence  of  haustoria  and  in  the  absence  of  chlorophyll,  but 


FIG.  1083.  —  Individuals  of  a  holoparasite,  Orobanche  fasciculata,  attached  to  the  roots 
of  an  Artemisia;  note  the  stages  between  young  seedlings  which  appear  like  root  swellings 
and  adult  plants  with  opening  flowers ;  note  also  that  the  parasite  is  attached  to  the  host 
at  but  a  single  point;  Gary,  Ind.  —  Photograph  by  FULLER. 

also  in  the  relative  absence  of  leaves,  whose  place  is  taken  by  insig- 
nificant and  probably  functionless  scales.  As  among  fungi,  the  higher 
holoparasites  may  be  plurivores  (e.g.  Cuscuta  Gronovii)  or  univores; 
examples  of  the  latter  are  the  flax  dodder  (Cuscuta  Epilinum)  and 
Orobanche  Hederae,  a  parasite  on  the  English  ivy.  The  European 
mistletoe,  Viscum  album,  usually  is  regarded  as  a  plurivore,  but  the 
presence  of  physiological  species  is  suggested  by  the  fact  that  the  fir 
mistletoe  does  not  grow  on  the  pine,  nor  vice  versa;  the  form  that  in- 


SAPROPHYTISM   AND    SYMBIOSIS 


771 


fests  deciduous  trees  also  appears  to  be  distinct  from  the  forms  that 
parasitize  the  conifers.  Even  plurivorous  species  of  Cuscuta  grow  very 
differently  on  different  hosts,  the  number  of  forms  on  which  they  flower 
and  fruit  being  much  less  than  the  number  on  which  they  can  develop 
vegetative  organs. 

Haustorial  structures.  —  The  haustoria  of  parasitic  seed  plants  are 
much  more  complex  than  are  those  of  the  fungi,  involving  various 
elements  in  the  cortical  and  vascular  tissues.  The  simplest  haustoria 
occur  in  such  partial  parasites  as  the  Euphrasieae,  where  contact  with 
the  host  often  seems  more  or  less  casual.  In  the  mistletoe,  also  a  partial 


FIG.  1084.  —  The  mistletoe,  a  partial  (water)  parasite:  A,  spiny  honey-locust  trees 
(Gleditsia  triacanthos),  many  of  whose  limbs  are  infested  with  mistletoe  (Phoradendron 
flavescens) ;  B,  a  close  view  of  a  single  mistletoe  plant;  Rome,  Ind  — Photographs 
supplied  by  LAND. 

parasite,  but  more  specialized,  the  haustoria  are  prominent  organs  which 
exhibit  some  structural  complexity,  their  hadrome  elements  coming 
in  close  contact  with  similar  elements  in  the  host.  Still  more  specialized 
are  the  haustoria  of  Cuscuta  (fig.  1082)  and  of  Orobanche,  their  terminal 
cells  deploying  in  all  directions,  and  coming  into  such  close  contact 
with  the  host  cells  that  sometimes  it  is  difficult  to  distinguish  one  from 
the  other,  though  the  frontier  cells  of  the  parasite  commonly  are  richer 
in  starch  than  are  the  adjoining  cells  of  the  host;  particularly  in  Oro- 
banche are  the  cortex,  leptome,  and  hadrome  of  the  haustorium  in  con- 
tact, respectively,  with  the  cortex,  leptome,  and  hadrome  of  the  host. 
Perhaps  the  acme  of  specialization  is  seen  in  Pilostyles,  where  the 
haustorial  elements  permeate  the  host  tissues  like  fungal  hyphae. 


772  ECOLOGY 

The  role  of  haustoria.  —  The  haustoria  of  parasitic  seed  plants  re- 
semble those  of  fungi  in  the  method  of  penetration.  In  the  mistletoe 
the  pressure  exerted  by  the  developing  haustorium  results  in  the  pene- 
tration of  weak  spots,  such  as  lenticels,  bark  rifts,  and  cellulose  walls; 
some  holoparasites  (as  Orobanche  and  Cuscuta)  also  secrete  substances 
that  dissolve  the  cell  walls  of  the  host.  In  Lathraea  the  outer  haus- 
torial  layer  is  glandular,  secreting  a  cement,  which  enables  the  parasite 
to  adhere  to  the  host. 

The  actual  substances  absorbed  by  haustoria  are  inadequately  known. 
In  Cuscuta  careful  cultures  indicate  that  glucose  is  the  chief  substance 
taken  from  the  host,  but  since  the  sieve  tubes  of  the  host  and  of  the  para- 
site are  in  contact  both  in  Cuscuta  and  in  Orobanche,  it  generally  has 
been  assumed  that  proteins  are  absorbed  also.  The  haustoria  of  such 
holoparasites  secrete  diastases  (such  as  amylase  and  cellulase),  which 
digest  the  starch  and  similar  foods  of  the  host.  In  the  mistletoe  it  is 
obvious  that  foods  or  food  materials  are  taken  from  the  host  tree.  Be- 
cause of  the  hadrome  contact  and  the  green  leaves,  the  usual  assump- 
tion is  that  food  materials  (i.e.  water  and  salts)  are  the  chief  things 
absorbed;  hence  such  plants  have  been  called  water  parasites. 

In  spite  of  the  possibility  that  the  chlorophyll  may  not  play  its  usual  role,  this 
assumption  probably  is  correct,  inasmuch  as  cultures  in  weak  light  result  in  im- 
poverished individuals.  However,  the  obvious  harm  caused  to  trees  by  the  mistle- 
toe, the  fact  that  it  does  not  grow  on  dead  trees,  and  the  fact  that  some  mistletoes 
(as  Arceuthobium)  are  essentially  holoparasitic,  give  some  support  to  the  view  that 
the  green  mistletoes  may  get  certain  foods  parasitically. 

Of  special  interest  are  the  Euphrasieae,  since,  in  addition  to  haus- 
toria and  leaf  chlorophyll,  there  are  root  hairs  which  are  attached  to 
soil  particles.  Careful  experiments  have  shown  that  in  this  group  there 
exists  every  gradation  from  autophytism  to  holoparasitism.  Nearly 
all  species  have  chlorophyll  and  all  have  haustoria,  thus  appearing  to 
indicate  a  double  nutritive  potentiality.  At  one  end  of  the  series  is 
Odontites  verna,  which  can  pass  its  entire  life  cycle  as  an  autophyte, 
producing  seeds  capable  of  germination;  and  almost  at  the  other  end 
is  Tozzia,  a  plant  without  root  hairs  which  requires  root  contact  for 
germination,  and  which  lives  nearly  two  years  as  a  subterranean  holo- 
parasite  before  sending  up  a  green  aerial  shoot  that  lives  but  a  few  weeks. 
At  the  extreme  end  of  the  series  is  the  holoparasite,  Lathraea.  The 
more  autophytic  species  have  abundant  root  hairs  and  few  haustoria, 


-T" 


SAPROPHYTISM   AND    SYMBIOSIS  773 

while  the  more  parasitic  species  have  few  root  hairs  and  abundant 
haustoria,  culminating  in  forms  like  Tozzia  and  Lathraea,  which  have 
no  root  hairs,  and  in  which  haustoria  develop  even  in  early  seedling 
stages.  Thus  there  is  a  reciprocal  relation  between  root  hairs  and 
haustoria  that  probably  indicates  the  degree  of  parasitism. 

While  the  parasitism  of  the  Euphrasieae  is  undoubted,  the  substances  obtained 
from  the  host  pJants  are  unknown,  except  in  such  forms  as  Tozzia  and  Lathraea; 
from  analogy  with  the  mistletoe,  it  might  be  thought  that  they  absorb  water  and  salts 
from  the  host  plants,  a  view  that  is  supported  by  the  reciprocal  relation  of  root 
hairs  and  haustoria.  In  favor  of  the  view  that  carbohydrates  are  absorbed  is  the 
fact  that  some  forms  (as  Melampyrum  pratense)  are  supposed  to  have  a  capacity 
for  saprophytism,  and  that  many  ordinary  green  plants  are  partial  saprophytes. 
It  has  been  claimed  that  in  some  species  (as  Euphrasia  officinalis)  the  chlorophyll 
has  almost  if  not  quite  lost  its  food-making  power;  this  view,  however,  seems  some- 
what improbable,  since  all  forms  except  Lathraea  require  light  for  their  optimum 
development,  and  since  most  species  grow  more  luxuriantly  as  autophytes  in  the 
light  than  as  parasites  in  the  shade;  furthermore,  careful  experiments  show  that 
starch  formation  in  these  partial  parasites  bears  quite  the  same  relation  to  abun- 
dant light  and  carbon  dioxid  and  to  open  stomata  as  in  autophytes.  In  any  event, 
the  fact  that  all  of  the  Euphrasieae  studied,  even  the  most  autophytic,  grow  more 
luxuriantly  with  than  without  pa'rasitic  attachment  shows  that  something  is  gained 
through  parasitism,  but  it  must  remain  for  further  experimentation  to  determine 
its  exact  nature. 

The  influence  of  external  factors  upon  development.  —  The  seeds  of 
autophytes  germinate  in  the  presence  of  oxygen  under  proper  conditions 
of  temperature  and  moisture,  but  those  of  extreme  holoparasites,  such 
as  Orobanche,  require  still  another  condition,  namely,  contact  with  the 
proper  host  plant;  this  condition  probably  means  a  particular  kind  of 
chemical  stimulation.  Cuscuta,  though  commonly  a  holoparasite,  germi- 
nates readily  in  ordinary  soil,  and  thus  its  parasitism  appears  to  be 
less  complete  than  is  that  of  Orobanche;  this  view  is  supported  also  by 
the  occasional  presence  of  chlorophyll  and  by  its  less  specialized  haus- 
toria. Even  in  Cuscuta  there  are  degrees  of  parasitism,  some  species 
(as  C.  lupuliformis)  germinating  readily  and  living  independently  for 
some  time,  whereas  other  species  (as  C.  europaed)  germinate  slowly 
and  soon  die  if  a  host  plant  is  not  present.  The  seeds  of  partial  para- 
sites, as  in  the  Euphrasieae,  germinate  generally  in  ordinary  soil,  though 
host  contact  seems  to  facilitate  germination;  Tozzia,  however,  requires 
host  contact  for  germination,  in  this  respect  alone  appearing  to  surpass  the 
dodder  in  parasitic  specialization.  The  seeds  of  Aeginetia,  a  relative  of 
Orobanche,  require  host  contact  for  germination,  the  roots  of  many 


774  ECOLOGY 

plants  serving  equally  well.  Upon  germination  peculiar  hair  tendrils 
arise,  and  further  development  depends  upon  their  coming  into  contact 
with  the  roots  of  certain  monocotyls. 

In  some  partial  parasites  (as  Melampyrum  pratense  and  Santalum) 
haustoria  appear  to  originate  without  special  contact  stimuli,  though 
their  full  development  usually  requires  contact  with  a  living  host;  for 
example,  in  Melampyrum,  tracheids  become  differentiated  only  after 
such  contact.  In  holoparasites  (as  Cuscuta  and  Lathraea)  and  probably 
in  most  partial  parasites  (e.g.  Odontites),  even  the  first  stages  of  haus- 
torial  development  require  contact  with  a  living  host.  That  the  ques- 
tion is  not  merely  one  of  contact,  however,  is  shown  by  the  fact  that 
non-nutritive  solid  bodies  do  not  stimulate  haustoria.  It  has  been 
claimed  that  when  an  Epifagus  plant  comes  in  contact  with  a  beech 
root,  haustoria  develop  on  the  host  rather  than  on  the  parasite.  Prob- 
ably the  factors  involved  in  haustorial  stimulation  are  chemical  in 
nature  and  exceedingly  complex. 

In  the  Euphrasieae  many  of  the  species  exhibit  a  high  degree  of  variability,  the 
amount  and  character  of  which  depends  upon  the  conditions  to  which  they  are 
exposed.  As  previously  noted,  some  of  this  group  (as  Lathraea)  may  be  either 
saprophytes  or  parasites,  and  others  (such  as  species  of  Melampyrum  and  Odontites) 
may  exhibit  varying  degrees  of  saprophytism,  parasitism,  and  autophytism.  When 
Odontites  verna  is  grown  in  humus,  only  10  per  cent  of  the  plants  are  attached  to 
hosts,  and  yet  the  plants  are  more  vigorous  than  in  sand,  where  43  per  cent  exhibit 
parasitic  attachment.  Parasitic  Euphrasieae  are  much  more  luxuriant  on  vigorous 
hosts  than  on  weak  hosts,  developing  larger  seeds  and  having  larger,  healthier,  and 
more  autophytic  progeny.  The  progeny  of  weak  parasites  is  much  more  likely 
to  be  albescent  than  is  the  progeny  of  strong  parasites.  The  latter  phenomena  are 
very  suggestive  from  an  evolutionary  standpoint.  The  culmination  of  variability 
appears  to  be  in  Alectorolophus,  where  many  so-called  species  have  been  found 
experimentally  to  be  merely  habitat  varieties. 

Holoparasites  are  much  more  specialized,  and  are  more  completely  dependent 
upon  their  hosts  than  are  partial  parasites;  hence  they  exhibit  much  less  variability. 
However,  Cuscuta  shows  interesting  variations  which  indicate  that  probably  it  has 
not  reached  the  utmost  bounds  of  parasitism;  for  example,  Cuscuta  monogyna 
has  been  grown  to  maturity  as  a  saprophyte  in  glucose  solutions.  While  the  pres- 
ence of  plastids  in  Cuscuta  has  been  both  affirmed  and  denied,  there  is  little  doubt 
of  the  presence  of  chlorophyll  when  the  dodder  is  grown  in  the  shade  or  on  starved 
hosts  ;  indeed,  there  is  evidence  that  the  dodder  is  able  to  manufacture  small  quan- 
tities of  carbohydrate  food.  Cassytha,  a  member  of  the  Lauraceae,  is  a  parasite 
of  similar  aspect,  and  likewise  is  yellow  in  the  sun  and  green  in  the  shade.  When 
Cuscuta  grows  on  members  of  the  Solanaceae,  the  haustoria  secrete  an  oily  substance 
not  noticeable  elsewhere;  since  the  poisonous  alkaloids  of  the  Solanaceae  do  not 
enter  the  parasite,  it  has  been  suggested  that  the  haustorial  secretions  may  have 


SAPROPHYTISM   AND   SYMBIOSIS 


775 


the  nature  of  an  antitoxin.  Certain  plants  appear  to  be  immune  from  parasitic 
attack,  possibly  through  the  secretion  of  toxins,  or  of  substances  which  induce  apo- 
chemotropic  reactions  in  haustoria,  or  through  the  absence  of  substances  which 
induce  prochemotropic  reactions  (e.g.  glucose,  in  the  case  of  Cuscuta).  When  the 
pear  is  attacked  by  the  mistletoe,  the  infected  branches  soon  die,  whereupon  the 
mistletoe  dies;  thus  the  pear  generally  is  free  from  this  parasite.  Some  trees  are 
immune  to  parasitic  attack  through  the  impenetrability  of  the  cork  layer,  and 
sometimes  infected  regions  of  host  plants  are  isolated  by  cork  formation. 

The  origin  of  parasitism  in  seed  plants.  —  Whatever  may  be  said  of 
the  fungi,  the  parasitic  seed  plants  obviously  form  a  series  of  disconnected 
groups,  each  of  which  is  more  or  less  clearly  related  to  some  autophytic 
group,  near  whose  level  the  parasitic  group  in  question  probably  orig- 
inated. The  Euphrasieae  appear  to  be  in  a  state  of  active  evolution, 
and  they  exhibit  all  stages  of  gradation  to  autophytic  Scrophulariaceae, 
a  family  to  which  they  clearly  still  belong.  The  parasitic  dodders  are 
so  close  to  the  autophytic  morning  glories  that  usually  they  are  regarded 
as  belonging  to  a  common  family.  Even  the  Orobanchaceae,  while 
commonly  accorded  separate  family  rank,  obviously  are  close  to  Gloxinia. 
Rafflesia  and  Balanophora  are  more  remote  from  any  known  autophytic 
stock,  and  yet  they  are  believed  to  be  not  very  far  distant  from  the  Aris- 
tolochiaceae.  In  no  case  is  there  a  long  genetic  series  of  heterotrophic 
forms,  as  in  the  fungi. 

While  thai lophy tic  parasites  probably  have  passed  through  an  intermediate 
saprophytic  stage,  this  may  not  have  been  the  case  in  seed  plants.  An  argument 
for  such  a  saprophytic  stage  is  furnished  by  facultative  forms  like  Lathraea  and 
Melampyrum,  and  also  by  the  probable  capacity  of  many  ordinary  autophytes  for 
partial  saprophytism.  Yet  such  parasitism  as  that  of  the  mistletoe,  where  only 
water  and  salts  appear  to  be  taken  from  the  host,  seems  on  the  whole  simpler  than 
the  saprophytic  absorption  of  organic  foods.  Practically  nothing  is  known  concern- 
ing the  exact  causes  underlying  the  development  of  parasitism  in  seed  plants,  and 
even  the  various  stages  cannot  as  yet  be  regarded  as  certainly  known. 

The  intergradations  among  the  Euphrasieae  are  very  suggestive  of 
possible  stages  in  parasitism,  and  the  following  may  be  hazarded  as 
a  possible  series  in  the  development  of  such  a  root  parasite  as  Orobanche. 
The  roots  of  plants  of  different  species  frequently  come  in  contact  in 
the  soil,  and  it  may  be  supposed  that  the  cells,  either  through  mechanical 
causes  or  through  chemotropic  reactions,  may  come  into  sufficiently 
close  contact  to  permit  of  osmotic  interchange.  Water  or  salts  or  both 
would  then  pass  from  regions  of  high  to  those  of  low  pressure,  the  plant 
in  which  the  pressure  is  low  being  the  incipient  parasite,  while  the  other 


776  ECOLOGY 

is  the  incipient  host.  If  some  substances  exist  in  greater  concentration 
in  one  plant  and  other  substances  in  the  other  plant,  the  condition  might 
give  rise  to  reciprocal  parasitism  (p.  786).  A  second  stage  might  be 
the  development  of  haustoria,  followed,  perhaps,  by  a  reduction  in  the 
number  of  root  hairs  and  finally  by  their  complete  elimination.  A  third 
stage  might  be  the  development  of  carbohydrate  absorption,  apparently 
corresponding  to  the  first  stage  of  saprophytism ;  this  in  turn  might  be 
followed,  as  in  thallophytes,  by  the  cessation  of  food  making  by  the 
chlorophyll,  or  even  by  the  entire  elimination  of  chlorophyll  and  of  the 
leaves  as  conspicuous  organs.  Finally,  there  might  come  the  develop- 
ment of  protein  absorption  through  sieve  tube  contact,  the  loss  of  ger- 
minating power  in  the  seed  except  in  contact  with  the  host,  and  the 
development  of  a  univorous  habit.  Parasites  on  aerial  organs  scarcely 
can  have  had  a  similar  origin;  the  ancestor  of  the  mistletoe  may  have 
been  an  epiphyte  and  that  of  the  dodder,  a  vine  with  adventitious  climb- 
ing roots.  In  the  tropics  the  mistletoe  family  has  many  parasitic  repre- 
sentatives that  are  scarcely  distinguishable  from  epiphytes,  since  they 
possess  roots  which  spread  over  the  bark  of  the  host  plants. 

Attempts  have  been  made  to  ascertain  the  factors  involved  in  parasitism  by 
growing  one  plant  on  another  instead  of  in  the  soil.  Peas  have  been  grown  on  beans 
in  such  a  way  that  they  could  obtain  water  and  salts  only  from  the  latter;  under 
these  conditions  they  not  only  developed  a  root  system  within  the  artificial  host, 
but  reached  maturity,  producing  flowers  and  even  seeds  capable  of  germination. 
Similar  and  equally  successful  experiments  have  been  carried  on  for  a  much  longer 
time  in  which  a  grape  (Cissus  digitala)  has  been  used  as  the  parasite  and  various 
cacti  (as  Opuntia  and  Cereus)  as  hosts.  In  this  experiment  the  grape  probably 
is  a  partial  parasite  comparable  to  the  mistletoe,  taking  chiefly  water  and  salts  from 
the  host  and  manufacturing  its  own  carbohydrates.  This  conception  is  supported 
by  the  fact  that  the  osmotic  pressure  of  the  cell  sap  of  the  grape  was  considerably 
higher  than  that  of  the  cactus.  In  all  cases  studied  the  osmotic  pressure  of  the 
sap  of  the  experimental  parasite  surpassed  that  of  the  host. 

Grafting.  —  General  phenomena.  —  Closely  allied  to  parasitism  are 
the  phenomena  of  grafting,  a  process  in  which  a  shoot  or  bud  (the  scion) 
is  inserted  in  another  plant  (the  stock)  in  such  a  way  that  the  former 
continues  to  grow  through  the  use  of  food  materials  derived  from  the 
latter.  Grafting  is  done  in  various  ways,  one  being  to  place  together 
oblique  surfaces  of  stock  and  scion  of  about  equal  size,  grafting-wax 
being  employed  to  hold  them  in  place  and  to  give  protection;  or  small 
scions  may  be  inserted  in  a  large  stock  which  has  been  cut  transversely 
(figs.  1087,  1088).  Budding  is  a  form  of  grafting  in  which  a  single  bud 


SAPROPHYTISM    AND   SYMBIOSIS 


777 


remaining  attached  to  a  small  piece  of  bark  is  inserted  in  a  T-shaped 
incision  in  the  stock  (figs.  1085,  1086).  The  grafting  of  fruit  trees  is 
of  great  commercial  advantage,  since  most  horticultural  varieties  do 
not  reproduce  true  to  seed,  and  since  even  those  varieties  which  "  come 
true  "  can  be  brought  into  bearing  some  years  sooner  by  grafting  than 
by  growing  from  seed.  In  the  Rosaceae,  grafting  commonly  is  possible 
only  among  closely  related  plants,  as  among  the  species  of  a  common 
genus;  for  example,  plums,  peaches,  and  apricots  may  be  readily  inter- 
grafted,  as  may  apples,  pears,  and  quinces,  while  intergrafting  is  im- 


1086 


FIGS.  1085-1088.  —  Budding  and  grafting:  1085,  1086,  shield-budding,  the  name 
being  derived  irom  the  shield-shaped  piece  of  bark  with  the  removed  bud,  b;  io8£.  the 
stock  in  which  a  T-shaped  incision  (i)  is  made;  the  bud  is  then  inserted  and  the  whole 
tied  securely;  1087,  1088,  cleft-grafting;  1087,' represents  two  scions  (b)  wedge-shaped 
below,  which  are  inserted  into  a  cleft  in  the  stock  (a),  and  then  fixed  in  place  by  grafting 
wax,  as  in  1088.  —  From  BAILEY. 

possible  between  apples  and  plums;  sometimes  closely  related  species 
cannot  be  intergrafted  (e.g.  Prunus  Padus  and  P.  Laurocerams).  In 
the  Solanaceae  and  the  Compositae  many  instances  are  known  where 
different  genera  can  be  intergrafted,  and  cases  are  on  record  of  inter- 
grafting  between  different  families.  The  latter  phenomenon  does  not 
seem  strange,  when  it  is  remembered  that  successful  grafting  depends 
upon  similarities  between  vegetative  characters,  and  that  these  are  not 
necessarily  correlated  with  the  reproductive  characters  upon  which 
plant  relationships  are  based. 

At  the  juncture  of  the  graft  symbionts  there  is  developed  a  callus,  in 
which  xylem  and  phloem  elements  arise,  uniting  the  two  plants  so  that 
they  appear  as  one.  However,  in  spite  of  the  close  union,  the  two  plants 


778  ECOLOGY 

in  large  part  retain  their  individuality  and  are  nearly  as  distinct  from 
one  another  as  are  a  parasite  and  its  host.  The  scion  may  well  be  com- 
pared to  such  a  water  parasite  as  the  mistletoe,  since  it  derives  water  and 
inorganic  salts  from  the  host  or  stock,  while  it  manufactures  its  own 
carbohydrates  and  proteins.  The  chief  difference  between  a  scion  and 
a  parasite  is  that  the  former  is  organically  united  to  the  stock,  while  the 
latter  has  haustorial  processes  which  ramify  through  the  host  tissues; 
however,  there  is  no  sharp  delimitation  between  the  phenomena  of 
parasitism  and  those  of  grafting,  since  scions  sometimes  develop  roots 
in  the  stock. 

The  influence  of  the  stock  and  the  scion  upon  each  other.  —  Many  cases 
are  now  known  in  which  either  the  stock  or  the  scion  is  influenced  by 
the  other,  so  that  a  part  of  the  original  individuality  is  lost;  particularly 
in  evidence  is  the  influence  of  the  stock  upon  the  scion.1  Such  differ- 
ences may  manifest  themselves  in  physiological  behavior,  as  in  changed 
respiration  and  synthesis,  and  particularly  in  reproductive  phenomena; 
or  there  may  be  changes  in  form,  in  color,  or  in  chemical  composition,  as 
when  an  apple  scion  grafted  on  the  wild  crab  bears  more  acid  fruit. 
The  best  known  changes  concern  the  time  of  fruiting;  a  variety  of  the 
apple  that  requires  ten  or  fifteen  years  to  come  into  bearing  from  seed 
may  bear  in  a  year  or  two  if  a  twig  from  a  sapling  is  grafted  on  an  old 
stock,  while  a  twig  from  an  old  stock  grafted  on  a  sapling  does  not  fruit 
for  years.  Some  late  apples  ripen  earlier  when  grafted  on  a  stock  of 
an  early  variety.  Certain  species  of  Citrus  are  more  productive  when 
grafted  on  Citrus  trifoliata  than  when  growing  independently.  When 
the  morning  glory,  which  is  an  annual,  is  grafted  on  the  sweet  potato, 
which  is  a  perennial,  the  latter  develops  its  characteristic  tuberous  roots 
much  earlier  than  otherwise,  thus  giving  an  excellent  illustration  of  the 
influence  of  the  scion  upon  the  stock.  When  the  sunflower,  which  is 
an  annual,  is  used  as  a  stock  for  the  Jerusalem  artichoke,  which  is  a 
tuberous  perennial,  the  artichoke  scion  develops  aerial  tubers  and  the 
sunflower  stock  is  characterized  by  a  large  development  of  secondary 
wood. 

Investigators  differ  as  to  whether  a  chemical  compound  that  is  char- 
acteristic of  a  given  species  or  variety  can  pass  into  a  stock  or  scion  of 

1  In  spite  of  the  great  array  of  evidence  against  it,  some  able  investigators  still  adhere 
to  the  theory  that  stocks  and  scions  maintain  the  individuality  of  their  respective  species; 
the  remarkable  changes  here  recorded  are  said  to  represent  only  such  differences  as 
might  be  obtained  by  growing  the  plants  in  different  soils  and  climates. 


SAPROPHYTISM   AND    SYMBIOSIS  779 

another  species  or  variety,  though  it  is  held  generally  that  readily  diffusi- 
ble organic  substances  may  thus  migrate,  in  addition  to  water  and  in- 
organic solutes.  For  example,  in  Abutifon,  albescence  may  develop 
in  a  green-leaved  scion  on  an  albescent  stock.  The  migration  of  such 
alkaloids  as  atropin  and  nicotin  from  one  graft  symbiont  to  the  other 
may  now  be  regarded  as  demonstrated.  For  example,  atropin  may 
accumulate  in  the  potato  or  in  the  tomato  when  intergrafted  with  Atropa 
or  Datura,  and  nicotin  passes  from  the  tobacco  plant  (Nicotiana  Taba- 
cum)  into  Nicotiana  alata  and  also  into  the  potato.  In  all  of  these  cases 
the  alkaloids  migrate  into  the  other  symbiont,  whether  the  alkaloid- 
producing  species  is  used  as  stock  or  as  scion.  However,  attempts  to 
induce  the  migration  of  hydrocyanic  glucosids  between  stock  and  scion 
in  Phaseolus  have  met  with  no  success.  In  a  number  of  cases  the  form 
of  a  plant  may  be  changed  by  grafting,  the  pear,  for  example,  becoming 
dwarfed  when  grafted  on  the  quince;  some  varieties  of  the  apple,  when 
used  as  scions,  exhibit  changes  in  the  compactness  of  their  branching. 
In  the  grape,  grafting  has  been  found  to  cause  the  modification  of 
many  characters,  such  as  the  size  of  the  vine,  the  form  of  the  leaf, 
the  size  of  the  seeds  and  of  the  fruits,  and  the  juiciness  of  the  fruits. 
French  vineyards  have  been  saved  from  the  ravages  of  the  destruc- 
tive plant-louse,  Phylloxera,  by  grafting  the  vines  on  immune  American 
stocks. 

Graft  hybrids  and  chimeras.  —  It  has  often  been  supposed  that  the 
stock  and  scion  sometimes  fuse  in  such  a  way  as  to  produce  new  shoots 
that  are  intermediate  between  the  two  graft  symbionts;  such  new  forms 
have  been  termed  graft  hybrids.  Famous  cases  of  supposed  graft  hybrids 
are :  the  Bizzaria  orange,  which  is  thought  to  have  arisen  from  a  graft 
of  Citrus  Aurantium  and  C.  medica;  Cytisus  Adami,  which  is  thought 
to  have  arisen  from  a  scion  of  C.  purpureus  on  C.  Laburnum;  and 
Crataegomespilus,  which  is  supposed  to  be  a  graft  hybrid  between 
Crataegus  monogyna  and  Mespilus  germanica.  A  case  has  been  reported 
where  there  arose  shoots  of  intermediate  character  when  a  pear  scion 
was  grafted  on  a  quince  stock.  Usually  seedlings  from  these  "  graft 
hybrids  "  revert  to  one  or  the  other  of  the  two  parent  forms,  but  in  at 
least  one  instance,  progeny  of  intermediate  character  has  been  reported ; 
for  example,  if  white  beets  are  grafted  on  red  beets,  about  a  fourth  of 
the  progeny  of  the  white  scion  is  red  or  reddish.  Recently  a  remark- 
able fusion  of  the  stock  and  the  scion  has  been  produced  by  a  graft 
between  the  tomato  (Solanum  Lycopersicum)  and  the  nightshade  (S. 


780  ECOLOGY 

nigrum)  in  which  a  bud  developed  where  the  two  calluses  fused,  growing 
subsequently  into  a  shoot  that  combined  the  characters  of  both  stock 
and  scion;  to  the  new  form  thus  produced  there  was  given  the  name 
Solatium  tubingense.  Several  such  forms  have  developed,  some  of  which 
are  nearly  intermediate  between  the  parent  species,  while  others  more 
closely  resemble  either  the  nightshade  or  the  tomato.  Seedlings  revert 
to  one  or  the  other  of  the  parent  forms.  Shoots  sometimes  arise  in 
which  a  part  is  like  the  nightshade  and  a  part  like  the  tomato;  such 
forms  have  been  termed  chimeras. 

Much  difference  of  opinion  has  arisen  concerning  the  interpretation  of  these 
striking  results.  One  theory  is  that  the  new  productions  (such  as  Solanum  tubin- 
gense), which  are  not  obvious  chimeras,  none  the  less  resemble  the  latter  in  main- 
taining the  individuality  of  the  two  components,  the  portions  of  each  being  aggre- 
gated into  a  sort  of  patchwork  or  mosaic;  the  new  forms  from  this  viewpoint  are 
regarded  as  hyper  chimeras.  Another  theory  has  resulted  from  a  study  of  forms 
of  Pelargonium  with  white  margined  leaves,  in  which  it  has  been  found  that  one 
of  the  graft  symbionts  may  serve  as  a  sort  of  mantle  for  the  other;  the  body  of  the 
new  form  is  composed  entirely  of  one  variety,  while  the  epidermis  alone,  or  the 
epidermis  with  the  hypodermis,  is  composed  entirely  of  the  other  variety.  These 
remarkable  forms  have  been  termed  periclinal  chimeras.  Already  it  has  been 
shown  that  Cytisus  Adami  is  a  periclinal  chimera,  the  body  being  composed  of  C. 
Laburnum  and  the  epidermis  of  C.  purpureus.  There  is  reason  to  believe  that 
Crataegomespilus  is  to  be  explained  similarly.  In  all  of  these  cases,  as  in  Solanum 
tubingense,  a  seedling  gives  rise  not  to  an  intermediate  form  but  to  a  form  like  one 
of  the  parent  symbionts.  While  most  of  the  supposed  graft  hybrids  thus  appear 
to  be  periclinal  chimeras,  there  are  some  investigators  who  still  hold  to  the  reality  of 
graft  hybrids;  in  the  last  analysis  a  graft  hybrid  should  differ  from  all  kinds  of 
chimeras  in  the  merging  of  the  protoplasm  of  the  two  graft  symbionts.  Such 
merging  has  not  as  yet  been  demonstrated. 

Galls.  —  The  influence  of  parasites  upon  their  hosts.  —  When  a  para- 
site attacks  another  organism,  the  activities  of  the  latter  may  be  ac- 
celerated or  diminished.  For  example,  respiration  and  transpiration 
commonly  are  increased  and  synthesis  commonly  is  decreased.  Para- 
sites often  secrete  deleterious  substances  which,  like  many  poisons,  at 
first  excite  various  activities,  while  an  increase  of  these  substances  causes 
depression  and  even  local  or  general  death.  In  other  cases  the  injury 
caused  by  parasites  consists  chiefly  in  the  removal  of  foods  and  food 
materials  from  the  host,  which  may  in  consequence  be  starved  and 
depauperate. 

Various  characteristics  of  galls.  —  The  most  conspicuous  influence  of 
parasites  on  hosts  is  in  connection  with  gall  formation.  A  gall  is  a  struc- 


SAPROPHYTISM    AND    SYMBIOSIS 


781 


tural  modification  of  a  tissue  or  of  an  organ  due  to  another  organism.1 
Commonly  the  affected  tissue  is  much  enlarged,  either  through  hyper- 


f 


FIG.  1089.  —  A  cross  section  through  the  edge  of  a  leaf  gall  of  Viburnum  Lantana, 
showing  striking  hyperplasy  and  hypertrophy;  p,  the  palisade  cells  of  the  uninfected 
portion;  p',  the  corresponding  cells  of  the  infected  portion;  /,  the  sponge  cells  of  the 
uninfected  portion;  /',  the  corresponding  cells  of  the  infected  portion;  e,  epidermis; 
h,  epidermal  hair;  considerably  magnified.  —  From  KUSTER. 

trophy  (cell  enlargement)  or  hyperplasy  (development  of  new  cells), 
or  through  both  combined  (fig.  1089).     Scarcely  less  common  is  the 

1  Galls  also  are  termed  cecidia  and  have  been  contrasted  with  domatia,  which  differ 
in  -that  the  organisms  inhabiting  them  are  thought  to  be  harmless  or  even  beneficial 
rather  than  detrimental  to  their  hosts.  An  example  of  domatia  is  afforded  by  the  root 
tubercles  of  clover;  sometimes  the  structures  inhabited  by  plant  lice  are  regarded  as 
domatia,  since  the  nitrogenous  animal  excreta  are  thought  to  benefit  the  plant.  It  seems 
wiser  to  call  all  such  structures  galls,  regardless  of  their  benefit  or  harm  to  the  host  plant. 


782 


ECOLOGY 


1093 


1091 


accumulation  of  food  in 
the  affected  tissues.  In 
some  cases  parasites  cause 
atrophy  (reduced  cell  size) 
or  hypoplasy  (reduced  cell 
number).  Often  gall  tis- 
sues remain  in  a  condi- 
tion more  primitive  than 
that  of  uninfected  tissues; 
the  primordia  of  wood  and 
bast,  for  example,  often 
remain  parenchymatous 
instead  of  developing  into 
complex  tissues. 

The  most  astonishing 
feature  of  galls  is  the  de- 
velopment of  nutritive 

FIGS.  1090-1093.  —  Cross  sec- 
tions of  galls,  showing  anatomical 
features:  1090,  a  solid  cynipid 
gall  from  an  oak  twig,  cut  in 
half;  note  the  radiating  lines  of 
tissue,  and  the  central  larval 
chamber  (c);  1091,  a  section  of 
an  oak  gall;  e,  epidermis;  0, 
cortex,  the  grains  representing 
plastids  (chloroplasts  above  and 
leucoplasts  with  prominent  starch 
grains  below) ;  s,  protective  layer 
of  sclerenchymatous  cells ;  /,  /', 
nutritive  layers  adjoining  the 
larval  chamber,  /  being  a  layer 
rich  in  starch,  and  /'  a  layer 
whose  cells  are  rich  in  proteins 
and  prominently  nucleated;  1092, 
part  of  the  nutritive  tissue  of  the 
gall  of  Nematus  gallorum;  note 
the  resemblance  to  the  tissues 
in  an  intumescence;  1093,  iso- 
lated nutritive  hairs  of  a  Cepha- 
loneon  gall  from  a  maple  (Acer); 
1091-1093,  highly  magnified.  — 
1090  after  KERNER,  1091  after 
LACAZE-DUTHIERS,  1092,  1093 
after  KCJSTER, 


SAPROPHYTISM  AND  SYMBIOSIS 


7*3 


layers,  in  which  food  accumulates  abundantly,  and  later  is  utilized  by 
the  parasite,  whether  fungus  or  insect.  A  somewhat  complex  situation 
is  found  in  the  cynipid  galls  (i.e.  galls  formed  by  members  of  the  Cyni- 
pidae,  a  group  of  highly  specialized  insects) ;  here  the  larval  chamber  is 
surrounded  by  well-defined  food  layers,  which  sometimes  are  differentiated 
into  an  inner  protein  layer  and  an  outer  carbohydrate  layer,  the  whole 
being  surrounded  by  a  layer  of  rigid  mechanical  cells  or  of  protective 
cork  cells  (figs.  1090-1093).  Sometimes  the  nutritive  layers  remain  meri- 
stematic,  continuing  to  regenerate  as  they  are  destroyed  by  the  insect 
larvae.  Galls  are  unusually  rich  in  tannins,  resinous  secretions,  and 
other  waste  products,  and  are  remarkably  resistant  structures;  if  an 
infested  plant  is  cut  down,  the  galls  often  remain  fresh  and  green  when 
the  other  parts  are  dead.  Not  infrequently  the  tissues  of  galls  are  much 
more  xerophytic  in  structure  than  are  the  tissues  of  uninfected  organs. 

Galls  occur  on  all  kinds  of  plant  organs  and  they  assume  a  wide  variety  of  forms, 
some  of  which  are  exceedingly  fantastic.  They  are  caused  by  many  species  of 
plants  and  animals,  but  chiefly  by 
fungi  and  insects.  Often  the  organ 
affected  is  greatly  modified  in  forn*, 
and  in  some  cases  structures  appear 
that  are  not  present  when  the  plant 
is  uninfected.  For  example,  the  rose 
gall,  formed  by  the  cynipid  insect, 
Rhodites  bicolor,  is  covered  with 
prickles,  even  if  the  rest  of  the  plant 
is  quite  smooth  (figs.  1094-1096); 
similarly,  the  grape-vine  gall  formed 
by  Cecidomyia  Vitis-pomum  (fig. 
823),  and  some  cynipid  oak  galls  are 
pubescent,  though  the  organs  con- 
cerned are  smooth  when  uninfected. 
In  the  common  oak  gall,  formed  by 
Amphibolips  inanis,  delicate  threads 
connect  the  larval  chamber  with  the 
gall  periphery.  Some  gall-forming 
organisms  injure  the  growing  point, 
thus  checking  elongation,  and  caus- 


1095 


1096 


FIGS.  1094-1096.  —  Gall  formation  in  a 
rose  (Rosa  blanda):  1094,  an  ordinary  leaf 
with  five  leaflets  (/)  and  a  pair  of  stipules  (s); 
1095,  a  leaf  which  has  been  attacked  by  a 
gall-producing  insect  (Rhodites  bicolor);  note 
the  prickly  galls  (g) ;  1096,  a  similar  leaf  still 


more  modified  through  gall  formation. 


ing  the  close  imbrication  of  the  de- 
veloping leaves,  as  in  the  conelike 
willow  gall  formed  by  Cecidomyia  strobiloides,  and  in  the  goldenrod  gall  formed 
by  C.  Solidaginis  (figs.  1097-1099).  A  great  many  root  galls  are  formed  by 
nematode  worms. 

An  interesting  group  of  galls  are  the  witches'  brooms,  which  are  formed  on  various 


784 


ECOLOGY 


trees  (as  the  white  birch,  the  hackberry,  and  various  conifers)  by  Exoascus  and  by 
other  fungi,  and  by  the  dwarf  mistletoe,  Arceuthobium  pusillum.  In  these  galls 
many  small  twigs  diverge  from  the  part  infected,  thus  manifesting  a  resemblance 
to  a  broom  or  brush.  In  the  case  of  Exoascus  the  mycelium  hibernates,  so  that 
the  fungus  recurs  season  after  season.  Other  important  fungus  galls  are :  the  black 
knot  of  the  cherry  (caused  by  Plowrightia,  fig.  noo);  the  ergot  of  rye  and  other 


1100 


FIGS.  1097-1100.  —  1097-1099  gall  formation  in  a  goldenrod  (Solidago  serotina): 
1097,  the  apical  portion  of  a  plant  that  has  been  attacked  by  an  insect  (Cccidomyia 
Solidaginis}\  such  galls  check  stem  elongation  and  prevent  flowering;  note  the  variation 
in  leaf  form;  1098,  an  ordinary  leaf;  1099,  a  gall  leaf  or  leaf-complex  made  up  of  a  num- 
ber of  coalesced  leaves;  uoo,  a  "black  knot"  on  a  branch  of  the  choke  cherry  (Prunus 
virginiana),  an  example  of  gall  formation  through  fungal  influence,  the  stimulating 
fungus  being  Ploivrjghtia  morbosa  ;  the  swollen  black  mass  is  known  as  a  stroma,  and  it 
contains  many  fructifications  known  as  perithecia. 


grasses  (caused  by  Claviceps),  in  which  black  protruding  bodies,  the  sclerotia, 
replace  the  grains  ;  the  cedar  apple  of  Juniperus  virginiana  (caused  by  Gymnospo- 
rangium);  and  the  leaf  and  flower  galls  of  the  Ericaceae  (caused  by  Exobasidium). 
Fungi  also  occasion  root  galls  in  the  cabbage  and  in  the  alder,  and  bacteria  occasion 
galls  on  the  roots  of  leguminous  plants  (p.  787).  Sometimes  fungi  cause  pro- 
nounced changes  in  plant  habit.  For  .example,  the  prostrate  herbs,  Euphorbia 
maculata  and  E.  polygonifolia,  become  erect  when  infested  with  a  certain  rust ; 


SAPROPHYTISM   AND    SYMBIOSIS  785 

comparable  modifications,  due  to  similar  causes,  occur  in  the  leaves  of  Anemone 
quinquefolid  and  Hepatica  acutiloba.  Parasitic  seed  plants  less  commonly  cause 
galls,  though  Conopholis  sometimes  occasions  conspicuous  enlargements  on  oak 
roots,  and  the  mistletoe  frequently  occasions  stem  swellings  and  variations  in  the 
form  of  the  branches. 

The  cause  of  gall  formation.  —  Although  galls  obviously  are  caused  by 
foreign  organisms,  the  exact  nature  of  the  stimuli  involved  is  unknown, 
and  can  scarcely  be  determined  precisely  until  galls  are  produced  arti- 
ficially under  controlled  conditions.  Some  investigators  regard  chemi- 
cal stimulation  as  the  ruling  factor,  while  others  think  that  mechanical 
irritation  is  more  important;  still  others  appeal  to  the  possibility  of 
osmotic  changes,  a  view  suggesting  analogies  with  tuberization.  Some 
investigators  think  that  a  brief  contact  with  a  foreign  structure  (as  with 
the  ovipositor  of  an  insect)  is  sufficient  for  gall  formation,  while  others 
think  that  long-continued  stimulation  (as  by  insect  larvae)  is  necessary. 
The  most  remarkable  galls  are  those  in  which  new  structures  appear,  as 
in  the  galls  of  the  rose,  the  grape,  and  the  oak,  with  their  prickles  or  hairs. 
In  some  cases,  as  in  various  so-called  domatia,  gall-like  structures 
appear  to  develop  without  stimulation  by  foreign  organisms.  For 
example,  the  myrmecophytes,  Hydnophytum  and  Myrmecodia,  have 
large  tubers  permeated  with  air  chambers  and  passages  that  are  in- 
habited by  ants,  but  it  has  been  shown  that  the  chambered  tubers  de- 
velop independently  of  ant  stimulation.  Recently  it  has  been  dis- 
covered that  some  witches'  brooms  (as  in  the  spruce)  can  be  propagated 
by  seed,  many  of  the  progeny  from  such  shoots  being  dwarf  and  bushy. 
There  has  been  advanced  the  somewhat  dubious  theory  that  structures 
of  this  sort  once  were  due  to  the  stimulation  of  the  foreign  organism, 
but  that  now  gall  formation  has  become  an  inherent  feature  of  the 
plant. 

The  advantages  of  galls.  —  Unlike  most  plant  structures,  galls  are 
obviously  disadvantageous  to  the  plants  of  which  they  form  a  part.  The 
energy  and  material  used  in  their  construction,  the  food  which  they 
accumulate  and  which  is  utilized  by  the  foreign  organism,  together  with 
many  activities  of  the  parasites  are  features  of  positive  detriment.  Thus 
galls  furnish  one  of  the  best  illustrations  of  the  fallacy  of  the  theory  of 
adaptation.  In  a  few  cases  galls  are  believed  to  be  advantageous  to  the 
host  as  well  as  to  the  parasite,  notably  in  the  root  tubercles  of  the  legumes 
(p.  790);  if  the  fungus  theory  of  tuberization  is  confirmed,  there  be- 
comes evident  another  striking  case  of  gain  through  gall  formation. 


786  ECOLOGY 

Fasciation.  —  Perhaps  to  be  classed  with  galls  are  the  peculiar  structures  known 
as  fasciations,  which  usually  are  manifested  in  the  form  of  flattened  stems  or 
branches.  The  form  may  arise  through  the  flattening  of  a  single  cylindrical  stem, 
or  a  number  of  stems  may  be  merged  into  a  single  fasciated  structure.  Often  the 
flowers  as  well  as  the  branches  are  modified  in  appearance.  The  phenomenon  is 
not  well  understood,  but  often  it  is  believed  to  be  associated  with  "  over-nutrition  "; 
sometimes  it  is  produced  by  mechanical  causes,  or  by  insect  or  fungal  activities 
(as  in  Oenothera).  Fasciation  sometimes  appears  to  be  inheritable,  but  this  re- 
mains to  be  established,  at  least  as  a  general  proposition. 

Autoparasitism.  —  Auto  parasitism  (i.e.  self-parasitism)  is  a  common  phenome- 
non, since  most  plants  contain  colorless  living  tissues  that  derive  their  food  from 
the  green  chlorenchyma.  In  some  cases  green  plants  bear  albescent  shoots  whose 
nourishment  is  parasitic;  in  the  redwood  such  shoots  occur  frequently,  and  when  they 
are  detached  and  planted  in  the  soil,  they  may  develop  chlorophyll.  In  plants  pos- 
sessing an  alternation  of  generations,  one  phase  commonly  is  parasitic  on  the  other. 
For  example,  in  liverworts  and  mosses  the  sporophyte  foot  (an  organ  resembling  a 
haustorium)  is  embedded  in  gametophyte  tissues,  and  in  Anthoceros  it  has  rhizoid- 
like  processes  (fig.  241);  since  the  moss  sporophyte  commonly  is  green,  it  probably 
is  a  water  parasite  comparable  to  the  mistletoe,  though  in  Sphagnum  the  sporophyte 
is  colorless  and  holoparasitic  (fig.  250).  In  the  seed  plants  the  gametophyte  is 
parasitic  on  the  sporophyte,  and  sometimes  there  are  haustorial  processes,  as  in 
Zamia ;  the  embryonic  sporophytes  also  are  parasitic,  the  suspensors  frequently 
resembling  haustoria  and  acting  as  organs  of  food  absorption  and  conduction  (figs. 
460,  510).  Parasitic  features  are  exhibited  by  many  germinating  seedlings,  partic- 
ularly among  the  monocotyls,  in  which  the  tip  of  the  cotyledon  often  is  a  haustorial 
organ  (figs.  1229,  1230)  and  secretes  digestive  enzyms.  Grass  seedlings  have  a 
peculiar  organ,  the  scutellum,  which  connects  the  embryo  with  the  region  of  accumu- 
lated foods;  often  the  scutellum  cells  are  elongated,  in  Briza  even  resembling  root 
hairs.  Perhaps  to  be  noted  under  autoparasitism  is  the  occasional  parasitism  of  a 
dodder  plant  upon  another  dodder,  of  mistletoe  upon  mistletoe,  or  of  strong  individ- 
uals upon  weak  individuals  among  the  Euphrasieae. 


3.    RECIPROCAL    PARASITISM,    HELOTISM,    AND 
ENDOSAPROPHYTISM 

Definitions.  —  The  topic  symbiosis  commonly  has  been  subdivided 
into  antagonistic  symbiosis  or  parasitism  and  cooperative  symbiosis  or 
mutualism.  Such  terms  as  mutualism  and  cooperation  are  humanistic, 
and  should  be  discarded.  Each  of  two  symbionts  may  be  benefited 
nutritively  or  otherwise  by  the  presence  of  the  other,  but  it  is  a  miscon- 
ception to  regard  two  symbionts  as  giving  one  another  food  or  assistance. 
That  form  of  symbiosis  in  which  each  of  the  symbionts  obtains  food 
from  the  other  may  be  termed  reciprocal  parasitism;  where  the  para- 
sitism of  the  two  symbionts  is  alternative  rather  than  simultaneous,  the 


SAPROPHYTISM   AND    SYMBIOSIS  787 

relationship  may  be  called  one  of  alternative  parasitism.  The  term 
helotism  (i.e.  slavery)  may  be  employed  in  cases  where  one  symbiont 
derives  appreciable  nutritive  benefit  without  conspicuous  gain  or  loss 
to  the  other.  Thus  helotism  is  intermediate  between  parasitism,  where 
one  symbiont  gains  and  the  other  loses,  and  commensalism,  where 
parasitism  is  absent.  Where  one  symbiont  lives  within  another,  obtain- 
ing food  saprophytically  rather  than  parasitically,  it  is  said  to  exhibit 
endosaprophytism. 

Root  tubercles  and  their  bacteria. — Structure  and  behavior.  —  Recip- 
rocal parasitism  is  best  illustrated  by  the  relation  existing  between  the 
Leguminosae  and  the  bacteria  which  inhabit  galls  on  their  roots ;  these  galls 
are  caused  either  directly  or  indirectly  by  the  bacteria  and  are  known 
as  root  tubercles  (fig.  noi).  The  tubercles,  like  most  galls,  are  com- 
posed chiefly  of  large  parenchymatous  cells,  many  of  which,  especially 
in  the  central  region,  contain  bacteria  belonging  to  the  species  Bacillus 
(or  Pseudomonas}  radicicola  (fig.  1102).  These  bacteria  are  facultative 
forms  that  are  present  somewhat  generally  in  the  soil  as  saprophytes, 
appearing  usually  as  minute  motile  rods.  They  enter  the  roots  of  legu- 
minous plants  through  the  root  hairs,  exhibiting  prochemotactic  reactions 
to  certain  root  excretions,  and  appearing  to  secrete  substances  which 
soften  or  dissolve  the  walls  of  the  root  cells.  Soon  after  the  bacteria  thus 
become  parasitic,  there  appear  hypha-like  infection  threads  or  bacterial 
tubes,  which  are  essentially  gelatinous  masses  of  minute  bacteria  known 
as  zoogloea.  The  bacteria  in  these  zoogloea  masses  bud  like  yeast,  giving 
rise  to  motile  or  immotile  rodlike  forms,  which  in  turn  produce  the  pecul- 
iar branched  forms,  known  as  bacteroids,1  that  are  characteristic  of  the 
tubercles.  Finally,  the  organisms  become  dissolved  and  incorporated 
into  the  body  of  the  green  plant.  Tubercle  formation  is  associated 
definitely  with  bacterial  infection,  and  starch  may  accumulate  in  the 
hypertrophied  cells  as  in  other  galls. 

While  the  root  bacteria  of  the  Leguminosae  commonly  are  referred  to  a  single 
morphological  species,  there  are  several  and  perhaps  many  well-defined  physio- 
logical species.  Commonly  it  is  easy  to  infect  the  root  of  a  legume  with  bacteria 
from  a  legume  of  the  same  species  or  even  genus,  but  usually  it  is  difficult  or  impos- 
sible to  do  this  if  the  bacteria  are  taken  from  another  genus;  however,  peas  are 
infected  readily  by  bacteria  from  vetches,  and  vetches  may  be  infected  similarly 
from  peas.  Bacteria  from  the  roots  of  peas  thrive  only  moderately  in  the  roots  of 

1  This  name  dates  back  to  the  time  when  the  organisms  were  regarded  as  an  albumi- 
nous product  of  the  green  plant.  The  term  Rhizobium  was  given  to  these  bodies  wheq 
they  were  supposed  to  be  organisms,  but  of  unknown  affinity. 


788 


ECOLOGY 


beans,  but  if  these  bacteria  are  used  to  inoculate  other  bean  roots,  they  grow  as  vig- 
orously as  did  their  immediate  ancestors  in  the  roots  of  peas.  Similarly,  bacteria 
from  legumes  of  calcareous  soil  do  not  infect  legumes  of  siliceous  soil,  nor  vice  "versa, 
but  if  the  acid  content  of  a  calcareous  soil  is  increased  slowly,  a  race  of  bacteria  can 
be  developed  (e.g.  on  alfalfa)  that  eventually  will  be  able  to  infect  lupines  growing 
in  siliceous  soil.  It  is  the  belief  of  bacteriologists  that  through  similar  progressive 


FlGS.  noi,  1 102.  —  noi,  roots  of  the  white  sweet  clover  (Melilotus  alba),  showing  the 
characteristic  root  tubercles  (0,  which  are  induced  by  a  special  bacterial  form.  Bacillus 
radicicola ,  note  the  grouping  of  tubercles  in  clusters ;  1 102,  a  longitudinal  section  through 
a  part  of  a  root  of  the  pea  (Pisum  sativum)  that  has  begun  to  tuberize  by  reason  of  the 
stimulating  influence  of  Bacillus  radicicola;  invasion  occurs  through  the  root  hairs  (r), 
where  infection  threads  (i)  are  formed;  these  penetrate  the  cortical  tissues  (c),  where 
branching  takes  place;  note  that  the  infected  cortical  cells  have  denser  cytoplasm,  larger 
nuclei  (n),  and  thicker  walls  than  the  uninfected  cells;  e,  epidermis;  highly  magnified. 
—  From  FRANK. 

variability  any  race  of  Bacillus  radicicola  can  be  grown  eventually  on  any  legume 
root,  and  this  view  is  favored  by  the  fact  that  when  a  leguminous  crop  is  introduced 
into  an  entirely  new  region,  the  roots  soon  become  infected  by  bacteria  which  induce 
tubercle  formation.  The  bacteria  of  legume  tubercles  thrive  only  where  free  nitro- 
gen and  an  abundant  supply  of  carbohydrates  are  available,  their  development 
being  retarded  by  an  abundance  of  nitrates.  Similarly,  the  tubercles  reach  their  best 
development  in  soils  that  are  poor  in  nitrates;  they  vary  from  the  size  of  a  pin  head 
jn  ordinary  clovers  to  the  size  of  a  pea  in  Strofhostyles  helvola,  a  plant  of  sandy 


SAPROPHYTISM    AND    SYMBIOSIS  789 

beaches.     The  tubercles  live  but  a  single  season,  hence  in  perennial  legumes  there 
is  a  new  bacterial  infection  each  year. 

Nitrogen  fixation  and  nitrification.  —  Although  it  is  an  abundant  and 
important  constituent  of  plants,  only  a  comparatively  small  number  of 
plants,  namely,  certain  bacteria  and  fungi,  are  known  to  be  able  to  utilize 
directly  the  free  nitrogen  that  exists  so  abundantly  in  the  air.  This 
incorporation  of  free  nitrogen  into  nitrogenous  compounds  within  the 
body  is  known  as  nitrogen  fixation,  and  it  is  a  process  of  vast  importance 
to  the  entire  organic  world.  The  first  organism  definitely  ascertained 
to  have  the  power  of  nitrogen  fixation  was  Clostridium  Pasieurianum,  one 
of  the  anaerobic  soil  bacteria.  A  number  of  nitrogen-fixing  organisms 
are  now  known,  embracing  various  widely  distributed  species  of  Azoto- 
bacter  (a  genus  of  aerobic  bacteria)  and  of  Bacillus,  such  as  B.  radicicola, 
the  organism  inhabiting  legume  tubercles,  and  B.  amylobacter,  which  is 
thought  by  some  investigators  to  include  forms  that  have  been  referred 
to  Granulobacter  and  Clostridium. 

Probably  the  most  important  of  all  nitrogen-fixing  organisms  is  Azotobacler 
chroococcum,  which  in  temperate  climates  is  abundant  in  nearly  all  soils  and  also  in 
fresh  and  salt  water,  being  absent  chiefly  in  bogs  and  in  some  virgin  soils.  Nitrogen 
fixation  by  this  organism  is  best  accomplished  in  aerated  soils  and  is  facilitated  by 
lime  and  phosphorus  and  retarded  by  acids.  Contrary  to  earlier  views,  humus 
facilitates  nitrogen  fixation  by  Azotobacter,  probably  because  of  its  microorganisms; 
it  has  been  suggested,  for  example,  that  cellulose-destroying  bacteria  furnish  carbo- 
hydrates in  available  form  for  nitrifying  organisms,  and  it  has  been  shown  that  the 
addition  of  sugar  to  cultures  facilitates  nitrogen  fixation.  In  addition  to  the  bacteria, 
some  yeasts  and  a  number  of  fungi  are  now  thought  to  be  able  to  fix  free  nitrogen 
(P-  797)-  Recently  some  investigators  have  claimed  that  the  hairs  of  many  plants 
are  able  to  fix  nitrogen;  the  fact  that  the  amount  of  nitrogenous  materials  in  such 
hairs  is  inconsiderable  and  that  this  small  amount  is  no  less  when  the  hairs  develop 
in  an  atmosphere  devoid  of  nitrogen  make  the  claim  very  doubtful.  At  the  same 
time  it  must  be  admitted  that  the  known  methods  of  nitrogen  fixation  seem  inade- 
quate to  account  quantitatively  for  the  large  and  relatively  constant  supply  of  avail- 
able nitrogen  in  the  soil,  particularly  in  view  of  the  considerable  amount  of  denitri- 
fication  that  is  effected  through  the  activity  of  various  bacteria. 

In  the  processes  of  organic  decay  the  complex  proteins  are  broken  up 
into  simpler  substances,  such  as  organic  acids,  amins,  and  ammonia. 
Some  of  these,  as  ammonia,  are  oxidized  through  the  agency  of  bacteria, 
constituting  a  process  that  is  known  as  nitrification.  The  first  step  in 
this  process  is  the  formation  of  nitrites,  and  from  these  by  further  oxida- 
tion are  formed  nitrates.  Different  organisms  are  concerned  in  the  two 


796  ECOLOGY 

stages,  Nitrosomonas,  for  example,  oxidizing  ammonia  into  nitrites,  and 
Nitrobacter  oxidizing  the  latter  into  nitrates.  The  nitrates  formed  by 
this  process  are  utilized  readily  as  a  source  of  nitrtigen  by  most  plants. 

Green  plants  on  the  one  hand  and  nitrogen-fixing  and  nitrifying 
bacteria  on  the  other  have  a  reciprocal  relation  of  vast  significance,  for 
the  former  produce  carbohydrates  and  the  latter  nitrates,  each  of  which 
is  of  great  importance  for  the  other,  as  well  as  for  all  living  organisms. 
Thus  there  is  a  sort  of  universal  symbiosis  between  the  carbohydrate- 
forming  and  the  nitrate-forming  organisms.  The  origin  of  this  sym- 
biosis is  unknown,  but  it  is  possible  that  the  first  plants  were  able  to  fix 
nitrogen  as  well  as  to  manufacture  carbohydrates,  and  that  divergent 
evolution  has  since  taken  place.  Even  among  plants  now  living  there 
are  some  bacteria,  notably  Nitrosomonas  and  Nitrobacter,  which  can 
manufacture  carbohydrates  as  well  as  nitrites  or  nitrates.  Sometimes 
nitrate-forming  and  carbohydrate-forming  organisms  are  in  somewhat 
close  symbiosis,  as  in  the  case  of  Azotobacter  and  Oscillatoria,  or  Azoto- 
bacter  and  Nostoc,  which  are  two  of  many  similar  associations,  where 
both  symbionts  grow  with  unusual  vigor;  similarly,  in  salt  water,  Azoto- 
bacter  lives  in  luxuriance  in  the  mucilage  which  coats  the  fronds  of 
Laminaria.  In  all  of  these  cases  the  amount  of  nitrogen  fixation  is 
greatly  beyond  the  usual,  whence  it  has  been  urged  that  algae  can  fix 
nitrogen,  though  it  seems  more  likely  that  such  symbiosis  stimulates 
the  bacteria  to  larger  activity  because  of  the  carbohydrates  which  the 
algae  manufacture.  An  interesting  but  poorly  understood  case  of  sym- 
biosis is  that  which  exists  between  bacteria  and  myxomycetes,  two 
groups  of  organisms  that  often  are  closely  associated;  it  is  claimed, 
even,  that  the  spores  of  some  myxomycetes,  for  example,  Dictyostelium 
mucoroides,  fail  to  germinate  except  in  the  presence  of  bacteria,  and 
that  the  food  of  myxomycetes  consists  largely  of  such  microorganisms. 

The  rdle  of  bacteria  in  legume  tubercles.  —  It  has  been  known  for  cen 
turies  that  leguminous  plants  enrich  the  land  when  their  roots  are  left 
in  the  soil;  furthermore,  the  high  nitrogen  content  of  the  root  tubercles 
was  known  long  before  the  tubercle  bacteria  or  their  power  of  nitrogen 
fixation  was  discovered.  Hence  it  is  not  strange  that  once  the  tubercles 
were  regarded  as  organs  which  manufacture  or  accumulate  protein. 
Even  before  it  was  empirically  proven  that  Bacillus  radicicola  fixes  free 
atmospheric  nitrogen,  a  comparable  conclusion  was  reached  by  elimina- 
tion, for  it  was  shown  that  nitrogenous  compounds  do  not  develop  in 
sterilized  soils,  and  that  the  legumes,  unlike  other  plants,  thrive  in  soils 


SAPROPHYTISM    AND    SYMBIOSIS 


791 


devoid  of  nitrogenous  compounds;    moreover,  tubercles  do  not  develop 
and  the  plants  are  depauperate  if  the  soil  is  sterilized. 

There  is  now  ample  direct  evidence  that  Bacillus  radicicola  can  fix 
free  atmospheric  nitrogen  to  some  extent  when  isolated  in  pure  cultures, 
and  much  more  abundantly  when  growing  in  the  roots  of  leguminous 
plants,  the  resulting  compounds  accumulating  within  the  bacterial  body. 
Such  nitrogen  fixation  has  been  shown  to  be  facilitated  by  the  presence 
of  an  abundance  of  sugar,  especially  maltose,  and  of  free  nitrogen,  and 
to  be  retarded  by  the  presence  of  an  abundance  of  nitrates  or  of  albu- 
minous substances.  It  is  scarcely  to  be  doubted  that  Bacillus  radicicola 
and  the  leguminous  plants  live  in  a  state  of  reciprocal  parasitism,  the 
bacteria  deriving  carbohydrates  from  the  legumes,  while  the  latter  derive 
nitrogenous  compounds  from  the  bacteria.  The  parasitism  of  the  two 
symbionts  appears  in  part  alternative.  The  bacteria  soon  after  entrance 
into  the  root  exhibit  great  vigor  and  activity,  probably  through  their 
bettered  food  relations,  while  the  legume  is  injured  rather  than  benefited 
by  their  presence,  since  in  the  cells  which  they  occupy,  the  bacteria  utilize 
the  starch  and  much  of  the  cytoplasm  and  cause  the  nuclei  to  become 
partly  disorganized.  After  a  time  the  leguminous  plant  appears  to 
overcome  the  bacteria,  and  it  enters  upon  a  state  of  vigor  because  of  the 
appropriation  of  nitrogenous  compounds,  while 
the  bacteria  enter  the  bacteroid  or  Rhizobium 
state  of  relative  inertness,  after  which  there  is 
no  recovery  of  the  power  to  fix  nitrogen  or  to 
infect  other  leguminous  roots;  probably  some 
nitrogenous  compounds  become  available  for  the 
legume  during  the  later  phases  of  bacterial  activity, 
though  it  is  through  the  final  dissolution  of  the 
bacteroids  that  the  main  supply  appears  to  be 
derived. 

Mycosymbiosis.  —  Ectotrophic  and  endotrophic 
mycorhiza.  —  Fungi  are  associated  habitually  with 
the  roots  of  many  plants,  such  as  the  oaks,  pines, 
orchids,  and  ericads  (Ericaceae).  The  root  in 
combination  with  its  fungus  is  known  as  a  myco- 
rhiza (i.e.  fungus  root),  and  the  phenomenon  may 
be  termed  mycosymbiosis.  If  the  fungal  hyphae 
invest  the  roots,  as  in  the  beech  (fig.  1103),  the 
mycorhiza  is  called  ectotrophic  (i.e.  nourished 


FIG.  1103.  —  A  root 
tip  of  the  European- 
beech  (Fagus  sylvatica), 
illustrating  ectotrophic 
mycorhiza;  the  fungal 
hyphae  (h)  form  a  dense 
layer  which  ensheathes 
the  root;  considerably 
magnified.  —  After 
FR'ANK. 


792 


ECOLOGY 


e  w 


outside),  while  if  the  fungi  occur  within  the  roots,  as  in  the  orchids 
(fig.  1106),  it  is  called  endotrophic  (i.e.  nourished  within).  Ectotrophic 
mycorhizas  vary  from  forms  with  loose  and  scattered  hyphal  threads 
which  come  into  casual  contact  with  the  roots  to  a  condition  like  that  in 
Monolropa,  where  the  root  system  usually  is  compacted  into  a  clump  or 

ball,  and  where  the  in- 
dividual rootlets  are  so 
closely  invested  by  fun- 
gal hyphaethat  thelatter 
when  sectioned  resem- 
ble a  compact  tissue 
(figs.  1104,  1105);  in 
such  a  case  the  root 
proper  does  not  come 
into  contact  with  the 
soil.  The  hyphae  com- 
posing the  fungal  root 
sheath  connect  with  the 
mycelia  that  permeate 
the  humus.  The  root- 
lets of  the  beech  and 
of  most  plants  with 
prominent  ectotrophic 
mycorhizas  are  rela- 
tively short  and  thick 
and  have  a  coraJloid 
aspect;  moreover, 
growth  is  relatively 
sluggish  and  root  hairs 
are  few  or  wanting  except  on  roots  or  on  parts  of  roots  that  are 
comparatively  free  from  fungi. 

Endotrophic  mycorhizas,  which  especially  characterize  the  orchids, 
are  in  many  respects  much  more  specialized  than  are  the  ectotrophic 
forms.  Orchid  roots  are  characteristically  fleshy  and  tuber-like,  differ- 
ing much  more  from  ordinary  roots  than  do  those  associated  with  ecto- 
trophic fungi;  furthermore,  the  endophytic  fungi  appear  to  be  specialized 
forms  rather  than  ordinary  soil  fungi.  Certain  cortical  cells  contain 
closely  interwoven  clumps  of  hyphae  which  commonly  enfold  the  nucleus, 
and  there  are  hyphal  connections  with  similar  clumps  in  adjoining  cells, 


FIGS.  1104,  1105.  —  Mycorhiza  of  the  Indian  pipe 
(Monotropa  uniflora):  1104,  the  basal  portion  of  a  stem 
with  its  clump  of  roots;  note  the  coralloid  root  system  (r), 
and  the  imbricated  scale  leaves  (s) ;  1 105,  a  cross  section 
through  Nthe  outer  part  of  one  of  the  coralloid  roots, 
showing  the  compact  arrangement  of  the  fungal  hyphae 
which  form  a  pseudo- parenchyma  (/) ;  note  that  the  fungus 
is  partly  ectotrophic  and  partly  endotrophic,  the  hyphae 
penetrating  into  the  epidermis  (e)  of  the  root  and  crowd- 
ing aside  the  cytoplasm  (c)  of  the  latter;  nt  the  nuclei  of 
the  epidermal  cells;  1105  highly  magnified. 


SAPROPHYTISM   AND    SYMBIOSIS 


793 


c 


f, 


so  that  the  mycelium  often  is  continuous  (figs.  1106,  1107);  frequently 
also  haustoria  are  present,  and  sometimes  the  internal  hyphae  are  con- 
tinuous with  the  hyphae 
which  ramify  the  soil. 
Root  hairs  commonly  are 
scarce  and  are  more  or 
less  filled  with  hyphae. 
Many  root  cells  are  free 
from  fungi,  including 
some  of  the  outer  cells 
as  well  as  those  of  the 
vascular  tract.1 


Endotrophic  fungi  are  as- 
sociated with  the  tuber-like 
gametophytes  of  Lycopodium 
and  Botrychium  (fig.  1108). 
Transitional  forms  between 
ectotrophic  and  endotrophic : 
mycorhizas  are  not  rare, 
being  characteristic  of  the 
Ericaceae ;  in  Monotropa, 


1107 


FIGS.  1106,  1107.  — EndotropAic  mycorhiza  of  orchid 
roots:  1106,  a  cross  section  through  a  part  of  a  root 
of  A  plectrum  hyemale,  showing  dense  clumps  of  fungal 
hyphae  (/)  in  some  of  the  larger  cortical  cells,  several 
rows  inside  of  the  epidermis  (e)\  r,  root  hairs;  v,  vas- 
cular tract;  1107,  a  single  cortical  cell  from  the  root 
of  Spiranthes  cernua,  showing  fungal  strands  (/),  cyto- 
plasm (c),  and  the  nucleus  (n);  1106  considerably,  and 
1107  highly  magnified. 


for    example,    hyphae    from 
the  fungal  sheath  invade  and  modify  the  epidermal  layer  (fig.  1105).     Even  ecto- 
trophic fungi  may  penetrate  into  the  root,  though  in  that  event  they  commonly 

are  intercellular  rather  than  intra- 
cellular,  as  in  most  endotrophic 
forms.  In  some  pines  there  are 
both  ectotrophic  and  endotrophic 
mycorhizas,  the  relative  develop- 
ment of  the  two  kinds  varying 
with  the  habitat.  In  some  climb- 
ing plants  (as  Vanilla)  the  fungus 
is  both  ectotrophic  and  endo- 
trophic and  is  said  to  penetrate 

even  the  tissues  of  the  supporting 
FIG.  1108. — A  section  through  the  gameto- 

phyte  of  Botrychium,  showing  endotrophic 
mycorhiza,  the  fungi  inhabiting  the  ventral 
region;  considerably  magnified.  —  After 


JEFFREY. 


plant  as  well  as  those  of  the  liana. 
There  is  little  doubt  that  the 
coralloid  aspect  of  roots  associated 
with  ectotrophic  fungi  and  the  tu- 


1  In  Corallorhiza,  which  has  no  roots,  fungi  occur  in  the  rhizome  and  in  its  "root 
hairs,"  and  in  A  plectrum,  which  has  ordinary  endophytic  root  fungi,  the  removal  of  the 
corm  is  followed  by  the  development  of  coralloid  rhizomes  that  bear  "root  hairs  "  and 
contain  fungi,  precisely  as  in  Corallorhiza. 


794  ECOLOGY 

berization  of  orchid  roots  are  due  to  some  stimulus  instituted  by  the  fungal 
symbiont,  so  that  the  resulting  structures  are  to  be  regarded  as  galls.  As  in  other 
galls,  the  cells  are  much  hypertrophied  and  the  nuclei  often  assume  enlarged  or 
otherwise  unusual  forms.  The  identity  of  the  mycorhiza  fungi  is  in  considerable 
doubt,  owing  to  the  usual  absence  of  reproductive  organs  and  the  difficulty  in  mak- 
ing artificial  cultures.  Originally  they  were  thought  to  be  truffle  mycelia,  and  it  is 
not  unlikely  that  this  is  a  correct  identification  for  some  forms,  while  others  may 
belong  to  the  agarics,  the  molds,  or  to  various  other  fungal  groups.  In  Fagus  and 
Carpinus  there  is  evidence  that  the  mycorhiza  fungi  are  molds,  such  as  Penicillium 
and  Citromyces.  In  the  orchids  it  is  probable  that  there  are  specific  forms  com- 
parable to  the  legume  bacillus.  Recent  studies  seem  to  indicate  that  the  orchid 
fungi  include  at  least  three  species  which  appear  to  belong  to  the  genus  Rhizoctonia; 
in  Phalaenopsis  there  is  a  peculiar  fungus  (perhaps  a  Rhizoctonia),  whose  hyphae 
anastomose  and  form  sclerotia. 

The  prevalence  of  mycosymbiosis .  —  Until  a  few  years  ago  mycosym- 
biosis  was  believed  to  be  a  somewhat  rare  phenomenon,  characterizing 
only  a  few  families,  such  as  the  conifers,  ericads,  orchids,  and  oaks ; 
but  now  it  is  believed  that  a  majority  of  ordinary  green  plants  are  myco- 
phytes  (i.e.  fungus  plants),  as  green  plants  with  root  fungi  may  be  called. 
In  Germany  seventy  out  of  a  hundred  and  five  species  taken  at  random 
had  root  fungi,  and  in  Java  sixty-nine  out  of  seventy-five.  Doubtless 
the  ectotrophic  fungi  are  more  abundant  than  the  endotrophic,  though 
the  latter  occur  in  numerous  forms,  as  in  aroids,  lilies,  and  many  trees, 
as  well  as  in  orchids  and  ericads;  it  is  probable  also  that  many  of 
the  former  are  mere  contact  forms  without  particular  significance. 
Endotrophic  fungi  are  now  well  known  in  some  mosses  (as  Buxbaumia 
and  Tetraplodori)  and  in  many  liverworts  (notably  in  the  Jungerman- 
niales,  but  also  in  Fegatella  and  Marchantia).  Mycorhizas  with  char- 
acteristic coralloid  rootlets  and  hyphal  clumps  have  been  detected  in 
certain  Carboniferous  plants,  as  in  Cordaites,  thus  attesting  to  the  an- 
tiquity of  mycosymbiosis.  For  the  most  part  root  fungi  are  absent  in 
sedges,  pinks,  crucifers,  most  ferns  (i.e.  Polypodiaceae)  and  legumes, 
though  the  last  are  characterized  by  bacterial  symbiosis.  As  might  be 
expected,  mycorhizas  are  associated  abundantly  with  plants  rooted  in 
forest  mold.  They  are  rare  in  water  and  in  wet  soils  (except  in  bogs) 
and  are  almost  universal  in  bulbous  and  tuberous  plants. 

The  role  of  root  fungi.  —  While  root  fungi  have  long  been  known, 
it  originally  was  supposed  that  their  contact  with  roots  is  merely  casual, 
or  that  they  represent  ordinary  parasites.  Some  years  since  it  was 
suggested  that  Monotropa  is  likely  to  have  a  nutritive  relation  with  its 
fungus,  since  the  latter  completely  invests  the  root  system.  Careful 


SAPROPHYTISM   AND    SYMBIOSIS  795 

experiments  on  beech  seedlings  demonstrated  that  while  the  plants 
developed  vigorously  in  ordinary  soil,  ten  out  of  fifteen  seedlings  grown 
in  sterilized  soil  died  within  two  years,  in  spite  of  a  greater  than  usual  . 
development  of  root  hairs.  Experiments  with  pine  seedlings  resulted 
similarly,  although  no  difference  appeared  between  the  cultures  the  first 
year.1  In  contrast  to  these  results  it  was  claimed  that  species  without 
root  fungi  grow  more  luxuriantly  in  sterilized  than  in  ordinary  soil; 
it  should  be  said,  however,  that  some  investigators  regard  this  last  experi- 
ment as  inconclusive. 

Recently  it  has  been  demonstrated  that  orchids  are  dependent  upon 
their  symbiotic  fungi  to  a  surprising  degree.  As  a  group,  orchids  have 
been  regarded  as  difficult  of  cultivation,  and  for  a  long  time  their  seeds 
were  supposed  to  be  incapable  of  germination.  However,  it  has  been 
discovered  that  the  thing  requisite  for  germination  is  contact  with  the 
appropriate  fungus,  in  which  respect  orchids  are  comparable  to  such 
holoparasites  as  Orobanche.  Of  great  interest  from  the  evolutionary 
standpoint  is  the  fact  that  various  species  of  orchids  differ  in  the  degree 
of  their  dependence  upon  their  fungi.  For  example,  the  seeds  of  Bletia 
germinate  without  fungus  contact,  and  the  seedlings  continue  to  grow 
as  autophytes  for  several  months,  when  growth  ceases,  never  to  be  re- 
sumed, unless  the  appropriate  fungus  comes  in  contact  with  the  orchid. 
In  Laelia  and  Cattleya  the  autophytic  seedling  period  is  much  shorter, 
while  in  Cypripedium  and  Neottia,  fungus  contact  is  necessary  for  the 
initial  phase  of  germination.  In  nature  the  fungus  almost  always  enters 
the  young  seedling  at  the  outset.  When  fungi  are  introduced  into  cul- 
tures of  minute  orchid  seedlings,  a  synthesis  of  hitherto  separate  indi- 
viduals takes  place  that  is  altogether  comparable  to  the  formation  of 
lichens  through  the  synthesis  of  algae  and  fungi  (p.  800)  .2 

In  the  orchid  mycorhizas  there  is  a  marked  parallelism  between  the  development 
of  the  fungi  and  that  of  the  orchids,  the  more  generalized  fungi  being  associated  with 
those  orchids  in  which  symbiosis  is  intermittent  and  most  nearly  facultative.  The 
higher  orchids,  on  the  other  hand,  are  the  most  obligate  of  mycophytes,  and  their 
fungal  symbionts  are  relatively  specialized  forms.  Apparently  the  development  of 
the  two  symbionts  has  been  parallel,  each  becoming  more  intimately  associated 
with  the  other,  as  its  evolution  has  progressed.  The  more  generalized  fungi,  such 
as  Rhizoctonia  repens,  can  infect  a  number  of  the  lower  orchids,  while  the  more 

1  Pines  may  thrive  even  for  several  years  without  mycorhiza  when  grown  in  sterilized 
humus. 

2  A  similar  synthesis  has  been  effected  by  inoculating  the  roots  of  beech  seedlings  with 
fungi  taken  from  other  beech  roots. 


796 


ECOLOGY 


specialized  fungi,  such  as  Rhizoctonia  lanuginosa  or  R.  mttcoroides,  can  institute 
a  long-lived  symbiosis  in  but  a  few  of  the  higher  orchids;  if  other  orchids  are  arti- 
ficially infected  with  the  latter  fungi,  one  or  the  other  of  the  symbionts  soon  dies, 
since  the  parasitism  is  one-sided  and  highly  detrimental  rather  than  reciprocal  and 
moderate.  The  far-reaching  influence  of  the  fungal  symbiont  is  well  illustrated 
by  the  fact  that  the  structure  and  behavior  of  the  orchid  may  vary  if  it  is  infected 
by  a  new  fungus;  for  example,  Bletilla  develops  a  corm,  when  it  is  infected  by  the 
specialized  fungus  of  Cattleya.  The  precise  influence  of  the  fungus  upon  germina- 
tion is  not  known;  it  has  been  claimed  recently  that 
it  secretes  diastase,  which  transforms  the  starch  of 
the  seeds  into  sugar.  The  orchids  are  not  alone 
in  requiring  the  presence  of  fungi  at  germination, 
since  the  spores  of  Lycopodium  are  unable  to  de- 
velop beyond  a  five-celled  stage  in  cultures  that  are 
free  from  fungi. 

The  data  just  given  appear  to  indicate 
that  some  mycophytes,  at  any  rate,  are  para- 
sitic or  at  least  dependent  upon  their  root 
fungi.  There  is  evidence  also  of  parasitism 
in  the  opposite  direction.  Gall  formation, 
cell  hypertrophy,  and  nuclear  disorganiza- 
tion have  already  been  mentioned,  and  they 
indicate  the  probable  parasitism  of  root 
fungi  upon  green  plants;  since  these  fungi 
also  live  as  saprophytes  in  the  soil,  they 
appear  to  belong  to  the  group  of  facultative 
parasites.  Careful  cytological  study  in 
Neottia  has  shown  that  in  certain  cells  (fun- 
gal host  cells,  fig.  1109)  the  hyphae  have 
haustoria  and  are  vigorous  and  healthy, 


1110 


FIGS.  1109,  1 1 10.  —  Cells 
from  the  root  of  an  orchid 
(Neottia  Nidus-avis}:  1109,  a 
fungal  host  cell  in  which  the 
hyphae  (h)  are  vigorous  and 
apparently  parasitic  upon  the 
orchid;  n,  nucleus;  mo,  a 
digestive  cell  in  which  the  hy- 
phae are  being  disorganized 
and  digested  by  the  orchid ; 
highly  magnified.  —  After 
MAGNUS. 


clearly  living  parasitically  on  the  orchid, 
while  in  other  cells  (digestive  cells,  fig.  mo),  the  orchid  appears  to  be 
destroying  and  digesting  the  fungal  hyphae.  Similarly  contrasting  host 
cells  and  digestive  cells  have  been  observed  also  outside  of  the  orchids, 
as  in  Podocarpus  and  Psilotum.  These  are  representative  instances 
of  reciprocal  parasitism,  and  it  is  rather  likely  that  similar  nutritive 
relations  occur  in  many  other  mycorhiza  plants,  though  how  widely 
it  is  not  yet  possible  to  say. 

The  substances  appropriated  from  one  another  by  the  two  symbionts  are 
not  certainly  known,  though  there  is  strong  presumptive  evidence  in 
certain  cases  that  the  interrelations  in  part  resemble  those  existing  be- 


SAPROPHYTISM   AND   SYMBIOSIS  797 

tween  leguminous  plants  and  their  tubercle  bacteria.  Careful  study  in 
certain  forms  has  shown  that  starch  and  other  carbohydrates  disappear 
from  the  root  cells  coincidently  with  an  obvious  increase  in  the  fungal 
cytoplasm.  In  some  forms  there  is  evidence  that  food  passes  from 
the  root  to  hyphae  in  the  soil,  one  observer  regarding  the  root  tannins  as 
the  source  of  food.  On  the  other  hand,  it  has  been  observed  that  the 
entrance  of  fungi  into  a  root  may  be  followed  by  increased  activity  in 
the  root  cells  and  by  nuclear  enlargement,  and  also  by  a  considerable 
increase  in  proteins;  in  Podocarpus  proteolytic  enzyms  appear  upon 
fungal  entrance.  It  is  now  thought  that  a  number  of  fungi  are  able  to 
fix  nitrogen;  among  these  are  such  common  forms  as  Aspergillus  and 
Penicillium.  It  has  been  shown  that  the  endotrophic  fungus  of  Podo- 
carpus is  able  to  fix  nitrogen,  and  several  species  of  Phoma  that  have  been 
taken  from  the  roots  of  ericads  have  been  found  able  to  fix  nitrogen  more 
economically  than  Azotobacter.  Thus  it  is  becoming  increasingly  likely 
that  root  fungi  are  important  sources  of  nitrogenous  compounds  for 
mycophytes.  However,  experiments  on  the  orchid  fungi  as  yet  give 
negative  results,  and  it  may  be  that  nitrogen  fixation  is  a  function  of  cer- 
tain root  fungi  rather  than  of  root  fungi  in  general.  Also  of  interest 
in  this  connection  is  the  demonstration  of  nitrogen  fixation  by  the  para- 
sitic fungi  that  frequently  inhabit  the  aerial  organs  of  Lolium,  result- 
ing in  increased  nitrogen  content  and  greater  vigor  in  the  latter. 

From  the  foregoing  it  seems  likely  that  the  fungus  appropriates  carbon 
compounds  from  the  green  plant,  while  the  latter  probably  appropriates 
nitrogenous  substances  from  the  fungus.  It  has  been  suggested  also 
that  ectotrophic  fungi  and  even  endotrophic  fungi  may  take  the  place 
of  root  hairs  as  organs  of  absorption  of  water  and  salts;  in  any  event 
root  hairs  are  scarce  in  mycophytes.  Since  fungal  hyphae  are  better 
absorptive  structures  than  are  root  hairs,  mycophytes  may  be  better  off 
nutritively  than  autophytes,  especially  if  they  are  able  to  utilize  the 
substances  absorbed  from  the  soil  by  their  fungi. 

In  cases  of  mycosymbiosis  where  neither  symbiont  is  green  (as  in  Monotropa  and 
Corallorhiza),  the  gain  to  the  fungus  is  less  evident  and  to  the  larger  symbiont  more 
evident  than  usual.  Monotropa  appears  usually  to  be  entirely  dependent  upon  its 
fungus,  since  its  roots  are  completely  invested;  sometimes,  however,  Monotropa  has 
elongated  roots  without  fungal  sheaths,  a  fact  which  seems  to  indicate  that  its 
nourishment  then  is  derived  saprophytically.  Corallorhiza  must  also  depend  upon 
its  fungus,  unless  it  is  able  to  absorb  food  saprophytically  from  the  humus,  a  matter 
that  is  as  yet  uncertain. 

Recently  there  have  come  into  prominence  two  further  theories  concerning 


798  ECOLOGY 

mycosymbiosis.  One  theory  is  merely  the  revival  of  the  old  hypothesis  that  the 
fungus  alone  is  parasitic;  favoring  this  view  is  the  fact  that  in  the  bryophytes  fungal 
symbiosis  seems  to  cause  diminished  rather  than  increased  luxuriance,  and  that  the 
endotrophic  mycorhizas  of  various  plants  (as  the  olive)  is  best  developed  on  the 
weaker  rather  than  on  the  stronger  trees;  also  it  has  been  observed  that  the  seed- 
lings of  oaks  and  chestnuts  have  been  killed  in  large  numbers  when  their  ectotrophic 
fungus  is  more  vigorous  than  usual,  and  especially  when  it  becomes  endotrophic. 
The  latter  instances,  however,  may  be  explained  in  line  with  the  prevailing  theory 
as  merely  a  disturbance  of  the  usual  balance  between  the  two  symbionts.  The 
other  theory  is  that  the  fungus  is  a  harmless  endosaprophyte,  living  upon  such  root 
excreta  as  tannin  or  upon  such  foods  as  sugar.  Supporters  of  the  theory  of 
parasitism  and  of  that  of  endosaprophytism  generally  agree  that  the  fungus  alone 
is  benefited  by  the  symbiosis,  the  digestion  of  the  fungus  by  the  root  cells  being 
regarded  merely  as  the  destruction  of  a  foreign  organism,  that  is,  as  phagocytosis. 
It  is  conceivable  that  all  the  theories  here  mentioned  are  valid,  for  the  possibility 
must  be  recognized  that  in  the  many  cases  of  mycosymbiosis  there  is  opportunity 
for  fundamental  differences  in  the  nutritive  relations  of  the  symbionts. 

Probably  to  be  compared  with  the  mycorhizas  are  the  fungus-containing  root 
tubercles  of  Alnus,  Shepherdia,  and  Elaeagnus ;  as  in  the  Leguminosae,  cultures 
of  these  plants  without  tubercles  result  in  depauperate  individuals,  and  there  is 
supplementary  evidence  that  the  fungi  are  able  to  fix  nitrogen.  As  in  the 
orchids,  the  fungi  may  occur  in  hyphal  clumps  and  may  exhibit  various  stages  of 
disintegration,  thus  indicating  digestion  or  phagocytosis  by  the  root  cells.  A  recent 
investigator  regards  the  organisms  of  the  alder  tubercles  as  bacteria  rather  than  as 
filamentous  fungi,  and  he  claims  to  have  demonstrated  the  existence  of  infection 
threads,  rodlike  forms,  and  bacteroids,  as  in  the  tubercles  of  the  Leguminosae; 
Myrica  and  Ceanothus  are  said  to  have  similar  root  swellings  containing  similar 
organisms.  The  roots  of  Cycas  revoluta  have  coralloid  galls  which  contain  colonies 
of  bacteria,  fungi,  and  blue-green  algae.  Recent  studies  appear  to  demonstrate 
the  existence  here  of  reciprocal  parasitism,  the  host  cells  showing  structural  altera- 
tions, nuclear  degeneration,  and  the  loss  of  starch,  while  there  also  occur  clumps  of 
fungal  hyphae  in  various  stages  of  degeneration;  after  the  digestion  or  phagocyto- 
sis of  the  fungi  has  taken  place,  the  host  cells  contain  various  excretory  products. 
Unlike  legume  tubercles,  cycad  and  alder  tubercles  are  perennial. 

The  significance  of  mycosymbiosis.  —  The  discovery  of  mycosymbiosis 
alters  previous  conceptions  concerning  the  prevalence  of  autophytism. 
Apparently,  ordinary  green  plants  may  be  divided  into  autophytes  and 
mycophytes ;  the  latter  also  include  a  few  plants  without  chlorophyll.  The 
former  are  not  only  free  from  root  fungi,  but  they  also  reach  their  opti- 
mum development  in  sterilized  soil,  probably  because  their  root  hairs 
are  more  imperfect  absorptive  organs  than  are  fungal  hyphae.  Con- 
trasted with  these  are  the  mycophytes,1  which  attain  their  optimum 

1  The  relation  here  pictured  also  has  been  termed  symbiotic  saprophytism,  since  the 
relation  of  the  symbiotic  complex  to  the  soil  is  saprophytic.  Most  cases  of  so-caljed 


SAPROPHYTISM   AND    SYMBIOSIS 


799 


development  in  association  with  root  fungi.  Between  extreme  auto- 
phytes  such  as  the  crucifers  and  obligate  mycophytes  such  as  the  orchids 
there  exist  all  gradations;  for  example,  liverworts  such  as  Preissia  and 
Fegatella  are  facultative  mycophytes,  growing  either  with  or  without 
fungi.  Generally  in  autophytes  roots  and  root  hairs  are  well  developed, 
starch  is  abundant,  transpiration  is  relatively  intense,  and  growth  is 
rapid,  while  in  obligate  mycophytes  root  development  is  weak,  root  hairs 
are  few  or  wanting,  transpiration  is  slight,  and  growth  is  slow. 

Even  though  the  total  number  of  mycophytes  may  surpass  that  of 
the  autophytes,  it  is  not  to  be  inferred  that  fungal  hyphae  surpass  root 
hairs  in  importance  in  green  plants  as  a  whole,  for  the  majority  of  my- 
cophytes have  root  hairs  and  only  a  relatively  sparse  development  of 
root  fungi;  probably  many  of  these  plants  are  facultative  mycophytes 
in  which  the  fungal  relation  is  relatively  casual  and  is  attended  with  no 
particular  benefit  or  injury  to  either  symbiont.  But  the  importance  of 
fungi  in  the  nutrition  of  the  higher  plants  probably  is  much  greater  than 
generally  has  been  suspected,  in  spite  of  the  fact  that  the  matter  may  be 
one  of  great  economic  importance.  It  is  very  likely  that  the  well-known 
difficulty  in  cultivating  many  of  the  Pinaceae,  Fagaceae,  Orchidaceae, 
and  Ericaceae,  as  contrasted  with  the  ready  cultivation  of  the  Cruci- 
ferae  and  Rosaceae,  may  be  due  to  the  fungal  symbiosis  of  the  former; 
many  orchids  are  now  grown  successfully  by  taking  care  that  conditions 
are  made  suitable  for  their  fungi.  Very  probably  the  ability  of  many 
plants  (such  as  the  ericads)  to  flourish  in  bogs,  where  nitrogen  fixation 
or  nitrification  by  bacteria  is  relatively  rare,  is  because  they  live  symbi- 
otically  with  fungi  which  are  capable  of  fixing  nitrogen.  It  is  common 
to  picture  organic  life  chains  leading  from  such  dependent  forms  as  ani- 
mals or  fungi  back  to  green  plants,  which  in  the  sunlight  manufacture 
foods  from  inorganic  raw  materials.  A  truer  picture  of  the  plant  king- 
dom is  one  that  recognizes  symbiosis,  bringing  out  the  dependence  of 
mycophytes  upon  their  fungi,  of  legumes  upon  their  bacteria,  and  of 
other  green  plants  upon  the  soil  bacteria,  as  well  as  the  dependence 
of  all  of  these  lower  forms  upon  the  green  plants. 

The  origin  of  myco symbiosis.  —  There  is  no  adequate  evidence  upon  which  to 
base  speculations  concerning  the  origin  of  mycorhizal  phenomena  Very  probably 
the  initial  phases  resembled  those  postulated  for  the  origin  of  parasitism,  and  chem- 
otropic  reactions  may  well  have  played  an  important  part  ;  indeed,  it  has  been 

holosaprophytism  in  the  higher  plants  (such  as  in  Lycopodium,  Monotropa,  and  Corallo- 
rhiza)  are  to  be  referred  to  symbiotic  saprophytism  or  mycophytism. 


8oo 


ECOLOGY 


clearly  demonstrated  that  some  root  fungi  are  prochemotropic  with  respect  to  certain 
substances  that  are  within  or  about  roots.     Probably  fungus  contact  with  roots 

originally  was  casual,  and  the  first 
mycosymbiosis  doubtless  was  faculta- 
tive ;  later,  it  may  be  supposed,  came 
obligate  mycosymbiosis,  reaching  its 
culmination  in  the  orchids  and  ericads, 
and  especially  in  those  species  that  re- 
quire fungus  contact  for  germination, 
and  in  such  forms  as  Neottia  and  Mono- 
tropa,  which  contain  no  chlorophyll 
and  thus  depend  entirely  upon  outside 
sources  for  their  food.  It  is  to  be  ob- 
served that  generally  the  fungus  does 
not  become  thus  dependent  upon  the 
other  symbiont,  but  remains  faculta- 
tive, although  there  is  evidence  of  considerable  dependence  upon  specific  symbionts 
among  the  orchid  fungi. 


FIG.  mi.  —  A  foliose  lichen  (Physcia) 
on  tree  bark;  note  the  marginal  vegetative 
propagation  characteristic  of  lichens,  also  the 
numerous  fruiting  structures,  the  apothecia. 
—  From  COULTER  (Part  I). 


Lichens.  —  Structural  relations.  —  A  lichen  is  a  plant  complex  made 
up  of  a  fungus  body  in  which  algae  are  enclosed.  Formerly  lichens  were 
supposed  to  be  in- 
dividual plants,  and 
the  green  cells,  now 
known  to  be  algae, 
were  called  gonidia 
(figs.  1111-1113). 
The  dual  nature  of 
lichens  was  discov- 
ered by  making  sep- 
arate cultures  of  the 
constituent  algae 

FIG.  1 112.  —  A  section  through  an  apothecium  of  a  lichen 


(Anaptychja),  showing  the  spore-bearing  layer  (hymenium), 


and    fungi    through 

entire  developmental  beneath  which  is  the  loose  myceiium  Of  the  lichen  body ;  these 

cycles.      Also  Spores  portions  are  in   large  part  invested    by  the  dense  cortical 

from  the  funffUS  ele-  mycelium  with  its  numerous  groups  of  algae;    considerably 


ment  of   the  lichen 


magnified.  —  After  SACHS. 


were  sown  among  algae  that  had  been  growing  separately  in  nature, 
and  the  developing  fungus  mycelium  enclosed  the  latter,  forming 
a  lichen  of  the  usual  kind.  Commonly  the  algal  symbionts  are 
well-known  forms,  such  as  Pleurococcus  and  Nostoc,  but  the  fungi 
are  most  diverse  and  generally  unlike  other  fungi,  suggesting  that 


SAPROPHYTISM   AND   SYMBIOSIS 


801 


considerable  change  may  have  been 
wrought  in  the  latter  through  symbi- 
osis. In  fact,  experiments  have  shown 
that  lichen  fungi  when  grown  independ- 
ently differ  in  form  and  in  chemical 
composition  from  the  same  fungi  when 
grown  in  symbiosis  with  algae.  While 
the  lichen  body  is  that  of  the  fungal 
symbiont,  it  is  generally  quite  unlike 
ordinary  fungus  bodies,  being  flat,  com- 
pact, and  expanded  like  a  liverwort. 
Since  the  algae  make  up  what  may  be 
called  the  synthetic  tissue,  the  advantage 
of  the  flat  expanded  surface  is  as  evident 
as  in  liverworts. 

The  autonomous  features  of  lichens.  — 
In  spite  of  the  proven  duality  of  lichens, 
there  are  various  things  which  suggest 
that  they  possess  a  high  degree  of  auton- 
omy or  unity.  It  is  quite  conceivable 
that  this  autonomy  might  ultimately  be- 


FIG.  1113.  —  A  cross  section  of 
a  stem  of  the  beard  lichen  (Usnea 
barbata)  at  the  point  of  origin  of  a 
branch  (5),  the  latter  being  shown 
in  longitudinal  section;  c,  cortical 
absorptive  layer;  a,  algal  layer;  /, 
loose  internal  mycelium;  w,  an  axial 
strand  of  densely  placed  hyphae, 
forming  the  highly  elastic  mechan- 
ical tissue  of  the  lichen;  highly 
magnified.  —  From  SACHS. 


come  so  complete  as  to  make  impossible 
the  separate  cultivation  of  the  two  symbionts.     Perhaps  the  most  strik- 
ing evidence  of  autonomy  is  afforded  by  the  soredia  (figs.  1114-1116), 

which  are  unique  reproductive  or- 
gans consisting  of  a  group  of  algal 
cells  invested  by  fungal  hyphae;  at 

«~    0  maturity  the  soredial  structure  buds 

1U4  ij^  nte'         °ff  fr°m  tne  ^cnen  thallus  like  a 

FIGS.  1  1  14-1116.  -Soredia  from  the   gemma  (p.  808),  forming,  perhaps, 

beard  lichen  (Usnea  barbata):  1114,  a  the  most  efficient  means  of  repro- 
simple  soredium  consisting  of  an  algal  duction  ssessed  by  lichens  since 
cell,  surrounded  by  a  web  of  fungal  hy- 

phae;  1115,  a  soredium  in  which  the  algal   the  fungal  spores  are  of  value  only 

cell  has  reproduced  by  division;  1116,  a  when  they  happen  to  fall  among 
germinating  soredium  in  which  the  algae  appropriate  algae.  This  is  almost 
are  dividing,  the  hyphae  forming  an  apex 

of  growth;  all  figures  highly  magnified,  the  only  case  where  two  symbionts 
—  From  SCHWENDENER.  have  a  common  reproductive  body.1 


1  The  fungal  symbiont  of  Lolium  is  scattered  with  the  seeds,  the  mycelia  occupying 
a  definite  layer  ;   the  bacterial  galls  of  Ardisia  also  are  propagated  by  seed.  • 


8o2  ECOLOGY 

Another  indication  of  autonomy  is  seen  in  the  geographic  distribution  of 
lichens.  As  a  class,  they  are  among  the  most  xerophytic  and  autophytic 
of  plants,  many  species  growing  on  the  driest  and  barest  of  rocks,  where 
few  other  plants  can  maintain  themselves.  Yet  lichens  are  made  up  of 
algae,  which  as  a  group  are  characteristically  hydrophytic,  and  of  fungi, 
which  as  a  group  are  characteristically  mesophytic  and  dependent; 
the  symbiotic  union  of  two  such  diverse  elements  appears  to  result  in  a 
form  widely  different  from  either,  and  more  resistant  and  independent 
than  is  to  be  found  in  almost  any  other  group  of  plants. 

The  nature  of  lichen  symbiosis.  —  The  parasitism  of  the  fungal  sym- 
biont  upon  the  algal  layer  is  undoubted,  but  there  are  various  theories 
concerning  the  relation  of  the  alga  to  the  fungus. 
A  common  theory  is  that  the  relationship  is  one 
of  helotism  or  slavery,   the  algal  symbiont  being 
represented  as  indifferent  to  the  fungus.     Another 
common    theory  is  that  of  reciprocal    parasitism, 
FIG. my.— Analgal    allied  to  which  is  the  recently  proposed  theory  of 
cell  of  a  lichen  (Cla-   endosaprophytism,  and  also  the  older  view  that  the 

donia  furcata),    which       j          fe     ,.^        ^  ^      ±      f  bod     ^  better 

is  closely  embraced  by  J  J 

hyphal  filaments  of  the  protected  than  when  separate  and  thus  are  enabled 
lichen  fungus;  highly  to  live  in  drier  habitats.  Still  another  view  is  that 

BORNET  Cd'  "'•  Fr°m    the  alga  is  merely  the  host  °f  an  ordinary  Parasite- 
The   parasitism   of    the   fungus    is    demonstrated 

clearly  by  a  number  of  facts,  such  as  the  close  embracement  of  algal 
cells  by  fungal  hyphae  (fig.  1117),  by  the  frequent  entrance  of  hyphae 
into  algal  cells,  by  the  occasional  development  of  haustoria,  by  the 
disorganization  and  subsequent  emptying  of  the  contents  of  many 
algal  cells,  and,  perhaps,  by  the  apparent  restriction  upon  algal  activity, 
which  is  evidenced  by  increased  vigor  when  released  from  symbiosis. 
As  for  the  algal  symbiont,  the  theory  of  endosaprophytism  on  fungal 
excreta  seems  most  tenable  in  view  of  the  fact  that  lichen  algae  are  de- 
cided mixophytes  which  thrive  particularly  on  peptones;  water  and 
salts  must  also  be  derived  through  the  medium  of  the  fungus. 

The  origin  of  lichens.  —  There  is  almost  no  experimental  knowledge 
concerning  the  origin  of  lichens,  and  most  of  the  common  species  appear 
to  be  well-defined  lichens  without  obvious  indications  of  their  evolution. 
In  some  instances  the  fungal  symbiont  lives  saprophytically  on  bark  or 
on  humus  or  parasitically  on  the  plant  which  gives  it  mechanical  support, 
as  well  as  on  its  algal  layer ;  such  species  are  most  abundant  in  the  tropics 


SAPROPHYTISM   AND    SYMBIOSIS  803 

(p.  659),  but  some  forms  of  temperate  regions  ( Usnea,  for  example) 
may  be  partially  parasitic  on  trees.  A  remarkable  tropical  lichen  is 
Cora  Pavonia,  in  which  the  fungal  symbiont,  one  of  the  Thelephoraceae, 
may  live  entirely  apart  from  algae,  or  symbiotically  either  with  the  alga, 
Chroococcus,  or  with  the  alga,  Scytonema,  the  body  form  differing  in  each 
case.  The  shape  of  this  lichen  varies  also  with  the  proportional  develop- 
ment of  the  symbionts,  the  so-called  genus  Dictyonema  representing 
a  bracket-like  form  in  which  the  fungal  element  dominates,  while  the 
so-called  genus  Laudatea  represents  a  felt  of  filaments  in  which  the  alga 
(i.e.  Scytonema)  dominates.  Botrydina  vulgaris,  which  commonly  is 
regarded  as  an  alga,  is  invested  by  fungal  hyphae,  and  is  thought  by 
some  investigators  to  be  a  primitive  lichen.  A  lichen  such  as  Cora  sug- 
gests that  the  fungal  symbionts  of  other  lichens  may  once  have  had 
facultative  relations  with  algae,  and  also  that  the  body  form  is  very 
likely  to  have  resulted  from  the  symbiosis. 

Green-celled  animals.  —  Among  the  most  remarkable  of  organisms  are  certain 
green-celled  animals  (such  as  Spongilla,  Hydra,  and  Convoluta),  which  in  some 
important  respects  resemble  lichens.  It  has  long  been  believed  that  the  green  cells 
represent  enslaved  algae,  though,  in  contrast  with  the  algal  symbionts  of  lichens, 
separate  cultivation  generally  is  impossible.  Indeed,  the  resemblance  to  algae  is 
much  less  than  in  lichens,  consisting  of  little  more  than  the  presence  of  chlorophyll ; 
usually  even  the  nuclei  are  absent  from  the  cells,  and  the  chlorophyll  may  be  dis- 
seminated through  the  cell  sap  instead  of  being  in  plastids.  Much  light  has  been 
thrown  on  these  strange  organisms  by  a  careful  study  of  Convoluta  roscoffensis,  one 
of  the  flat-worms.  This  animal  is  colorless  when  hatched,  but  in  the  first  few  days 
it  becomes  infected  by  motile  algae  (appearing  to  belong  near  Carteria),  which  seem 
to  exhibit  prochemotactic  reactions  to  substances  in  the  egg  capsules.  At  first  the 
animal  has  a  mouth  and  feeds  like  other  flat-worms,  but  soon  the  opening  becomes 
closed  and  the  worm  henceforth  is  completely  dependent  upon  its  symbiotic  alga; 
even  excretory  organs  are  wanting  in  the  adult,  it  being  supposed  that  the  algae 
utilize  the  excreta  as  a  source  of  nitrogenous  food.  If  the  appropriate  alga  is 
absent  in  a  culture  of  the  worms,  the  animals  soon  die,  even  in  the  continued  presence 
of  such  food  as  they  previously  have  used.  After  imprisonment  the  algae  lose  their 
motility,  though  active  cell  division  takes  place  for  some  time.  The  new  cells  have 
no  cell  walls,  and  eventually  they  become  distorted  ;  finally  the  nuclei  disappear  and 
all  activity  ceases.  Such  modified  algae  are  unable  to  live  apart  from  the  worm, 
and  the  worm  cannot  live  apart  from  the  algae;  indeed,  the  algae  of  adult  worms 
cannot  infect  young  worms,  so  that  when  they  die,  they  leave  no  progeny.  All 
doubt  as  to  the  reality  of  symbiosis  has  been  removed  by  synthesizing  the  composite 
organism  from  pure  cultures  of  the  alga  and  the  worm,  precisely  as  lichens  previously 
were  synthesized  from -cultures  of  algae  and  fungi.  While  the  exact  nutritive  inter- 
relations are  not  certainly  known,  it  has  been  shown  that  the  mature  animal  with  its 
green  cells  is  prophototropic,  and  that  starch  is  manufactured  and  oxygen  given  off 


804  ECOLOGY 

in  the  sunlight.  Since  the  animal  has  no  other  source  of  food,  it  clearly  is  parasitic 
on  the  alga.  At  first  the  animal  uses  the  food  (probably  sugar)  manufactured  by 
the  green  cells,  but  ultimately  it  destroys  the  cells  themselves  and  brings  on  thereby 
its  own  destruction.  While  it  is  possible  that  the  alga  utilizes  substances  in  the 
animal  (in  which  event  the  relation  is  one  of  reciprocal  parasitism),  it  is  quite  as  likely 
that  the  relation  is  to  be  regarded  as  a  sort  of  destructive  helotism,  destructive  be- 
cause the  enslaved  organism  is  weakened  and  finally  destroyed.  In  another  flat- 
worm,  Convoluta  paradoxa,  the  symbiotic  alga  is  a  unicellular  brown  species. 
Here  the  parasitism  of  the  animal  seems  less  obligate  than  in  C.  rosco/ensis,  since 
it  continues  to  use  its  mouth  in  taking  food,  even  after  the  symbiotic  algae  are  well 
established  in  its  body.  However,  the  animal  dies  if  it  is  kept  in  the  dark  until 
the  algae  are  destroyed.  While  the  origin  of  such  symbiosis  is  unknown,  it  may  be 
noted  that  in  Noctiluca,  one  of  the  infusorians,  symbiosis  with  green  algae  is  faculta- 
tive rather  than  obligate,  thus  suggesting  a  more  primitive  condition.  Some  of  the 
sea  anemones  contain  algae  which  are  believed  to  be  of  nutritive  importance  to  the 
animals,  since  the  reactions  of  the  latter  to  light  resemble  the  reactions  of  algae 
rather  than  those  of  such  sea  anemones  as  are  without  chlorophyll.  It  is  likely 
that  the  plants  utilize  the  carbon  dioxid  given  off  by  the  animals,  and  that  the  animals 
in  turn  utilize  the  oxygen  given  off  by  the  plants. 


CHAPTER  V  — REPRODUCTION  AND  DISPERSAL 
I.  REPRODUCTIVE  BEHAVIOR  IN  THE  SEEDLESS  PLANTS 

General  remarks.  —  The  process  by  which  organisms  give  rise  to 
others  of  their  kind  is  known  as  reproduction.  The  essential  element 
in  reproduction  is  the  organization  of  a  cell  or  a  group  of  cells,  which, 
if  detached,  possesses  a  capacity  for  independent  development,  and  hence 
may  be  called  offspring.  Closely  associated  with  reproduction  is  dis- 
persal, which  makes  possible  the  development  of  organisms  in  new 
territory,  and  without  which  reproduction  would  be  of  small  significance.1 
Detachable  structures,  however  produced,  if  capable  of  dispersal,  may 
be  called  disseminates,  and  it  is  obvious  that  upon  such  capacity  for  de- 
tachability  and  for  subsequent  mobility,  the  effectiveness  of  dispersal 
and  therefore  the  success  of  a  species  must  largely  depend. 

Most  plants  give  rise  to  many  new  individuals  within  their  lifetime, 
but  only  a  few  of  these  come  to  maturity  and  have  progeny.  The  vast 
majority  of  plant  disseminules  fail  to  lodge  in  places  suitable  for  de- 
velopment, while  of  those  that  make  a  start,  but  a  very  few  ever  reach 
maturity.  The  preemption  of  S£ace  by  other  plants,  the  submergence 
of  the  weaker  individuals,  and  untoward  physical  conditions  cause  the 
destruction  of  most  plant  offspring  and  prevent  the  otherwise  rapid 
advance  of  any  given  species  over  the  face  of  the  earth.  Three  kinds 
of  reproduction  may  be  distinguished,  each  with  its  characteristic  dis- 
seminules, namely,  vegetative  reproduction  or  propagation,  reproduction 
by  asexually  formed  spores,  and  sexual  reproduction. 

Vegetative  reproduction.  —  General  characteristics.  —  In  vegetative 
reproduction  or  propagation,  new  plants  are  formed  in  connection  with 
the  vegetative  organs,  and  the  offshoots,  sometimes  known  as  propa- 
gules,  more  or  less  resemble  the  parts  from  which  they  issue.  Vegetative 
reproduction  is  associated  with  periods  of  activity,  while  other  forms  of 
reproduction  commonly  terminate  such  periods.  Vegetative  dissemi- 

1  Dispersal  without  reproduction,  though  a  conspicuous  feature  in  animals,  is  relatively 
rare  in  plants;  however,  it  is  illustrated  by  certain  motile  algae  (as  Chlamydomonas  and 
Volvox,  figs.  21-29)  and  bacteria  (figs.  14-20),  and  also  by  the  amoeboid  movements  of 
myxomycete  plasmodia  (fig.  3). 

805 


8o6  ECOLOGY 

nules  differ  from  most  other  sorts  in  the  relative  absence  of  protective 
structures. 

Vegetative  reproduction  in  the  algae.  —  The  simplest  form  of  repro- 
duction is  by  fission  or  cell  division,  and  is  well  illustrated  by  various 
unicellular  algae,  in  which  the  mature  cell  divides,  producing  two  or 
more  new  cells,  which,  whether  cohering  or  becoming  detached,  repre- 
sent new  plant  individuals  (figs.  858,  4,  34).  Inmost  unicellular  species 
the  entire  body  takes  part  in  propagation,  so  that  the  disappearance  of 
the  adult  organism  necessarily  is  coincident  with  the  development  of  its 
progeny,  though  the  original  cell  wall  may  remain  for  some  time.  Simple 
fission  of  this  character  is  the  only  form  of  reproduction  in  unicellular 
blue-green  algae  and  in  some  green  algae  (as  Pleu rococcus) . 

Many  algae  are  filamentous,  since  division  takes  place  only  in  parallel 
planes,  and  since  the  newly  formed  cells  cohere  (figs.  859,  6).  While 
such  filaments  have  a  certain  individuality,  the  cells,  at  least  in  the 
lower  algae,  are  essentially  independent,  and  therefore  to  be  regarded 
as  potential  individuals;  hence  as  in  unicellular  forms,  cell  division  may 
here  be  called  vegetative  reproduction,  whether  or  not  the  new  cells 
become  detached  from  the  old.  True  filamentous  forms  differ  from 
unicellular  species  in  that  the  adult  does  not  necessarily  disappear  as  its 
vegetative  progeny  develops.  In  the  higher  filamentous  algae,  cell 
coherence  becomes  a  fixed  feature,  and  the  individuality  of  single  cells 
is  less  marked,  so  that  commonly  a  filament  as  a  whole  rather  than  one 
of  its  cells  is  regarded  as  a  plant.  Here  the  propagules  rarely  are  single 
cells,  but  rather  pieces  of  filaments  that  become  detached.  Even  with- 
out such  detachment,  however,  filamentous  algae  (such  as  Spirogyra) 
by  continued  elongation  may  spread,  so  as  to  fill  a  pond  in  a  relatively 
short  time;  branched  forms  like  Cladophora  (fig.  63)  would  seem  partic- 
ularly suited  for  rapid  propagation  of  this  sort.  In  many  blue-green 
algae  the  filaments  break  in  rather  definite  places,  the  limits  of  the  new 
filaments,  the  so-called  hormogonia,  being  determined  by  cells  differing 
from  the  rest,  and  known  as  heterocysts  (fig.  8). 

Vegetative  reproduction  in  bacteria,  fungi,  and  lichens.  —  Bacteria 
reproduce  only  by  fission,  yet  they  increase  more  rapidly  than  does  any 
other  group  of  plants.  The  possibility  of  rapid  vegetative  increase 
among  the  fungi  is  well  illustrated  by  the  growth  of  molds  in  moist 
chambers,  where  the  hyphae  spread  quickly  in  all  directions  from  the 
original  center  (fig.  1078).  Often  the  fungus  mycelium  dies  at  its 
original  growth  center,  perhaps  because  of  the  exhaustion  of  its  food 


REPRODUCTION    AND   DISPERSAL  807 

supply,  but  quite  as  likely  because  of  the  inhibitory  effect  of  its  own 
excretions.  Subsequently  the  living  mycelium  forms  an  obvious  ring 
of  constantly  increasing  circumference,  but  whose  thickness  may  not 
increase  on  account  of  the  death  of  the  inner  hyphae  pari  passu  with 
the  advance  of  those  outside.  Such  rings  are  common  on  pots  in  moist 
greenhouses.  In  some  of  the  agarics,  circles  of  toadstools,  which  are 
known  as  fairy  rings,  arise  from  subterranean  mycelial  rings,  producing 
a  striking  effect.  Sometimes  fairy  rings  recur  from  a  given  mycelial 
center  for  many  years;  in  a  colony  of  Hydnum  suaveolens  that  was 
observed  for  nine  years,  the  diameter  of  the  ring  increased  from  seven- 
teen to  twenty-one  meters,  whence  the  age  of  the  colony  was  estimated 
at  about  forty-five  years.  Sometimes  similar  circles  of  reproductive 
bodies,  arising  from  rings  of  hidden  mycelia,  are  associated  with  parasitic 
fungi  (e.g.  Puccinia  Pyrolae).  Lichen  thalli  commonly  spread  radially 
from  the  original  center  of  establishment,  the  advancing  edge  being 
lobate  by  reason  of  differential  growth  (fig.  mi);  frequently  the  older 
portions  die  as  the  newer  parts  spread  out  radially,  producing  living 
rings  or  bands,  as  in  fungi. 

Vegetative  reproduction  in  the  bryophytes.  —  Thallose  liverworts,  such 
as  Marchantia  and  Riccia,  spread  radially  from  their  original  growth 
center  (figs.  742,  743),  after  the  manner  of  lichens.  As  the  branches 
radiate  outward  and  the  posterior  portions  die,  a  number  of  individuals 
may  arise  from  one  by  isolation;  it  has  been  shown  that  liverwort  frag- 
ments which  are  only  two  millimeters  in  diameter  can  develop  into  a  plant. 
Essentially  similar  is  the  propagation  of  foliose  liverworts  and  of  creeping 
mosses.  Mosses  increase  vegetatively  to  a  notable  extent  through  the 
activity  of  their  protonema,  which  is  composed  of  branched  alga-like 
filaments  that  creep  along  the  soil  surface  much  as  do  rhizomes.  The 
protonemal  filaments  bear  buds  that  grow  into  leafy  plants,  so  that  the 
area  occupied  by  moss  colonies  is  subject  to  constant  radial  extension. 
Mosses  possess  a  wonderful  capacity  for  propagation,  almost  any  part 
of  the  leaves  or  stems  (either  gametophyte  or  sporophyte)  being  capable 
of  giving  rise  to  protonemal  filaments  under  suitable  conditions. 

Vegetative  reproduction  in  the  vascular  plants  is  of  great  significance,  but  has 
been  considered  in  connection  with  roots  and  leaves,  and  particularly  in  connection 
with  stems  (p.  667). 

Gemmation.  —  There  is  another  kind  of  reproduction  which  generally 
is  regarded  as  vegetative,  although  it  grades  insensibly  into  reproduction. 


8o8 


ECOLOGY 


1118  1119 

FIGS.  1118,  1119.  —  Liver- 


by  asexual  spores.     Gemmation  may  be  defined  as  the  organization  of 
vegetative  buds  that  readily  become  detached  from  the  parent   plant; 

such  disseminules  are  called  gemmae.  The 
simplest  gemmae  are  unicellular,  and  they 
differ  from  asexual  spores  chiefly  in  the 
absence  of  a  protective  wall  and  of  the 
resting  period  usually  associated  therewith, 
although  there  exist  all  intergradations  be- 

wort  gemmae:  1118,  a  thailus    tween  the  two.    Simple  gemmae  of  this  char- 
lobe  of   Lunularia   vtdgaris,    acter  are  found  in  yeast  (figs.  168-173),  and 

also  in  Mucor  and  in  Vaucheria  *—'*• 

1119,  a  single  gemma  from  a  Some  species  of  liverworts  also  have  uni- 
cupule  of  Marchantia  polymor-  cellular  gemmae  on  the  leaf  margins;  in 
ed"  Aneura  the  gemmae  are  two-celled,  and  in 
Marchantia  and  Lunularia  they  are  multicellular  and  are  borne  in  clus- 
ters in  special  cupules  (figs.  1118,  1119). 

Multicellular  gemmae  occur  also  on  various  mosses  (as  Georgia  pellucida),  on 
fern  prothallia,  and  on  some  algae  (as  Sphacelaria  and  Char  a).  The  soredia  of 
lichens  (figs.  1114-1116)  also  may  be  classed  with  gemmae.  Certain  structures  in 
the  vascular  plants,  such  as  the  gemmae  of  Lycopodium,  the  leaf  bulbils  of  ferns, 
the  stem  bulbils  of  lilies,  and  the  inflorescence  bulbils  of  the  onion  (p.  902)  are  com- 
parable to  the  gemmae  of  the  lower  plants. 

Sclerotia.  —  In  autumn  the  mycelium  of  the  ergot  fungus  (Claviceps)  becomes 
enveloped  in  a  dense  and  relatively  impermeable  protective  layer  of  dark,  thick- 
walled  cells,  within  which  the  vegetative  hyphae  remain 
dormant  over  winter ;  the  entire  structure,  which  is  richly 
packed  with  food,  is  called  a  sclerotium  (fig.  1120).  In 
spring  the  sclerotium  germinates,  and  ordinary  vegetative 
activity  is  resumed.  Somewhat  similar  to  the  sclerotia  of 
ergot  are  those  of  Peziza  sderotiorum  and  of  various  other 
fungi.  Many  fungi  (e.g.  Sclerotinia)  have  subterranean 
tuber-like  sclerotia  richly  packed  with  food,  which  endure 
through  unfavorable  periods,  and  other  forms  have  tough 
sclerotial  strands  resembling  shoestrings.  In  the  myxomy- 
cetes  the  plasmodium  or  a  part  of  it  may  become  encysted 
into  a  sclerotial  mass  and  remain  dormant  even  for  years. 
While  strictly  vegetative  tissue  is  involved  in  the  formation 
of  sclerotia,  they  agree  with  spores  and  seeds  in  being  formed 
at  the  close  of  vegetative  periods  and  in  being  fitted  for 
existence  in  a  dormant  state  during  severe  periods.  Com- 
parable to  sclerotia  are  the  resting  cells  of  bacteria,  the  thick- 
walled  resting  cells  of  Nostoc  which  are  closely  packed  with  food,  the  starchy  tubers 
pf  Chara,  and  the  subterranean  resting  buds  of  liverworts  and  mosses  (fig.  251). 


FIG.  1 1 20.  —  A 
sclerotium  (s)  of  the 
ergot  fungus  (Clavi- 
ceps purpurea),  grow- 
ing from  a  spikelet  of 
the  sand-reed  (Am- 
mo phUa  arenaria). 


REPRODUCTION   AND   DISPERSAL  809 

Auxospores.  —  In  the  diatoms  vegetative  reproduction  takes  place  by  longitudinal 
splitting,  but  each  new  individual  often  is  shorter  than  the  last,  because  it  is  formed 
within  the  old  and  rigid  silicious  wall.  In  time  progressive  diminution  ceases,  and 
the  protoplast  escapes,  whereupon  it  enlarges  to  the  original  size  and  again  becomes 
incased  by  rigid  silicious  walls.  The  enlarging  protoplast  is  called  an  auxospore. 

The  significance  of  vegetative  reproduction.  —  Vegetative  propagation 
is  the  most  universal  kind  of  reproduction.  Some  plants  (as  the  bac- 
teria and  lower  algae)  have  no  other  kind,  while  very  few  plants  (e.g. 
some  annuals,  biennials,  and  trees)  are  altogether  without  it.  Many 
plants  that  are  capable  of  producing  spores  or  sex  organs  nevertheless 
spread  almost  wholly  by  vegetative  means;  among  such  plants  are  many 
mosses,  some  liverworts  (as  Lunularid),  and  even  some  of  the  higher 
plants  (as  the  duckweeds).  In  far  northern  regions  many  plants  are 
said  to  reproduce  as  a  rule  only  vegetatively,  the  summer  being  too  short 
for  seed  production.  Even  those  plants  that  fruit  regularly  usually 
spread  much  more  by  vegetative  propagation  than  by  spores  or  seeds. 
Thus  there  can  be  no  doubt  that  vegetative  reproduction  is  the  chief 
factor  in  the  maintenance  of  species  and  in  the  enlargement  of  their  areas. 

The  chief  disadvantage  associated  with  vegetative  reproduction  is  that 
prppagules  rarely  are  fitted  for  distant  dispersal.  Hence  the  invasion 
of  new  areas  by  this  means  alone  is  slow;  the  ultimate  establishment  of 
a  species  by  vegetative  reproduction  in  a  distant  region  is  even  im- 
possible unless  favorable  habitats  are  continuous,  since  propagules 
rarely  are  able  to  cross  barriers.  For  example,  the  rhizomes  of  meso- 
phytes  (such  as  the  Solomon's  seal  and  various  ferns)  are  unable  to 
migrate  over  bodies  of  water  or  dry  ridges,  although  such  migration  may 
be  accomplished  quickly  by  most  spores  and  by  many  seeds.  Some- 
times, however,  propagules  are  true  disseminules,  notably  among  the 
water  plants,  where  portions  of  plants  may  become  detached  and  float 
for  great  distances,  thus  equaling  seeds  in  mobility  and  in  the  wide- 
spread invasion  of  new  regions.  Among  land  plants  the  distant  dispersal 
of  vegetative  disseminules  is  confined  chiefly  to  gemmae;  the  minute 
gemmae  of  various  liverworts  may  be  scattered  for  some  distance  by 
wind,  and  lichen  soredia  are  scattered  almost  as  effectively  as  are  spores. 

Reproduction  by  asexually  formed  spores.  —  General  characteristics.  — 
A  sexually  formed  spores  l  commonly  are  unicellular  structures  (multinu- 
cleate  in  Vaucherid),  produced,  as  a  rule,  by  specialized  spore-bearing 
organs  (sporangia,  etc.;  see  Part  I).     Generally  they  differ  from  gemmae 

1  Often  these  are  called  for  convenience  asexual  spores  or  simply  spores. 


8io 


ECOLOGY 


in  their  unicellular  nature,  in  their  production  by  specialized  organs,  and 
in  their  capacity  for  endurance,  which  often  is  increased  by  the  presence 
of  thick  protective  walls;  however,  hard  and  fast  lines  are  not  to  be 
drawn,  since  some  spores  are  incapable  of  enduring  severe  periods,  while 
gemmae  may  be  unicellular  or  borne  by  special  organs. 

Reproductive  structures  generally  have  one  or  more  of  three  char- 
acteristics: capacity  for  increasing  the  number  of  individuals  in  a  species 
(which  is,  of  course,  the  primary  feature  of  reproduction) ;  capacity  for 
endurance  through  severe  periods;  and  capacity  for  dispersal.  Asexual 
spores  are  efficient  in  all  three  respects,  thus  contrasting  with  propagules, 
which  have  been  seen  to  be  relatively  ineffective  as  disseminules  and 
often  unfitted  for  endurance,  though  they  are  the  most  efficient  of  all 
means  of  multiplying  individuals.  Asexual  spores  occur  in  nearly  all 
plant  groups,  though  they  are  unknown  in  various  algae  (as  in  the  Con- 
jugales,  Fucales,  and  Charales),  and  are  practically  absent  in  some 
higher  plants  (as  in  various  mosses  and  in  the  duckweeds). 

Asexual  spores  in  the  algae.  —  The  most  representative  asexual  spores 
among  the  algae  are  the  zoospores  or  swarm-spores,  which  differ  from 
most  spores  in  being  without  protective  walls,  and 
whose  chief  distinguishing  character  is  the  power 
of  locomotion  in  water;  usually  they  move  by 
means  of  variously  arranged  cilia,  which  may  be 
single  (as  in  Botrydium,  fig.  92),  two  (as  in  II y- 
drodidyon,  fig.  1121),  four  (as  in  Ulothrix,  fig. 
1133),  or  many  (as  in  Oedogonium  and  Vaucheria, 
figs.  76,  96).  While  most  characteristic  of  green 
algae,  swarm-spores  occur  in  some  of  the  brown 
algae  (as  in  Ectocarpus,  fig.  121).  Zoospores  are 
among  the  most  efficient  of  reproductive  struc- 
tures, partly  because  commonly  they  are  produced 
in  large  numbers,  but  particularly  because  they 
differ  from  almost  all  other  disseminules  in  ex- 
hibiting directive  dispersal.  For  example,  they  are 
prophototactic,  hence  they  usually  move  to  a  well- 
lighted  situation  where  they  may  germinate  into 
new  plants  under  favorable  conditions.  The  lack  of  protective  walls  is 
hardly  a  disadvantage,  since  zoospores  are  not  exposed  to  transpiration, 
nor  are  they  obliged  to  live  over  unfavorable  seasons.  In  the  red  algae 
true  zoospores  are  wanting,  though  a  few  species  have  spores  that  exhibit 


FIG.  1 12 1.  —  A  zoo- 
spore  or  swarm  spore 
of  the  water-net  (Hy- 
drodictyori) ;  note  the 
two  cilia  which  effect 
locomotion;  highly 
magnified.  —  After 

TlMBERLAKE. 


Jr" 


REPRODUCTION   AND   DISPERSAL 


811 


amoeboid  movements;  much  more  characteristic  are  non-motile  car- 
pos pores  and  tetras pores  (figs.  150,  151),  which,  like  zoospores,  are 
devoid  of  protective  walls.  Non-motile  spores  may  occur  also  in  the 
green  algae  (as  in  the  aplanospores  of  Botrydium,  fig.  93). 

Asexual  spores  in  the  fungi.  —  Perhaps  the  culmination  of  asexual 
spore  development  occurs  in  the  fungi.  A  few  forms  that  grow  in  water 
or  in  wet  places  have  ciliated  zoospores  (as  in  Saprolegnia,  fig.  156); 
in  certain  myxomycetes  there  are  zoospores  which  swim  for  a  time,  and 
then  lose  their  cilia  and  creep  with  an  amoeboid  movement.  In  general, 
however,  fungus  spores  are  not  self-motile,  and  are  invested  with  con- 
spicuous walls.  They  may  be  borne  within  a 
special  spore-bearing  organ,  for  example,  a  spo- 
rangium (as  in  Mucor,  fig.  1122),  or  an  ascocarp 
(as  in  Peziza,  figs.  175,  176),  or  they  may  be 
developed  externally,  as  in  the  conidia  of  Peni- 
cillium  (fig.  179)  and  in  basidios pores1  (fig. 
201). 

Fungus  spores  commonly  are  dispersed  by 
wind,  and  their  minute  size  and  their  resistance 
to  wetting  make  possible  the  remarkable  effi- 
ciency of  this  agent;  even  the  very  slightest 
movements  of  the  air  are  sufficient  to  initiate 
dispersal.  Many  species  of  fungi  are  common 
to  widely  separated  regions,  and  it  is  thought 
that  this  cosmopolitanism  is  due  in  large  part  to 
the  effectiveness  with  which  their  spores  are  dis- 
persed by  wind.  The  abundance  of  spores  and  the  ease  with  which  they 
are  carried  is  shown  by  the  readiness  with  which  cultures  of  various 
fungi  may  be  made  anywhere  by  exposing  to  the  air,  bread  or  cheese, 
properly  moistened,  so  as  to  insure  good  conditions  for  germination. 
Fungi  surpass  all  other  plants  in  the  number  of  new  individuals  that 
may  be  produced  from  a  single  plant  by  asexual  spores.  A  single  large 
puffball  (as  Lycoperdon  giganteum)  may  produce  several  trillion  spores, 
and  in  other  large  fungi  their  number  may  run  well  into  the  billions. 
The  production  of  spores  in  such  great  numbers  is  advantageous,  since 
generally  only  a  single  spore  in  many  millions  falls  in  a  place  where  it 
can  develop  into  a  plant. 


FIG.  1 122.  —  The  spo- 
rangiumof  a  mold  (Mucor), 
showing  the  columella  (c) 
and  numerous  spores  (s); 
highly  magnified.  —  From 
COULTER  (Part  I). 


1  Basidiospores,  however,  though  actually  external,  in  the  mushrooms  are  considerably 
protected  by  the  fruit  body  on  which  they  develop. 


8l2 


ECOLOGY 


Various  features  of  structure  or  habit  facilitate  spore  dispersal.  As 
previously  noted,  many  fungi  bear  spores  externally,  so  that  they  are 
readily  blown  away  as  soon  as  they  are  abstricted.  In  the  fleshy  fungi, 
the  spore-containing  organs  often  are  borne  on  conspicuous  apogeo- 
tropic  stipes,  which  thus  elevate  the  spores  into  a  good  position  for  wind 
dispersal  (figs.  1078,  2,  197).  In  many  of  these  forms  the  spores  are  dis- 
charged from  the  gills,  after  which  they  drop  into  positions  where  they 
may  be  wafted  off  by  air  currents.  In  Coprinus  (figs.  198,  199),  which 
has  a  cylindrical  fruit  body,  the  spores  mature  first  in  the  lower  part, 
which  then  curves  outward,  and  hence  does  not  hinder  the  dispersal 
of  those  which  ripen  later.  Where  spores  are  borne  within  sporangia 
or  similar  organs,  there  often  are  no  special  features  which  facilitate 
spore  removal,  it  being  necessary  for  the  enveloping  organs  to  rot  away 

before  the  spores 
can  be  dispersed. 
In  some  cases  there 
is  definite  dehis- 
cence,  as  in  Geaster, 
where  the  sporan- 
gial  wall  (peridium) 
has  two  layers,  of 
which  the  outer 
splits  into  star- 
shaped  segments 
(whence  the  com- 
mon name,  earth 
star),  while  the  in- 
ner has  an  apical 
opening  (fig.  1124);  in  the  related  puff  balls  the  outer  layer  breaks 
irregularly.  In  Geaster  the  hyphae  are  arranged  at  right  angles  to  the 
surface  in  the  inner  (i.e.  upper  when  open)  part  of  the  ray  and  parallel 
to  the  surface  in  the  outer  part.  Hence  in  moist  weather  the  inner  part 
absorbs  the  more  water  and  the  rays  open  (fig.  1124),  while  they  close  in 
dry  weather  (fig.  1123),  since  the  inner  part  loses  the  more  water.  This 
hygroscopic  mechanism  has  been  thought  to  facilitate  spore  dispersal ; 
the  dry  closed  structure  is  bowled  along  by  the  wind  like  a  tumbleweed, 
and  the  rain  washes  out  spores  from  the  opened  structure. 

In  a  few  fungi,  spores  are  scattered  by  agents  other  than  wind.  In  Pilobolus 
(fig.  630)  the  columella  of  the  sporangium  ultimately  bursts  by  reason  of  increasing 


1123 


FIGS.  1123,  1124.  —  An  earth  star  (Geaster  hygrometricus); 

1123,  a  fructification,  as  seen  in  dry  weather,  the  peridium 
rays  (r)   being  incurved    about   the  spore-bearing   portion; 

1124,  a  fructification,  as  seen  in  moist  weather,  the  peridium 
rays  being  expanded;   note  the  aperture  (a)  through  which 
the  spores  escape. 


REPRODUCTION   AND    DISPERSAL 


turgor,  whereupon  the  escaping  water  tears  loose  the  sporangium  and  expels  it  with 
the  enclosed  spores  for  some  distance.  In  a  somewhat  similar  fashion  are  expelled 
the  conidia  of  Entomophthora  and  the  ascospores  of  Ascobolus  and  of  Peziza  repanda. 
In  the  ergot  fungus  (Claviceps)  a  sweetish  substance,  known  as  honey  dew,  is  se- 
creted as  the  conidia  ripen,  and  insects  visiting  the  fungus  for  the  honey  dew  scatter 
the  spores.  In  the  stinkhorn  fungus  (Phallus  impudicus)  the  spore-bearing  portion 
deliquesces  into  a  vile-smelling 
mass  that  attracts  flies,  which 
scatter  the  spores.  Doubtless 
many  fungus  spores  also  ad- 
here to  the  slimy  surface  of 
slugs  and  thereby  are  scattered. 
Flies  are  among  the  most  effi- 
cient scatterers  of  spores,  which 
become  attached  te  various 
parts  of  the  body,  and  cccur 
abundantly  in  the  excreta  ;  the 
spores  or  propagules  of  more 
than  fifty  species  of  fur  gi  and 
bacteria  have  been  found  in 
a  single  fly  speck. 

Many  fungus  spores  are  able 
to  endure  severe  conditions. 
For  example,  the  spores  of 
Mucor  and  of  Aspergillus  have 
been  dried  for  two  years,  after 
which  they  were  exposed  for 
three  weeks  to  a  temperature 
of  —  1 80°  C.,  and  for  three  days 
to  —  253°  C.,  without  impair- 
ing their  capacity  for  germi- 
nation. Desiccated  bacteria 
have  been  known  to  retain 
their  vitality  for  nearly  a  hun- 
dred years.  It  is  concluded 
from  such  experiments  that  all 
vital  activity  may  be  suspended 
for  long  periods  of  time  (p.  909). 
In  part  this  endurance  is  due 
to  unexplained  features  in  the  resting  protoplasm,  but  there  are  also  many  instances 
of  protective  structures  or  habits.  In  most  ascomycetes  the  spores,  though  thin- 
walled,  are  protected  within  the  ascocarps  (as  in  lichens  and  mildews,  figs.  181, 182), 
while  in  many  hymenomycetes  the  thin-walled  basidiospores  are  protected  by  the 
pileus  ;  some  of  the  so-called  bracket  fungi  are  hard  and  woody  and  capable  of 
enduring  the  winter.  In  the  heteroecious  rusts  there  are  borne  in  spring  and  sum- 
mer basidiospores  (fig  194),  aecidiospores  1  (fig.  196),  and  uredospores  (fig.  1125), 

1  Sometimes  aecidiospores  and  uredospores  are  regarded  as  sexually  formed  spores. 


FIGS.  1125-1127.  —  Spores  of  the  wheat  rust  (Puc- 
cinia  gramims):  1125,  uredospores;  1126,  young 
teleutospores ;  1127,  mature  tele utos pores;  note  that 
the  uredospores  are  one-celled,  and  the  teleutospores 
two-celled;  highly  magnified;  1125,  1127  from 
COULTER;  1126  from  CHAMBERLAIN;  (Part  I). 


8i4  ECOLOGY 

all  with  relatively  thin  cell  walls,  while  toward  the  close  of  the  season  there  are 
developed  teleutospores  (figs.  1126,  1127),  which  are  thick-walled  and  are  capable 
of  enduring  the  winter. 

Asexual  spores  in  the  bryophytes.  —  In  most  liverworts  and  mosses 
there  is  a  well-defined  alternation  of  generations  (p.  822),  spores  being 
characteristic  of  one  generation,  the  sporophyte,  and  sex  organs  being 
equally  characteristic  of  another  generation,  the  gametophyte.  The 
spores  are  scattered  chiefly  by  the  wind,  their  minute  size  and  the 
generally  stalked  and  therefore  elevated  capsules  facilitating  such  dis- 
persal (figs.  977,  231,  254).  The  spores  contain  chlorophyll,  so  that 
independence  is  possible  from  the  outset,  if  the  sporelings  (i.e.  the  ger- 
minating spores)  are  exposed  to  light.  Sometimes  (as  in  Riccia  and 
Phascum)  the  spores  are  exposed  to  dispersing  agents  only  upon  the 
decay  of  the  capsule  wall,  but  more  commonly  there  is  definite  dehis- 
cence.  In  the  Jungermanniales  and  in  Anthoceros  the  capsule  wall 
splits  into  valves  (figs.  235,  239),  and  in  the  Marchantiales  and  in  most 
mosses  there  is  a  lid  or  operculum  (fig.  250).  Most  moss  capsules  are 
fringed  toward  the  tip  with  a  peristome  (figs.  263,  264),  whose  hygro- 
scopic teeth  open  when  the  weather  is  dry  and  close  when  it  is  moist; 
these  movements  effect  the  detachment  of  the  operculum,  and  probably 
are  of  some  value  in  facilitating  the  removal  of  spores  from  the  capsule. 
In  most  liverworts  long,  fiber-like,  spirally  thickened  bodies,  known  as 
elaters  (fig.  230),  occur  among  the  spores,  and,  like  the  peristome  teeth 
of  mosses,  they  exhibit  hygroscopic  movements  which  are  thought  to 
facilitate  spore  removal.  As  a  rule,  the  spores  of  liverworts  soon  lose 
their  capacity  for  germination,  but  the  spores  of  mosses  may  retain  their 
vitality  for  a  long  time;  cases  are  on  record,  where  moss  spores  have 
germinated,  after  having  lain  dry  in  a  herbarium  for  fifty  years. 

Asexual  spores  in  the  pteridophytes.  —  In  the  Filicales  the  spores  com- 
monly are  borne  in  sporangia  on  the  backs  of  ordinary  foliage  leaves 
(figs.  1128,  1129),  but  in  some  cases  (as  in  Onoclea  and  Osmunda) 
special  leaf  regions  or  entire  leaves  are  spore-bearing,  while  other  leaf 
regions  or  entire  leaves  are  foliage  organs;  comparable  to  the  latter  are 
the  Ophioglossales  (figs.  352-354).  In  Equisetum  the  sporangia  are 
borne  on  a  special  structure,  the  strobilus  (fig.  332),  and,  as  in  liverworts, 
there  are  elaters  (figs.  337,  338)  which  assist  to  some  extent  in  dispersal. 
In  Lycopodium  the  sporangia  may  be  arranged  in  the  axils  of  foliage 
leaves  (fig.  265)  or  in  a  strobilus  (fig.  266).  In  the  above  pteridophytes 
all  the  spores  are  alike,  that  is,  homosporous,  but  in  the  water  ferns 


REPRODUCTION    AND    DISPERSAL 


(i.e.  Marsilea,  Salvinia,  and  Azolld)  and  in  Selaginella  and  Isoetes  there 
are  two  kinds  of  spores,  namely,  small  spores  or  microspores,  and  large 
spores  or  megas pores;  such  a  condition  is  known  as  heterospory  (fig.  303). 
Upon  germination  the  microspores  give  rise  to  male  plants  and  the 


FIGS.  1128,  1129.  —  Reproduction  by  asexual  spores  in  a  fern  (Aspidium);  1128,  a 
leaf  segment  (pinnule)  with  fruit  dots  (sori),  each  with  a  shield-shaped  cover  (indusium) ; 
1129,  a  cross  section  through  a  sorus,  showing  the  indusium  (i)  and  long-stalked  sporangia 
(s) ;  1 129  considerably  magnified.  —  After  WOSSIDLO. 

megaspores  to  female  plants,  whereas  the  spores  of  most  homosporous 
ferns  give  rise  to  plants  that  bear  both  male  and  female  organs. 

The  spores  of  most  pteridophytes  are  scattered  by  the  wind,  and  they 
are  well  fitted  for  such  dispersal  by  their  small  size,  by  their  resistance 
to  wetting  (particularly  in  Lycopodium),  and  by  their  elevation  upon 
foliage  leaves  or  special  stalks  (figs.  266,  332,  353).  Fern  sporangia 
dehisce  in  a  somewhat  complicated  manner  (p.  351),  a  ring  of  dead 


FIGS.  1130-1132.  —  Dehiscence  of  a  sporangium  in  a  fern  (Polystichumacrostichoides): 
1130,  the  sporangium  cracked;  a,  the  annul  us;  1131,  position  of  reversal,  exposing  the 
spores;  1132,  position  after  recoil,  the  sporangium  having  been  emptied;  highly  magnified. 
—  After  ATKINSON. 

tissue,  the  annulus,  springing  back  and  releasing  the  spores  when  a 
certain  stage  of  desiccation  is  reached  (figs.  1130-1132)..    Probably  no 


816  ECOLOGY 

other  vascular  plants  equal  homosporous  pteridophytes  in  their  capacity 
for  dispersal;  the  great  wealth  of  ferns  on  oceanic  islands  commonly  is 
explained  by  the  easy  dissemination  of  their  spores  by  wind. 

In  Salvinia  there  is  no  true  dehiscence,  the  whole  sporangium  being  shed  and  the 
spores  germinating  within.  In  Azolla  the  sporangial  wall  slowly  decays,  setting  free 
the  spores.  In  Marsilea  the  spores  are  contained  within  a  hard-walled  structure, 
the  sporocarp;  when  moistened  the  internal  mucilaginous  tissue  absorbs  water  and 
it  swells  to  such  an  extent  as  to  burst  the  sporocarp  wall  and  protrude  into  the  water, 
carrying  with  it  the  attached  sporangial  masses  (fig.  411).  In  heterosporous  pteri- 
dophytes the  microspores  have  the  mobility  characteristic  of  the  spores  of  homo- 
sporous forms,  but  the  megaspores  are  much  less  mobile;,  indeed,  in  some  species 
of  Selaginella  mobility  is  entirely  absent,  and  the  megaspore  no  longer  is  a  dis- 
seminule  (fig.  308).  With  regard  to  protection  and  endurance,  spores  may  vary 
from  the  relatively  delicate  chlorophyll-containing  spores  of  Equisetum,  which  die 
unless  germination  occurs  at  once,  to  the  remarkably  protected  spores  of  Selaginella 
(fig.  303),  which  commonly  germinate  only  after  a  long  resting  period.  Remarkable 
resistance  to  severe  conditions  is  shown  by  the  spores  of  Marsilea,  which  have 
been  known  to  germinate  when  sporocarps  that  had  been  kept  dry  for  eighteen  years 
were  placed  in  water;  much  of  this  capacity  for  endurance  is  due  to  the  impermea- 
bility of  the  sporocarp  wall,  as  is  shown  by  the  fact  that  the  spores  in  sporocarps  that 
have  been  kept  in  alcohol  for  three  years  may  still  remain  capable  of  germination. 

In  the  seed  plants  there  is  an  extremely  complicated  situation  (p.  256  ff.).  Het- 
erospory  is  there  universal,  and  the  microspores  (better  known  as  pollen  grains) 
are  scattered  by  various  agents.  The  megaspores,  however,  always  are  retained, 
having  no  longer  the  character  of  disseminules.  The  ecological  features  of  these 
organs  will  be  considered  in  connection  with  flowers  (p.  825  ff.). 

Sexual  reproduction.  —  Significant  features.  —  The  chief  feature  of 
sexual  reproduction  is  the  union  or  fusion  of  two  cells,  known  as  gametes, 
resulting  in  the  production  of  a  sexually  formed  spore.  Usually  the  two 
gametes  may  be  distinguished  as  male  cells  or  sperms,  and  female  cells 
or  eggs.  The  spore  resulting  from  fusion,  upon  germination,  develops 
into  a  structure  called  the  embryo. 

Isogamy.  —  In  those  thallophytes  in  which  sexuality  seems  to  be' just 
beginning  (e.g.  Ulothrix,  figs.  1133,  1134),  the  two  gametes  are  similar 
in  size  and  in  structure  and  usually  in  activity;  such  a  condition  is  called 
isogamy;  the  spore  resulting  from  the  fusion  of  equal  gametes  is 
called  a  zygospore  (figs.  49,  50).  Isogamous  gametes  may  be  ciliated 
and  actively  motile  (as  in  Uloth'ix),  non-ciliated  but  somewhat  motile  (as 
in  the  diatoms),  or  almost  inrnotile,  that  is,  not  leaving  the  plant  body 
(as  in  Spirogyra,  fig.  109) .  Although  isogamous  gametes  exhibit  no  struc- 
tural differences,  there  is  some  evidence  of  unlikeness;  for  example,  in 
Ulothrix  and  in  Acetabularia,  fusion  takes  place  only  between  gametes 


REPRODUCTION   AND   DISPERSAL 


817 


from  different  gamete-producing  organs  (gametangid) ,  and  in  Dasy- 
cladus,  only  between  gametes  from  different  plants,  though  it  is  impossible 
in  any  of  these  to  distinguish  male  and  female 
characters.  However,  in  the  Conjugates,  one  of 
the  gametes  often  is  immotile,  while  the  other 
migrates  from  a  neighboring  filament  through  a 
passageway  made  by  the  fusion  (or  conjugation) 
of  two  lateral  outgrowths  (figs.  107-109).  From 
analogy  with  the  higher  plants,  the  immotile 
gamete  may  be  called  female  and  the  motile 
gamete,  male.  In  some  conjugating  forms,  as 
Mucor,  there  is  no  such  distinction,  the  two 
gametes  moving  equally  and  meeting  in  the 
passageway  between  the  filaments  (figs.  163- 
166).  Usually  zygospores  are  thick-walled  rest- 
ing cells  closely  packed  with  food  and  well  able 
to  exist  over  severe  periods  (figs.  50,  no,  166). 

Heterogamy.  —  In  the  great  majority  of  plants, 
including  many  thallophytes  and  all  the  higher 
plants,  the  two  gametes  are  unequal;  this  con- 
dition is  known  as  heterogamy,  and  the  spore  re- 
sulting from  the  fusion  of  unequal  gametes  is 
called  an  oospore.  It  is  in  the  heterogamous 
plants  that  one  may  speak  of  true  sex  differenti- 
ation and  of  the  development  of  male  gametes  or 
sperms  and  of  female  gametes  or  eggs  (fig.  1135). 
In  nearly  all  bryophytes,  pteridophytes,  and 
heterogamous  algae  the  sperms  are  relatively 
small,  ciliated,  actively  motile  bodies  (figs.  28, 
119,  320,  349,  415),  whereas  in  the  seed  plants 
(except  in  Ginkgo  and  in  the  cycads,  fig.  455), 
they  are  non-ciliated,  and  exhibit  but  little  true 
locomotion  (fig.  479).  Eggs  commonly  are  much 
larger  than  sperms,  and,  except  in  the  case  of  a 
few  algae  where  they  float  freely  in  the  water, 
they  are  essentially  immotile  (figs.  31,  77,  481). 
Often  the  male  and  female  gametes  are  borne  in  special  organs,  the 
antheridia  and  the  oogonia  (or  archegonia),  respectively.  In  many 
thallophytes  the  oospore  is  a  thick-walled  resting  cell  (fig.  70). 


FIGS.  1133,  1134.  - 
Zoospores  and  isoga- 
mous  gametes  in  Ulo- 
thrix :  1133,  a  part  of  a 
filament  from  which  a 
4-ciliate  zoospore  and 
several  smaller  biciliate 
gametes  are  escaping  ; 
1134,  gametes  pairing 
and  fusing  ;  highly  mag- 
nified.—From  COULTER. 


1135  r 


FIG.  1135.  —  Heterog- 
amy ;  an  egg  of  Fucus, 
surrounded  by  a  swarm 
of  biciliate  sperms  ; 
highly  magnified.  — 
After  THURET. 


8i8  ECOLOGY 

Of  chief  ecological  interest  in  connection  with  heterogamy  are  the 
factors  concerned  in  facilitating  the  fusion  of  gametes  and  particularly 
the  fusion  of  gametes  of  different  immediate  ancestry.  In  the  algae  as  a 
group,  fusion  is  comparatively  easy,  since  the  eggs  are  immersed  and  the 
sperms  capable  of  locomotion.  In  the  bryophytes  and  pteridophytes  the 
difficulty  is  greater,  because  usually  they  are  land  plants.  In  the  liver- 
worts and  ferns  the  gametophytic  generation  commonly  grows  close  to 
the  moist  soil,  where  there  often  is  sufficient  moisture  for  the  swimming 
of  such  minute  bodies  as  sperms.  In  many  cases  the  female  organs  con- 
tain substances  which  occasion  prochemotactic  reactions  in  sperms,  thus 
greatly  facilitating  the  fusion  of  gametes;  among  such  substances  are 
cane  sugar  (as  in  mosses)  and  malic  acid  (as  in  ferns). 

Monoecism  and  dioecism.  —  When  the  same  individual  bears  correl- 
ative kinds  of  reproductive  organs  (e.g.  antheridia  and  archegonia,  or 
stamens  and  pistils),  the  species  is  called  monoecious  if  the  organs  are 
borne  on  separate  branches,  and  hermaphroditic  if  the  organs  are  borne 
together  on  a  common  branch;  if  the  two  kinds  of  organs  are  borne  on 
separate  individuals,  the  species  is  called  dioecious.  In  the  heteros- 
porous  pteridophytes  dioecism  occurs  regularly,  male  gametophytes 
developing  from  microspores  and  female  gametophytes  from  mega- 
spores.  In  most  homosporous  pteridophytes  the  gametophytes  are 
monoecious,  but  they  are  chiefly  dioecious  in  Equisetum,  and  there  are 
many  dioecious  species  among  the  algae  and  bryophytes.  Obviously 
the  movement  of  sperms  to  the  female  organs  is  easier  in  monoecious 
than  in  dioecious  species.  It  has  been  supposed  that  the  chief  advantage 
of  dioecism  is  that  it  prevents  close  inbreeding  (i.e.  fusion  between  closely 
related  sex  cells),  it  being  believed  oftentimes  that  certain  advantages 
are  associated  with  the  fusion  of  gametes  of  different  immediate  ancestry 
(see  p.  820).  In  the  homosporous  pteridophytes  the  fusion  of  related 
gametes  often  is  impossible,  since  many  species  are  dichogamous,  that 
is,  with  the  correlative  organs  on  the  same  individual  maturing  con- 
secutively; commonly  the  male  organs  develop  first. 

In  some  dioecious  species  there  are  features  that  facilitate  the  germination  of 
male  and  female  plants  in  close  proximity;  for  example,  the  elaters  of  Equisetum 
(figs.  337,  338)  often  cause  a  group  of  spores  to  become  intertangled  and  thus  to 
fall  and  germinate  together,  and  in  Azolla  the  microspores  cohere  in  masses  and 
often  have  hooks,  the  so-called  glochidia,  which  become  caught  in  the  projecting 
filaments  of  the  megaspores  (fig.  403).  However,  in  many  cases  dioecism  doubtless 
is  disadvantageous  because  of  the  difficulties  in  the  way  of  fusion  between  male  and 
female  gametes.  In  many  mosses  sporophytic  generations  rarely  are  seen,  partly, 


REPRODUCTION   AND   DISPERSAL  819 

perhaps,  because  of  dioecism  and  partly,  it  may  be  supposed,  because  the  elevated 
female  organs  make  more  uncertain  the  presence  of  sufficient  water  for  sperm  mo- 
tility.  In  hermaphroditic  mosses  dichogamy  is  rare,  hence  close  inbreeding  is  the 
rule  rather  than  the  exception. 

The  advantages  and  disadvantages  of  heterospory.  —  In  heterosporous 
pteridophytes  the  proximity  of  male  and  female  gametes  is  a  matter  of 
uncertainty,  since  it  depends  upon  the  chance  of  microspores  and  meg- 
aspores  lodging  near  one  another.  In  most  living  species  the  difficulty, 
perhaps,  is  slight,  since  the  plants  are  so  small  that  often  the  spores  must 
fall  near  the  parent  plant  and  hence  near  each  other;  furthermore, 
a  number  of  the  species  are  hydrophytes,  and  hence  the  motile  sperms 
have  a  favorable  medium.  In  past  ages,  however,  there  have  been  many 
heterosporous  trees  among  the  pteridophytes,  and  the  waste  of  both 
microspores  and  megaspores  must  have  been  enormous.  This  is  the 
only  known  ecological  group  of  past  ages  that  is  unrepresented  among 
living  forms,  and  it  well  may  be  that  its  disappearance  was  due  in  part, 
at  least,  to  its  disadvantageous  heterospory,  coupled,  perhaps,  with 
extensive  land  emergence  and  with  the  consequent  lessening  of  habitats 
favorable  for  the  fusion  of  gametes.  In  contrast  with  this  extinct  group 
are  the  seed  plants,  whose  greater  success  probably  is  due  in  part  to  the 
retention  of  the  megaspores  instead  of  their  dispersal,  with  the  enormous 
consequent  waste;  the  waste  even  of  microspores  is  reduced  largely  in 
the  great  group  of  insect-pollinated  plants.  However,  the  seed  plants 
do  not  equal  the  homosporous  ferns  and  the  lower  plants  in  ease  of 
dispersal,  as  appears  from  the  fact  that  the  homosporous  constituents  of 
any  two  widely  separated  floras  are  much  more  alike  than  are  their 
heterosporous  constituents.  Obviously,  the  great  advantage  of  heteros- 
pory, if  such  there  be,  must  be  sought  along  other  lines,  even  in  the 
seed  plants. 

The  significance  of  sexual  reproduction.  —  In  the  simplest  cases  (as  in 
Ulothrix)  the  result  of  the  fusion  of  gametes  is  a  decrease  of  potential 
individuals,  since  two  cells  resembling  zoospores  and  having,  perhaps, 
the  possibility  of  growing  into  two  plants  unite  and  form  a  spore  that  can 
grow  into  but  one  plant.  However,  since  a  single  algal  filament  may 
produce  a  number  of  gametes,  considerable  multiplication  is  possible 
through  sexuality;  this  is  conspicuous  especially  in  those  groups  which 
are  without  asexual  spores  (viz.  the  Conjugales,  Charales,  and  Fucales). 
But  in  plants  with  a  well-defined  alternation  of  generations  (viz.  in 
bryophytes,  ferns,  and  seed  plants)  sexual  reproduction  rarely  results 


820  ECOLOGY 

in  a  significant  increase  of  individuals,  asexual  spores  or  propagules 
chiefly  being  responsible  for  such  increase.1  Thus  multiplication,  which 
is  the  feature  of  chief  significance  in  other  forms  of  reproduction,  usually 
is  not  conspicuous  in  sexual  reproduction.  Gametes  and  the  spores 
resulting  from  their  fusion  (except  in  the  thallophytes)  are  among  the 
most  delicate  of  plant  structures,  so  that  fitness  for  endurance  through 
severe  periods  is  not  one  of  their  characteristics,  as  it  is  of  many  asexual 
spores.  Furthermore,  neither  the  gametes  nor  the  resulting  spores  are 
particularly  efficient  disseminules ;  the  female  gamete,  in  particular, 
whose  position  determines  the  place  of  the  next  generation,  is  for  the 
most  part  immotile.  Hence  none  of  the  three  features  commonly  as- 
sociated with  reproduction,  namely,  multiplication,  endurance,  and 
dispersal,  are  of  especial  significance  in  sexual  reproduction. 

It  is  believed  commonly  that  sexual  reproduction  makes  possible  the 
advantageous  merging  in  one  individual  of  the  qualities  of  two  races, 
hence  sometimes  the  phenomenon  is  known  as  amphimixis.  In  Ulo- 
thrix  the  advantage  gained  has  been  thought  to  be  one  of  size,  since  plants 
developing  from  zygospores  are  larger  than  those  which  develop  from 
gametes  that  fail  to  fuse.  In  other  cases  (apart,  perhaps,  from  seed 
plants,  p.  866)  the  advantages  of  sexuality  appear  more  hypothetical 
than  real,  but  even  hypothetically  the  crossing  of  two  races  might  fairly 
be  expected  to  introduce  into  a  given  strain  disadvantages  as  well  as 
advantages.  The  chief  reason  for  believing  that  sexuality  is  of  no  partic- 
ular advantage  (at  least  in  the  lower  plants)  is  that  its  absence  seems 
to  bring  no  disadvantage.  In  the  bacteria  and  blue-green  algae,  in  some 
green  algae,  and  in  many  fungi,  true  sexuality  is  absent,  but  no  plants 
are  more  successful  than  these ;  in  many  fungi  (as  in  Saprolegnia  and  in 
the  Ascomycetes) ,  and  in  some  liverworts  and  mosses  there  is  excellent 
evidence  of  diminishing  sexuality  (see  p.  883),  but  none  of  diminishing 
success.2  Even  in  the  higher  plants,  where  sexuality  is  much  more 

1  Such  increase  as  there  is  among  these  plants  is  most  conspicuous  among  the  bryo- 
phytes,  where  single  gametophytes  may  bear  several  (rarely  many)  sporophytes.     Some- 
times (especially  in  gymnosperms)  two  or  more  embryos'  develop  from  one  sexual  spore  or 
from  one  sporiferous  center  of  a  gametophyte,  a  phenomenon  known  as  polyembryony  ; 
this  is  of  little  significance,  however,  inasmuch  as  but  one  embryo,  as  a  rule,  is  able  to 
mature. 

2  Somewhat  recently  there  have  been  discovered  modified  forms  of  sexuality  in  the  rusts 
and  smuts  and  in  various  other  fungi,  but  in  these  cases  there  is  no  crossing,  so  that 
true  .amphimixis  with  its  supposed  advantages  necessarily  is  excluded.     In  many  algae, 
there  occurs  inbreeding   or  automixis,  which  is  well  illustrated  in  Spirogyra,   where 
fusion  may  take  place  between  gametes  of  adjoining  cells  in  the  same  filament,  and  in 


REPRODUCTION   AND   DISPERSAL  821 

general,  no  demonstrable  loss  comes  from  its  elimination  in  those  cases 
where  vegetative  reproduction  is  well-developed,  and  such  cases  con- 
stitute the  vast  majority.  In  various  seed  plants  (as  the  duckweeds) 
sexual  reproduction  rarely  occurs,  and  many  economic  plants  (such  as 
the  banana,  the  fig,  and  the  sweet  potato)  have  been  propagated  almost 
from  time  immemorial  solely  by  vegetative  means  and  yet  without  obvious 
deterioration.  A  second  theory  claims  that  sexuality  insures  rejuvenes- 
cence. However,  this  is  insured  much  more  generally  and  economically 
through  propagation  and  asexual  reproduction.  A  third  theory  is  that 
sexuality  favors  variation  and  therefore  evolution  through  the  repeated 
mingling  of  new  elements,  thus  giving  rise  to  new  combinations  of  char- 
acters and  hence  to  new  species.  This  view  seems  reasonable,  but  there 
is  little  positive  evidence  in  its  favor.  Furthermore,  variation  is  known 
to  be  of  frequent  occurrence  in  the  bacteria  and  blue-green  algae  and  in 
other  sexless  groups;  indeed,  many  investigators  hold  that  crossing 
promotes  fixity  rather  than  variation,  and  it  has  been  shown  that  in 
inbred  races  of  Spirogyra  and  Phaseolus  the  amount  of  variation  is  as 
great  as  or  greater  than  in  cross-bred  races.  Sometimes  the  theory  is 
advanced  that  the  significance  of  sexuality  lies  in  the  fusion  of  kinetic 
and  trophic  (i.e.  nutritive)  elements;  the  egg  is  regarded  as  having  the 
food  necessary  for  development,  while  the  sperm  adds  the  requisite 
developmental  stimulus.  These  kinetic  and  trophic  roles  are  not  to  be 
doubted,  but  they  furnish  no  clew  to  the  significance  of  sexuality,  giving 
rather  an  explanation  of  embryo  development.  In  propagules  and  in 
asexual  spores  both  kinetic  and  trophic  elements  are  present  in  sufficient 
degree  to  insure  development,  so  that  in  these  respects  sexuality  adds 
nothing  new.  At  present  no  theory  as  to  the  role  of  sexuality  has  much 
support.  It  is  not  impossible  that  it  is  a  necessary  accompaniment  of 
evolution  but  without  particular  significance,  although  in  the  entire 
plant  kingdom  there  probably  is  no  other  equally  widespread  phenomenon 
which  is  without  conspicuous  advantage.  The  most  that  can  be  said 
with  certainty  concerning  the  advantage  of  sexual  reproduction  among 
the  lower  plants  is  that  it  supplements  the  other  and  more  successful 
kinds.1  As  to  the  plants  above  the  thallophytes,  there  remains  to  be 
considered  the  alternation  of  generations. 

Ulva,  where  fusion  may  take  place  between  sister  gametes  arising  from  a  common  cell. 
In  some  ferns  (as  Lastrea)  and  in  various  fungi,  the  fusing  structures  are  thought  to  be 
vegetative  rather  than  sexual ;  in  contrast  to  amphimixis  and  automixis  such  fusion 
has  been  termed  pseudomixis. 

1  In  Paramoecium,  one  of  the  infusorians,  individual  animals  reproduce  ordinarily  by 


822  ECOLOGY 

The  significance  of  alternating  generations.  —  The  chief  advantage 
in  the  alternation  of  generations  has  been  supposed  to  be  that  one  genera- 
tion, the  sporophyte,  produces  asexual  spores  in  great  abundance,  thus 
facilitating  multiplication  and  dispersal,  while  the  other  generation,  the 
gametophyte,  produces  gametes,  thus  facilitating  the  merging  of  char- 
acters of  different  individuals.  The  advantages  of  the  sporophytic 
generation  are  obvious  enough,  but  those  of  the  gametophytic  generation 
are  less  apparent,  depending  solely  upon  such  advantages  as  may  inhere 
in  sexuality.  There  are  some  obvious  disadvantages  in  alternation; 
for  example,  in  certain  mosses,  as  previously  noted,  the  conditions  for 
the  fusion  of  gametes  often  are  lacking,  hence  development  is  impossible 
for  the  sporophyte  with  its  numerous  asexual  spores,  however  well- 
fitted  they  may  be  for  multiplication  and  dispersal.  In  the  ferns,  where 
considerable  moisture  is  required  by  the  gametophytic  generation  and 
particularly  for  the  fusion  of  gametes,  the  sporophytic  generation,  which 
often  is  well-suited  for  xerophytic  situations,  can  grow  only  where  a 
gametophytic  generation  has  preceded  it.1  In  the  seed  plants,  the 
alternation  of  generations  means  that  seed  formation,  in  addition  to 
favorable  conditions  in  the  soil  and  the  climate,  depends  upon  polli- 
nation, and  therefore  upon  various  pollinating  agents,  such  as  wind 
and  insects. 

Apogamy  and  apospory.  —  In  some  cases  one  of  the  alternating  gener- 
ations in  whole  or  in  part  is  eliminated;  if  the  eliminated  structure  or 
process  is  gametophytic,  the  phenomenon  is  called  apogamy;  if  sporo- 
phytic, it  is  called  apospory.  In  Pteris  cretica,  Nephrodium  molle,  and 
in  many  other  ferns  the  sporophyte  may  develop  from  a  bud  on  the 

simple  fission.  After  a  time,  individuals  of  one  line  of  ancestry  conjugate  with  those  of 
another,  and  there  appears  to  be  an  exchange  of  substance  between  the  individuals,  which 
later  separate  and  again  reproduce  by  fission.  Since  cultures  of  Paramoecium  in  which 
conjugation  does  not  take  place  show  a  gradual  decrease  in  size  and  activity  after  many 
generations,  it  has  been  urged  that  sexuality  thus  is  shown  to  be  advantageous,  at  least  in 
animals.  Indeed,  in  the  older  experiments,  cultures  in  which  conjugation  was  prevented 
could  not  be  maintained  for  more  than  140  generations  (i.e.  about  three  months),  although 
parallel  cultures  with  conjugation  remained  vigorous  indefinitely.  More  recently  it  has 
been  shown  that  it  is  not  lack  of  conjugation  which  causes  death,  but  probably  some 
deterioration  in  the  culture  media,  since  by  varying  the  media  from  time  to  time,  1500 
generations  have  been  secured  without  conjugation  and  with  no  loss  of  vigor.  It  is  now 
believed  that  cultures  of  Paramoecium  thus  can  be  kept  indefinitely  without  conjugation, 
and  it  is  to  be  noted  that  the  changes  introduced  in  the  culture  media  probably  are  much 
less  than  are  those  which  occur  in  natural  habitats. 

1  Of  course  such  a  plant  as  Pteris  may  migrate  from  its  place  of  origin  by  rhizome 
propagation. 


REPRODUCTION    AND   DISPERSAL  823 

gametophyte;  here  gametophytes  but  not  gametes  appear  to  be  necessary 
for  sporophyte  development.  In  Coelebogyne  (one  of  the  seed  plants) 
certain  cells  of  the  sporophyte  nucellus  are  able  to  develop  into  embryos, 
the  entire  gametophytic  generation  thus  being  unnecessary  for  seed 
production.  Apogamy  is  now  recognized  in  a  number  of  seed  plants, 
as  in  Euphorbia,  Allium,  Elatostema,  and  Balanophora;  in  Coelebogyne, 
in  Balanophora,  and  in  Euphorbia  dulcis  there  is  no  necessity  for 
sexual  fusion;  in  Elatostema  there  is  no  egg,  and  in  Balanophora 
globosa  even  the  staminate  flowers  are  wanting.  Apospory  is  illustrated 
in  certain  ferns,  in  which  the  gametophytic  generation  may  develop 
from  sporangia  (as  in  Asplenium)  or  even  from  vegetative  parts  of  the 
leaf  (as  in  Polystichum) .  There  are  some  varieties  of  ferns  and  of  seed 
plants  which  exhibit  both  apogamy  and  apospory,  their  reproduction 
being  wholly  vegetative.  The  chief  advantages  of  apogamy  and  of 
apospory  would  appear  to  be  that  they  eliminate  the  disadvantages  of 
alternating  generations. 

Parthenogenesis.  —  The  development  of  a  gamete  into  a  plant  without 
fusing  with  another  gamete  is  known  as  parthenogenesis.  The  egg  is 
much  more  likely  to  develop  parthenogenetically  than  is  the  sperm,  prob- 
ably because  of  its  greater  size  and  more  abundant  food  supply.1  In 
such  forms  as  Ulothrix,  where  a  gamete  almost  indifferently  either  may 
fuse  with  another  gamete  or  develop  independently,  or  in  Ulva,  where 
small  gametes  commonly  fuse  and  certain  of  the  larger  gametes  usually 
develop  without  fusion,  parthenogenesis  probably  represents  the  retention 
of  a  primitive  character;  perhaps  the  same  is  true  in  Zygnema  (fig.  112). 
The  significance  is  quite  otherwise  in  Saprolegnia,  where  parthenogenesis 
is  accompanied  by  all  stages  in  the  abortion  of  the  male  organs,  from 
almost  complete  development  to  the  entire  absence  of  the  antheridium. 
Other  plants  in  which  parthenogenesis  has  been  reported  are  Chara 
crinita,  Marsilea,  Thalictrum,  Alchemilla,  Wikstroemia,  Hieracium, 
Antennaria,  and  Taraxacum,  the  last  six  being  seed  plants,  and  the 
final  three  being  Compositae.  In  these  cases  also  parthenogenesis 
undoubtedly  involves  the  loss  of  a  character  formerly  present;  in  Wik- 
stroemia and  Hieracium  the  pollen  often  is  imperfectly  formed  or.  im- 
potent. Parthenogenesis  occurs  in  many  animals,  as  in  rotifers  and 
in  a  number  of  insects  and  crustaceans.  Like  apogamy,  partheno- 

1  However,  male  parthenogenesis  has  been  reported  in  the  brown  alga,  Ectocarpus 
siltculosus,  though  here  the  sperms  are  relatively  large  and  the  plants  into  which  they 
develop  relatively  small. 


824  ECOLOGY 

genesis  is  advantageous  in  that  it  eliminates  the  disadvantages  of  alter- 
nating generations.  The  elimination  of  sexual  fusion,  though  often 
regarded  as  a  sign  of  degeneracy,  is  quite  as  likely  to  be  a  sign  of  pro- 
gressive evolution.  Furthermore,  the  theory  which  holds  that  sexuality 
leads  to  variability  has  little  support  from  the  facts  of  parthenogenesis, 
since  no  plant  genera  are  more  variable  than  are  Taraxacum  and 
Hieracium. 

Concluding  remarks.  —  So  far,  at  any  rate,  as  the  seedless  plants 
are  concerned,  the  significance  of  sexual  reproduction  is  in  doubt,  as 
has  been  indicated  in  the  preceding  paragraphs.  The  obvious  advan- 
tages appear  to  be  subsidiary,  and  not  at  all  commensurate  with  the 
amount  of  energy  and  material  that  is  involved.  The  appearance  of 
dioecism,  together  with  that  of  alternating  generations  and  of  heteros- 
pory,  multiplies  disadvantages  and  introduces  no  conspicuous  corre- 
sponding advantages,  unless  it  should  be  discovered  that  amphimixis 
is  inherently  advantageous ;  in  this  event  dioecism,  alternating  genera- 
tions, and  heterospory  are  highly  beneficial,  since  they  increase  the 
chance  of  fusion  between  gametes  that  differ  in  immediate  ancestry. 
In  the  seed  plants,  through  the  marked  subordination  of  the  game- 
tophytic  generation,  through  the  retention  of  the  megaspore,  and 
through  the  dispersal  of  the  embryo  (seed),  the  chief  disadvantages  of 
alternation  and  heterospory  are  eliminated.  To  a  small  extent  the  dis- 
advantages of  alternation  are  eliminated  through  apogamy,  apospory, 
and  parthenogenesis,  but  the  elimination  of  disadvantage  has  come 
chiefly  through  vegetative  reproduction,  which  in  the  great  majority  of 
plants  insures  the  perpetuation  of  species,  regardless  of  the  presence  or 
absence  of  sexual  reproduction.  The  almost  unlimited  capacity  for 
vegetative  reproduction  in  the  gametophyte  generation  of  bryophytes 
and  of  the  sporophyte  generation  of  ferns  and  seed  plants  doubtless 
has  been  the  means  of  preserving  many  species  that  otherwise  would 
have  perished.  Thus  it  is  not  to  be  assumed  that  the  progress  of 
evolution  necessarily  is  advantageous,  and  that  heterospory  and  alterna- 
tion must  be  an  improvement  over  homospory  and  lack  of  alternation. 
Probably  the  decadence  of  the  heterosporous  pteridophytes  and  of  many 
groups  of  animals  is  due  to  disadvantageous  trends  in  evolution.  Even 
in  the  seed  plants,  supremacy  is  due,  not  so  much,  probably,  to  heteros- 
pory and  alternation,  as  to  various  features  which  eliminate  their  disad- 
vantages and  most  of  all  to  their  high  capacity  for  vegetative  reproduc- 
tion, for  foliage  display,  and  for  the  development  of  secondary  wood. 


REPRODUCTION   AND    DISPERSAL 


825 


2.  FLOWERS 

General  characteristics  of  flowers.  —  The  parts  of  a  representative 
flower.  —  Ecologically  speaking,  &  flower  is  an  organ  whose  role  is  pol- 
lination, which  is  the 
initial  process  of  seed 
production.  Struc- 
turally, a  flower  is 
a  shortened  shoot 
with  spore-bearing 
organs,  which  usually 
(though  not  neces- 
sarily) are  subtended 
by  one  or  more  leaf- 
like  structures.1  In 
a  representative 
flower  the  outermost 
whorl  of  floral  leaves 
is  known  as  the  calyx, 
the  individual  leaves 
being  termed  sepals 
(s,&,figs.  1136, 1137). 
Next  within  this  is  the 

corolla,  which  may  or 

,  ,  FIG.  1136.  —  An  inflorescence  of  a  syringa  (Philadelphus), 

y  due  up    showmg  the  floral  organs  of  a  hypogynous,  monoclinous,  poly- 

of 


leaves,    petalous  flower;    note  the  calyx  with  its  individual  sepals  (-?) 
s    fp     and  the  corolla  with  its  individual  petals  (p),  the  calyx  and 


separate 
known  as 

corolla  together  forming  the  perianth;  note  also  the  stamens, 
C,  IlgS.  1136,  II37).  each  composed  of  a  filament  (/)  and  an  anther  (a),  and  the 
The  calyx  and  corolla  pistil,  of  which  there  are  here  to  be  seen  the  style  (/)  and  four 
together  form  the  per-  Sti8mas  (£)  J  this  inflorescence  is  a  cyme,  the  terminal  flower 

blossoming  first. 

ianth.     Next   within 

the  corolla  are  the  stamens,  each  of  which  consists  usually  of  a  slender 
stalk,  the  filament  (/,  fig.  1136),  and  a  spore-bearing  body,  the  anther  (a, 
figs.  1136,  1137) ,  the  spores  being  known  as  microspores  (fig.  1145).  At 


1  The  latter  statement  groups  the  strobilar  organs  of  many  pteridophytes  with  flowers, 
there  being  no  sharp  line  structurally  between  strobili  and  certain  floral  shoots  or  inflores- 
cences (see  p.  180);  however,  since  the  r61e  of  gymnosperm  and  pteridophyte  strobili  is 
fundamentally  different,  in  the  following  pages  gymnosperms,  but  not  pteridophytes,  will 
be  regarded  as  true  flower-producing  plants. 


826 


ECOLOGY 


the  center  of  the  flower  is  the  pistil  (or  pistils);  a  simple  pistil  or  one 
member  of  a  compound  pistil  is  called  a  carpel  (g,  fig.  1137).  Commonly 
a  pistil  is  composed  of  an  enlarged  basal  portion,  the  ovary  (o,  figs.  1180, 
1181),  and  a  slender  upper  portion,  the  style  (t,  fig.  1136),  which  is  sur- 
mounted by  the  somewhat  enlarged  and  sticky  stigma  (or  stigmas,  g, 
fig.  1136). 

Inside  of  the  ovary  are  ovules  (figs  581-584),  which  represent  incipient 
seeds,  and  within  each  ovule  is  the  megaspore  or  embryo  sac  (figs.  582, 


FIG.  1137.  —  A  longitudinal  section  through  the  flower  of  a  peony  (Paeonia),  showing 
the  calyx  with  its  sepals  (&),  the  corolla  with  its  petals  (c),  numerous  stamens  with  their 
filaments  and  anthers  (a),  and  the  pistils  or  carpels  (g);  the  broadened  end  of  the  axis 
just  below  the  carpels  is  the  receptacle.  —  From  STRASBURGER. 

589) ,  which  develops  into  the  minute  female  gametophyte  that  is  char- 
acteristic of  seed  plants  (figs.  590-594).  The  entire  life  of  the  female 
gametophyte  is  passed  within  the  ovule,  and  after  the  fusion  of  the 
gametes,  the  sexually  produced  spore  (oospore)  germinates  into  the  em- 
bryo, whose  subsequent  development  is  the  most  conspicuous  feature 
of  seed  formation  (figs.  600-613).  Usually  the  minute  male  gameto- 
phyte begins  to  develop  from  the  microspore  \vithin  the  anther,  forming 
a  structure  of  two  or  more  cells  which  with  the  persisting  microspore 
wall  forms  the  mature  pollen  grain  (fig.  1146).  The  pollen  grains, 
lodging  upon  the  stigma,  germinate,  developing  elongated  structures, 
known  as  pollen  tubes,  which  penetrate  the  pistil  to  the  female  game'to- 


REPRODUCTION   AND    DISPERSAL  827 

phyte,  thus  permitting  the  migrating  male  cells  to  reach  the  neighbor- 
hood of  the  egg  (figs.  533,  599). 

The  more  or  less  broadened  terminal  part  of  the  axis,  which  bears  the 
floral  organs,  is  the  receptacle  (fig.  1137).  Most  flowers  are  subtended 
by  leaflike  organs,  known  as  bracts  (b,  fig.  1141),  into  which  foliage 
leaves  often  grade  imperceptibly;  a  group  of  whorled  or  closely  arranged 
bracts  is  called  an  involucre  (figs.  1193,  1194)-  Although  flowers  often 
are  solitary,  they  more  commonly  are  grouped  into  an  inflorescence 
(fig.  1141). 

Differences  in  floral  structure.  —  While  the  sort  of  flower  described 
above  is  as  representative  as  any,  there  are  divergences  in  almost  all 
respects,  and  since  these  divergences  are  relatively  fixed,  whatever  the 
environmental  conditions,  they  have  been  made  the  chief  basis  for  sep- 
arating seed  plants  into  subdivisions.  The  kind  of  flower  that  is  most 
fundamentally  different  from  the  one  above  pictured  is  that  of  the 
gymnosperms,  which  has  no  ovary,  style,  or  stigma,  the  ovules  being 
exposed  directly  to  falling  pollen.  Any  one  of  the  parts  of  a  flower  may 
be  wanting  or  even  all  the  parts  except  either  stamens  or  pistils.  Often 
there  is  but  one  kind  of  floral  leaves  which  in  the  dicotyls  is  arbitrarily 
regarded  as  the  calyx  (figs.  1159,  1160),  but  which  in  the  monocotyls  is 
termed  the  perianth ;  sometimes  there  are  no  floral  leaves,  as  in  the  cat- 
tails, peppers,  and  hazels  (fig.  1161),  and  in  most  gymnosperms.1  Even 
where  the  perianth  is  lacking,  one  or  more  bracts  commonly  are  present. 
The  simplest  flower  is  that  of  the  duckweeds,  in  which  the  only  organ 
present  is  a  single  stamen  or  pistil.  In  the  dicotyls  the  corolla  may  be 
made  up  of  separate  petals  (figs.  1136,  1137),  or  the  parts  may  be  united 
(as  in  the  Sympetalae,  fig.  1185).  Most  flowers  are  monoclinous,  that 
is,  with  pistils  and  stamens  occurring  in  the  same  flower  (figs.  1136, 
1137),  but  some  are  diclinous,  that  is,  with  stamens  and  pistils  occurring 
in  separate  flowers ;  diclinous  species  may  be  monoecious,  having  the 
two  kinds  of  flowers  on  the  same  plant  (fig.  1161),  or  dioecious,  having 
the  two  kinds  on  separate  plants  (fig.  1165). 

While  the  floral  whorls  commonly  are  sharply  delimited,  the  calyx 
and  corolla  often  are  much  alike,  as  in  many  monocotyls  and  in  some 
dicotyls  (e.g.  Poly  gala}.  A  striking  instance  of  intergrading  parts  is 
found  in  the  white  water  lily  (Castalia],  where  the  stamens  pass  gradually 
into  petals,  suggesting  to  some  observers  that  stamens  are  transformed 

1  Spikes  or  catkins  of  such  flowers  do  not  differ  essentially  in  structure  from  pterido- 
phyte  strobili,  though  their  r61e  is  radically  different. 


828  ECOLOGY 

petals  and  to  others  that  petals  are  transformed  stamens,  neither  view 
having  adequate  support.  When  the  calyx,  corolla,  and  stamens  are 
inserted  on  the  receptacle  below  the  ovary,  the  flower  is  called  hypogynous 
(figs.  1137,  1138);  when  the  corolla  and  stamens  are  inserted  on  the 
calyx  at  the  level  of  the  ovary,  the  flower  is  called  perigynous  (fig.  1139); 
and  when  the  calyx  appears  to  be  inserted  on  the  ovary,  the  flower  is 
called  epigynous  (fig.  1140).  A  determinate  inflorescence  is  one  in  which 


1139 


FIGS.  1138-1140.  —  Diagrams,  showing  the  position  of  the  floral  organs  in  hypogy- 
nous (1138),  perigynous  (1139),  and  epigynous  (1140)  flowers;  in  1138  the  calyx,  corolla, 
and  stamens  are  attached  to  the  receptacle;  in  1139  the  corolla  and  stamens  are  attached 
to  the  calyx  tube;  in  1140  the  other  floral  organs  appear  to  be  attached  to  the  ovary.  — 
From  GANONG. 

the  terminal  flower  blossoms  first,  while  an  indeterminate  inflorescence 
is  one  in  which  the  lateral  flowers  blossom  first,  so  that  a  shoot  may  con- 
tinue to  bloom  somewhat  indefinitely  (fig.  1141).  Cymes  are  a  represen- 
tative form  of  determinate  inflorescence  (fig.  1136),  and  common  forms 
of  indeterminate  inflorescences  are  spikes  (fig.  1163),  catkins  (fig.  1161), 
racemes  (fig.  1199),  corymbs  (fig.  1173),  umbels  (figs.  1196,  1197),  pan- 
icles (fig.  1162),  and  heads  (fig.  1193). 

The  significance  of  differences  in  floral  structure.  —  The  floral  diver- 
gences heretofore  noted  are  of  great  convenience  in  classification,  because 
they  are  relatively  invariable,  but  they  appear  to  have  had  little  or  no 
significance  in  determining  the  success  or  failure  of  plants.  It  is  believed, 
for  example,  that  the  trend  of  plant  evolution  has  been  toward  epigyny, 
but  there  is  practically  no  evidence  that  epigyny  is  more  advantageous 
than  hypogyny.  Monocliny  or  dicliny  and  the  presence  or  absence  of 
a  perianth  may  be  of  greater  consequence,  and  they  will  be  considered 
later,  but  it  appears  that  floral  evolution  has  taken  place  in  large  part 
without  relation  to  role  or  to  ecological  advantage,  especially  in  those 
structures  most  used  in  classification.  In  many  other  respects,  how- 


REPRODUCTION   AND   DISPERSAL 


829 


ever,  flowers  possess  conspicuous  advantages,  and  these  will  now  be 
considered. 

The  r61e  of  flowers  and  the  essential  organs  involved.  —  Pollination.  — 
Pollination,  that  is,  the  transfer  of  pollen  grains  to  the  stigma  (or  to  the 
ovule  in  gymnosperms)  is  the  chief  activity 
associated  with  flowers.  When  pollen  is  trans- 
ferred from  a  flower  of  one  plant  to  a  flower  of 
another,  the  phenomenon  is  termed  cross  polli- 
nation or  xenogamy,  and  when  pollen  is  trans- 
ferred from  the  anthers  to  the  stigma  of  the  same 
flower,  it  is  termed  dose  pollination  or  autogamy. 
Geitonogamy,  in  which  pollen  is  transferred  from 
one  flower  to  another  on  the  same  plant,  is 
intermediate  between  xenogamy  and  autogamy, 
and  often  is  classed  with  the  former,  but  in  reality 
it  is  much  closer  to  the  latter.  In  many  species 
autogamy  is  the  only  kind  of  pollination  possible, 
and  in  other  species  (probably  a  greater  number) 
only  xenogamy  is  possible,  but  it  is  probable  that 
in  the  great  majority  of  plants  both  autogamy  (or 
geitonogamy)  and  xenogamy  are  possible,  though 
usually  it  is  believed  that  the  latter  is  the  more 
advantageous.  In  all  cases  xenogamy  is  possible 
only  through  the  action  of  external  agents,  of 
which  wind  and  insects  are  the  most  important. 
In  geitonogamy  and  autogamy  (especially  the 
latter)  pollination  may  occur  through  the  direct 
contact  of  anther  and  stigma,  but  gravity,  wind,  sympetalous  bilabiate  co- 

.  rolla,  composed  of  an  as- 

and  insects  often  effect  autogamy  and  geitonog-     cending  upper  lip  (v)  and 
amy  as  well  as  xenogamy;     in  some  cases  in- 
sects   are    as    necessary    for   autogamy   as    for 
xenogamy  (as  in  Yucca)  -1 

The  dehiscence   of  the   anthers.  —  When   the 
pollen  grains   are  mature,  the  anther  dehisces, 
usually  by  longitudinal  slits   (fig.  1142),  but  sometimes  by  transverse 
slits,  by  valves  (fig.  1176),  or  by  terminal  pores  (as  in  Solatium  and 


FIG.  1141.  —  A  com- 
pound raceme  of  Coleus, 
the  individual  flowers 
being  arranged  in  paired 
cymes;  c,  calyx;  cf ',  the 


a  boat-shaped  lower  lip 
(/) ;  note  the  partially  ex- 
serted  stamens  (a)  and 
style  (0;  the  developing 
cymes  are  subtended  by 
caducous  bracts  (6). 


1  On  this  account  the  term  self-pollination,  if  used,  should  be  restricted  to  contact  polli- 
nation, rather  than  be  made  synonymous  with  autogamy  in  general. 


83o 


ECOLOGY 


1f42 


1143 


1144 


FIGS.  1142-1144.  —  Stamens  of  angiosperms, 
showing  methods  of  anther  dehiscence:  1142, 
ordinary  stamens  with  longitudinal  dehiscence; 
1143,  a  stamen  of  Solatium  with  dehiscence  by  a 
terminal  slit  or  pore;  1144,  a  stamen  of  V actinium 
with  tubular  prolongations  of  the  pollen  sacs.  — • 
From  KERNER. 


in  the  Ericaceae,  figs.  1143, 
1144).  Dehiscence  is  occa- 
sioned by  tissue  desiccation. 
Beneath  the  epidermis  is  a 
layer  with  unequally  thick- 
ened fibers,  in  which  strains 
arise  when  the  water  content 
lessens;  rupture  then  occurs 
along  the  lines  (or  at  the 
spots)  of  weakness,  where- 
upon the  pollen  may  be 
shaken  out  by  such  agents 
as  wind  and  insects. 


Commonly  anther  desiccation 
is  due  to  the  great  transpiration 
to  which  open  flowers  are  exposed. 
Some  anthers,  however,  open  in 
the  bud  or  in  moist  weather  and  it  has  been  claimed  that  this  is  due  to  the  absorption 
of  water  from  the  anther  by  adjoining  nectaries  or  by  other  tissues  rich  in  sugar. 
Dehiscence  occurs  when  anthers  are  placed  in  contact  with  a  cane  sugar  solution, 
though  much  more  slowly 
than  in  dry  air.  Light  and 
the  pressure  of  growing  pol- 
len also  appear  to  facilitate 
dehiscence. 

1145 
The    pollen.  —  The 

pollen  grains  are  borne 
in  pollen  sacs  within  the 
anther,  where  they  com- 
monly are  produced  in 
fours  (tetrads}.  Usually 
the  grains  break  apart  at 
maturity,  scattering  in- 
dependently, but  in  some 
plants  they  cohere  in 

groups  (as  in  Mimosa},  development  in  a  rosin-weed  (Silphium):  1145,  a  micro- 
while  ill  Others  they  spore,  representing  the  one-celled  stage  of  a  developing 
pollen  grain;  1146,  a  mature  pollen  grain;  1147,  1148, 
germinating  pollen  grains,  showing  the  first  stages  of  pol- 
len tube  development ;  note  the  thick  and  spiny  outer  coat 


1147 
FIGS.   1145-1148.  —  Different  stages  of  pollen  grain 


cohere  in  large  and  defi- 

mte    masses,    known    as 


pollinia  (as  in  the  milk-     (exine)  ;  highly  magnified.  —  From  MERRELL. 


REPRODUCTION   AND   DISPERSAL 


831 


weeds  and  the  orchids). 
Pollen  grains  commonly 
have  a  thick  outer  layer, 
the  exine,  and  a  delicate 
inner  layer,  the  inline 
(figs.  1145-1148);  in 
cases  where  there  is  a 
single  layer,  it  may  be 
thick  and  cutinized  (as 
in  Senecio)  or  thin  and 
permeable  (as  in  sub- 
mersed aquatics).  Pol- 
len grains  differ  con- 
siderably in  shape,  the 
common  forms  being  spher- 
ical or  ellipsoidal  (figs. 
1149-1157),  and  also  in 
size,  those  of  some  mallows 
being  a  hundred  times  as 


1154        1155         1156 

FIGS.  1149-1157.  —  Pollen  grains:  1149,  grains  of 
Euphorbia  splendens,  both  dry  (a)  and  moistened  (b)  ; 
1150,  angular  grain  of  the  nightshade  (Solanum 
nigruni)  ;  1151,  grains  of  a  croton  (Codiaeum  varie- 
gatum),  both  dry  (a)  and  moistened  (6);  1152,  a 
germinating  pollen  grain  of  Oxalis  ;  1153,  ellipsoid 
grain  of  Impatiens  Sultani;  1154,  grain  of  Cuphea 
ignea  with  processes  at  the  angles;  1155,  grain  of  a 
nasturtium  (Tropaeolum)  with  prominent  angles; 
1156,  spiny  pollen  grain  of  Bidens  ;  1157,  grain  of 
Hibiscus  with  prominent  spiny  processes;  note  the 
relatively  gjgantic  size  J  a11  eclually  magnified. 


large  as  the  grains  of  many 

other  plants  (fig.  1157);  they  differ  also  in  surface  sculpturing,  most 
grains  being  smooth,  but  some  being  spiny,  as  in 
the  composites  and  the  mallows  (figs.  1156,  1157). 
Many  pollen  grains  have  thin  spots  which  upon 
germination  determine  the  position  of  the  develop- 
ing pollen  tubes;  in  some  cases  the  tube  forces  off 
a  part  of  the  spore  coat  as  a  lid. 

The  stigma.  —  The  essential  elements  of  the 
pistil  are  the  ovary  and  the  stigma,  the  style  often 
being  short  or  wanting,  though  its  presence  may 
be  advantageous  through  its  elevation  of  the  stigma 
into  a  region  of  optimum  exposure  to  pollen. 
When  mature,  the  stigma  secretes  mucilaginous 
substances,  which,  together  with  its  papillate  or 
spinescent  surface,  facilitate  the  adherence  of  pollen 
(fig.  1158).  Stigmas  also  secrete  substances  which 
facilitate  the  germination  of  pollen  grains,  and  in 
some  cases  they  secrete  very  specialized  substances 
which  stimulate  the  germination  of  pollen  from 


FIG.  1158.  —  Stig- 
matic  region  of  Hibis- 
cus; t,  the  upper  part 
of  a  style  branch  with 
scattered  hairs ;  g,  the 
stigma  with  its  hairy 
surface  to  which  pol- 
len grains  (p)  are  ad- 
hering; highly  mag- 
nified. 


832  ECOLOGY 

flowers  of  the  same  or  nearly  related  species,  but  which  have  either 
no  effect  or  a  detrimental  effect  upon  other  pollen. 

The  pollen  tube.  —  Under  suitable  conditions  pollen  grains  adherent 
to  the  stigmatic  surface  germinate,  and  the  developing  pollen  tube, 
which  is  the  bearer  of  the  male  cells,  penetrates  the  style  and  enters 
the  ovary;  ultimately  it  may  reach  the  female  gametophyte  in- 
side an  ovule,  where  the  fusion  of  the  gametes  takes  place.  Usually 
the  pollen  tube  enters  the  ovule  through  the  micropyle  (m,  fig.  594), 
which  is  a  narrow  channel  at  the  ovule  apex,  where  the  enveloping 
integuments  have  not  quite  grown  together.  In  some  species  the 
pollen  tube  penetrates  the  pistil  so  rapidly  that  the  gametes  fuse 
a  few  hours  after  pollination,  while  in  other  species  a  number  of 
months  elapse  between  pollination  and  gamete  fusion  (as  in  the  oaks 
and  pines). 

The  secluded  position  of  the  female  gamete  and  the  usual  non-motility 
of  the  male  gametes  make  the  pollen  tube  an  organ  of  the  first  impor- 
tance in  the  facilitation  of  sexual  reproduction  in  most  seed  plants,  since 
it  bears  the  male  cells  (sometimes  for  an  almost  incredible  distance) 
in  its  fungus-like  course  through  the  pistil  tissues,  from  which  it  derives 
food  parasitically.  This  method  of  bringing  the  male  gametes  into  the 
proximity  of  the  egg  seems  especially  suited  to  land  plants,  since  it  elim- 
inates the  necessity  of  a  liquid  medium,  such  as  is  required  by  motile 
sperms.  Pollen  grains  germinate  readily  in  various  liquid  media,  swell- 
ing rapidly  and  sending  out  tubes  for  a  short  distance.  In  respect  to 
conditions  favoring  germination,  pollen  grains  show  wide  diversity, 
especially  in  their  osmotic  relations  with  the  medium.  The  pollen  of 
a  number  of  species  germinates  readily  in  distilled  water,  but  in  other 
cases  this  medium  causes  the  grains  to  burst;  Canna  grains,  for  example, 
burst  in  water,  but  not  in  a  2  per  cent  cane  sugar  solution.  Most  pollen 
germinates  in  cane  sugar  solutions,  that  of  some  species  requiring  high 
concentration,  while  that  of  others  germinates  readily  in  solutions  of  low 
concentration.  Pollen  grains  that  are  difficult  to  germinate  (as  those  of 
the  grasses)  send  out  tubes  if  they  absorb  water  slowly.  Some  pollen 
(as  in  certain  umbellifers  and  composites)  has  never  been  seen  to  germi- 
nate except  on  stigmas.  Probably  because  of  the  presence  of  the  proper 
stimulating  substance  at  the  proper  degree  of  concentration,  germina- 
tion usually  takes  place  more  readily  on  stigmas  than  in  artificial  media, 
and  complete  development  does  not  occur  unless  germination  has  taken 
place  on  the  stigma  of  the  proper  plant  (viz.  of  the  same  or  of  a  closely 


REPRODUCTION   AND   DISPERSAL 


833 


related  species)  *;  in  some  species  pollen  is  essentially  impotent  on  the 
stigma  of  the  flower  in  which  it  was  produced  (p.  854).  Pollen  grains 
usually  retain  their  vitality  for  a  number  of  days,  but  those  of  Hibiscus 
Trionum  live  scarcely  more  than  three  days,  while  those  of  some  species 
(as  the  date  palm)  may  live  for  several  months,  especially  if  kept  dry. 
Usually  pollen  grains  that  have  been  moistened  and  subsequently  dried 
die  quickly,  but  some  pollen  is  so  resistant  that  submergence  for  a  num- 
ber of  hours  does  not  impair  its  vitality.  The  pollen  of  vernal  flowers 
is  especially  resistant,  not  only  to  moisture,  but  also  to  low  temperatures. 


1160 


FIGS.  1159,  1160.  —  The  dioecious  wind-pollinated  flowers  of  the  box  elder  {Acer 
Ncgundo) :  1 1 59,  fascicles  of  drooping  staminate  flowers  borne  on  long  stalks  or  pedicels 
(/»);  note  the  prominent  anthers  (a);  1160,  ascending  racemes  of  pistillate  flowers  from 
another  tree;  note  the  perianth,  consisting  only  of  a  calyx  (c),  and  also  the  two  prominent 
stigmas  (£);  note  also  the  transition  between  the  bud  scales  (6)  and  the  ordinary  foliage 
leaves  (/),  the  intermediate  leaves  having  a  prominent  flattish  petiole  (0)  and  a  small 
trifoliate  blade  (&')• 

In  general,  the  stigmas  are  more  sensitive  to  harmful  factors  than  are  the 
pollen  grains. 

Pollen  tubes  usually  take  a  more  or  less  direct  course  toward  the 
ovary.  Commonly  the  central  region  of  the  style  is  composed  of  delicate 
elongated  cells,  or  sometimes,  even,  it  is  hollow,  so  that  the  direct  course 
is  the  easiest ;  in  the  grasses,  however,  the  region  traversed  by  the  pollen 
tube  seems  no  more  easily  penetrable  than  do  the  adjoining  tissues. 
After  leaving  the  style  and  entering  the  ovary,  the  pollen  tube  commonly 

1  Often  germination,  but  not  the  later  stages  of  development,  may  take  place  on  the 
stigmas  of  unrelated  plants;  the  pollen  tubes  of  Ranunculus,  a  dicotyl,  have  been  seen 
penetrating  to  the  micro pyle  of  Scilla.  a  monocotyl.  It  may  be  noted  in  this  connection 
that  the  sperms  of  ferns  swim  into  the  archegonia  of  many  species  indifferently,  but  that 
fusion  with  the  egg  takes  place  only  in  the  same  or  in  a  closely  related  species. 


834 


ECOLOGY 


follows  the  inner  wall,  and  it  may  pursue  a  tortuous  course,  or  it  may 
grow  directly  toward  a  micropyle;  pollen  tubes  have  been  shown  to 
exhibit  prochemotropic  reactions  toward  certain  carbohydrates  and  pro- 
teins, including  those  that  are  secreted  by  stigmas. 

Wind  pollination.  —  Features  that  favor  the  scattering  of  pollen.  — 
The  simplest  form  of  pollination  and  the  one  most  closely  related 

to  spore  dispersal  in  the  lower 
plants  is  wind  pollination* 
and  wind-pollinated  plants 
have  many  features  which  re- 
semble those  of  the  fungi, 
bryophytes,  and  pteridophy  tes 
rather  more  than  they  do 
those  of  the  insect-pollinated 
seed  plants.  In  many  cases 
the  staminate  flowers  are 
arranged  in  catkins,  which 
usually  are  slender,  pendu- 
lous inflorescences  that  yield 
gracefully  to  breezes  (fig. 
1161).  Catkins  suited  for 
wind  pollination  are  especially 
characteristic  of  many  trees 
and  shrubs  (notably  the  pop- 
lars, oaks,  birches,  and  other 
Amentiferae,  and  also  most  of 
the  conifers),  which  perhaps 

FIG.    1161. — A   flowering    twig    of    a    hazel  m                                      .          . 

(Corylus  americana),  a  shrub  which  has  monoe-  IS    advantageous    in    View    of 

cious  wind-pollinated  flowers ;  note  that  the  stam-  the  relative  exposure  of  such 

inate   flowers   are  lowermost  and  are  in  catkins  ^^    ^   wind      jn    most    QJf 

(c)  which  sway  in  the  breeze,  the  pollen  grains  r 

O)  often  appearing  in  clouds;  s,  scale  leaves  which  these  plants,  also,  the  flowers 

protect  the  flower  buds  in  winter;  the  pistillate  develop     before     the     leaves, 

flowers  develop  from  scaly  buds  (6),  and  at  anthe-  ^    further    fadlitati         ex_ 

sis  the  stigmas  (g)  are  exserted.  °. 

posure  to  wind.  The  pistil- 
late flowers  sometimes  are  in  catkins  (as  in  poplars  and  birches),  but 
often  they  are  not  (as  in  oaks  and  hickories) ;  such  arrangement,  appar- 
ently, is  of  no  particular  advantage. 


1  Species  with  wind  pollination  often  are  called  anemophilous,  a  term  that  should  be 
discarded,  together  with  other  humanistic  words  as  applied  to  plants. 


REPRODUCTION   AND    DISPERSAL 


835 


In  many  species  with  wind  pollination,  and  especially  in  those  without 
catkins  that  move  readily  in  the  wind,  the  stamens  have  long  and  slender 
filaments,  which  so  expose  the  anthers  that  they  are  shaken  in  the 
gentlest  air  movements  (as  in  the  grasses  and  the  box  elder,  figs.  1159, 
1162,  1163).  In  the  nettles  the  pollen  is  discharged 
into  the  air  by  a  sudden  move- 
ment of  the  filaments.  In  many 
plants  pollen  that  falls  in  quiet 
weather  accumulates  in  pockets 
of  one  sort  or  another,  whence 
it  is  scattered  readily  by  the 
first  breeze.  In  most  wind- 
pollinated  species  (not,  how- 
ever, in  most  grasses  and 
sedges)  the  pollen  is  produced 
in  great  abundance;  this  is  a 
matter  of  much  advantage  in 
view  of  the  great  waste.  The 
abundance  of  pine  pollen  re- 
sults sometimes  in  the  so-called 
sulfur  showers,  and  the  abun- 
dance of  ragweed  pollen  in  the 
air  is  thought  to  be  a  factor  in 
causing  hay  fever. 

Wind-scattered  pollen  com- 
monly is  smooth,  light,  and  dry,  and  hence  easily 
blown  about  (fig. 
1161),  and  in  the 
pines,  dispersal  is 
facilitated  further 
by  the  presence 
of  a  wing  on  each 
side  of  the  grain 
(fig.  1164).  In 
wind-pollinated 
species  the  pollen 
grains  are  not 

easily  wetted,  thus  further  resembling  the  spores  of  fungi  and  ferns; 
this    is   highly   advantageous,   since    moistening    might   prevent  wind 


FIG.  1162.  —  A 

panicle  branch  of  the 
meadow  fescue  (Fcs- 
tuca  elatior),  a  plant 
with  monoclinous 
wind-pollinated  flow- 
ers ;  note  the  unopened 
spikelets  above  with 
their  imbricated 
scales;  below  to  the 
right  is  a  spikelet  in 
which  two  of  the  lower 
flowers  have  opened, 
each  disclosing  two 
plumose  stigmas  and 
three  stamens  whose 
long  and  slender  fila- 
ments expose  the  an- 
thers to  the  wind. 


FIG.  1163.  — The 
upper  part  of  a  plan- 
tain spike  (Plantago), 
illustrating  protogyny 
in  monoclinous  wind- 
pollinated  flowers; 
note  that  the  conspic- 
uous plumose  stigmas 
(g)  appear  before  the 
stamens  are  evident; 
in  the  older  flowers 
note  the  long  and 
slender  filaments  (/) 
and  the  triangular  an- 
thers (a);  c,  calyx;  cft 
corolla. 


FIG.  1164.  —  A  pollen  grain  of  a 
pine  (Pinus),  showing  the  two  wings 
which  aid  in  its  dispersal  by  wind; 
highly  magnified.  —  From  COULTER 
and  CHAMBERLAIN. 


836  ECOLOGY 

dispersal  and  lead  to  premature  germination.  Even  among  the 
submersed  aquatics  there  are  some  species  (e.g.  many  pondweeds)  that 
at  anthesis  develop  aerial  flowering  shoots,  which  produce  light  pollen 
that  is  not  easily  wetted  and  that  is  scattered  by  wind.1  The  fact  that 
anthers  dehisce  chiefly  when  dry  is  of  much  significance  in  the  pro- 
tection of  pollen  from  moisture. 

Features  that  favor  pollen  reception.  —  In  wind-pollinated  species  the 
stigmas  commonly  are  large  and  conspicuously  exserted  (figs.  1160, 
1.161),  and  sometimes  they  are  feathery  plumose  (as  in  the  grasses,  figs. 
1162,  1163),  the  "silk"  of  corn  being  a  familiar  and  conspicuous 
example  of  these  characters.  In  the  conifers,  where  there  is  no  stigma, 
the  pollen  may  be  caught  in  a  drop  of  mucilaginous  liquid  exuded 
from  the  ovule. 

Features  that  favor  cross  pollination.  —  While  wind-pollinated  flowers 
structurally  are  relatively  simple,  they  are  on  the  whole  as  wTell  fitted 
for  cross  pollination  as  are  insect-pollinated  species,  and  they  exhibit 
even  many  of  the  specialized  features  which  are  regarded  as  more 
characteristic  of  the  latter.  In  the  first  place,  many  and  perhaps  most 
wind-pollinated  plants  are  diclinous,  and  in  these,  of  course,  there  can 
be  no  autogamy.  A  large  number  of  the  diclinous  forms  are  dioecious 
(e.g.  the  poplar,  ash,  box  elder,  juniper,  date  palm,  and  meadow  rue), 
and  their  pollination  necessarily  is  xenogamous  (figs.  1159,  1160). 
Among  common  monoecious  forms  are  the  oaks,  hickories,  birches, 
alders,  pines,  nettles,  and  most  of  the  sedges  (fig.  1161);  while  geito- 
nogamy  as  well  as  xenogamy  might  occur  in  such  plants,  the  chance  of 
it  is  minimized  in  the  many  cases  in  which  the  pistillate  flowers  are 
higher  than  the  staminate  (as  in  the  hazel,  the  pine,  and  in  many 
sedges) .  Furthermore,  in  monoecious  species  the  pistillate  flowers  of 
a  given  individual  blossom  before  the  staminate,  and  sometimes  several 
days  before,  as  in  some  alders  and  cattails.  Even  in  dioecious  plants 
the  pistillate  flowers  commonly  mature  before  the  staminate. 

In  monoclinous  wind-pollinated  flowers  (as  in  the  grasses  and  plan- 
tains, figs.  1162,  1163),  cross  pollination  commonly  is  favored  by  the 
consecutive  maturity  or  dichogamy  of  the  anthers  and  stigmas.  In  the 
plantain  (fig.  1163)  the  stigmas  mature  first,  exhibiting  a  phenomenon 
known  as  protogyny,  while  the  earlier  maturation  of  the  anther  is  known 

1  This  phenomenon  is  especially  striking  in  Myriophyllum,  since  the  hitherto  flaccid 
and  submersed  main  stem  axis  becomes  at  the  tip  rigidly  erect  and  emersed  just  before 
anthesis. 


REPRODUCTION   AND   DISPERSAL 


837 


as  protandry;  both  protandry 
and  protogyny  are  seen  in 
maize.  The  most  special- 
ized means  of  preventing 
close  pollination,  namely, 
that  in  which  the  pollen  is 
impotent  on  the  stigma  of 
the  same  flower,  is  illustrated 
in  rye,  though  in  wheat  and 
barley,  and  probably  in  most 
monoclinous  species,  close 
pollination  is  not  necessarily 
excluded. 


Miscellaneous  features  of  wind- 
pollinated  flowers.  —  Wind-polli- 
nated flowers  usually  contrast  with 
those  that  are  insect-pollinated 
in  their  lack  of  showiness,  odor, 
and  nectar,  though  some  of  them 
are  conspicuously  colored  (as -in 
the  cottonwood  and  field  sorrel). 
The  perianth  mostly  is  inconspicu- 
ous (either  through  its  greenish 
or  brownish  color  or  its  small  size) 
and  often  it  is  absent;  when  pres- 
ent, it  consists  commonly  of  a 
calyx,  the  corolla  being  rarely  in 
evidence.  None  of  these  features 
would  occasion  comment,  but  for 
the  corresponding  presence  of 
showiness,  odor,  and  nectar  in 
insect-pollinated  flowers  and  for 
the  consequent  assumption  that 
in  the  latter  these  features  prob- 
ably are  advantageous.  The  dis- 
tribution of  species  with  wind-pol- 
linated flowers  has  been  thought 
to  differ  somewhat  from  that  of 
other  seed  plants.  For  example, 
the  percentage  of  the  former  is 
greater  in  windy  habitats  than 
elsewhere  (as  on  small  islands  and 
along  shores),  while  the  flowers 


C      f 

J 


FIG.  1165.  —  Pollination  in  the  tape  grass 
(Vallisneria  spiralis};  s,  staminate  plant;  p,  pis- 
tillate plant;  the  staminate  flowers  are  borne  in  a 
spike  (k)  •  upon  detachment  they  rise  (a)  to  the 
surface,  open  out  (6),  and  float  on  the  water;  the 
pistillate  flowers  (/)  are  borne  in  spathes  (d)  on 
long  scapes  (e\  just  reaching  the  water  surface, 
where  the  floating  staminate  flowers  may  come  in 
contact  with  them  (c) ;  note  also  the  vertical  ribbon- 
like  leaves  (/)  and  the  stolon  (r),  o  representing  a 
new  potential  plant  or  offset.  —  After  KERNER. 


838  ECOLOGY 

of  our  northern  trees  contrast  with  those  of  tropical  trees  in  being  predominantly 
wind- pollinated. 

The  advantages  and  disadvantages  of  wind  pollination.  —  The  ques- 
tion of  advantage  is  here  largely  one  of  speculation.  Undoubtedly  a 
great  disadvantage  in  wind  pollination  is  the  enormous  waste  of  pollen. 
Probably  not  more  than  one  out  of  a  thousand  or  even  out  of  many  thou- 
sand grains  ever  reaches  the  proper  stigma.  Perhaps,  on  the  other  hand, 
the  chance  of  a  favorable  wind  is  greater  than  that  of  a  visit  by  the  proper 
insect.  The  dominance  of  wind  pollination  in  such  plants  as  the  oaks, 
pines,  grasses,  and  sedges  at  once  suggests  that  wind  pollination  certainly 
is  not  detrimental.  However,  the  great  abundance  of  such  plants 
(especially  the  grasses  and  sedges)  is  quite  as  likely  to  be  due  to  vegeta- 
tive as  to  reproductive  organs. 

Water  pollination.  —  Pollination  through  the  agency  of  water  is  a  relatively  rare 
occurrence  but  it  is  of  much  interest.  In  plants  that  are  completely  submersed 
(as  in  several  of  the  Potamogetonaceae  and  Najadaceae)  the  pollen  grains  are  fila- 
mentous structures  that  are  as  heavy  as  water  or  heavier,  and  the  thick  exine  char- 
acteristic of  aerial  pollen  is  lacking;  such  pollen  grains  upon  release  float  below  the 
surface  and  may  come  into  contact  with  the  long  exserted  stigmas. 

In  the  tape  grsiSs(Vallisneria)  and  in  some  of  its  relatives,  pollination  takes  place 
at  the  water  surface.  Vallisneria  (fig.  1165)  is  a  dioecious  plant,  whose  pistillate 
flowers  are  single  and  are  borne  on  long  scapes  that  bring  the  flower  at  the  time  of 
stigmatic  maturity  just  to  the  water  level.  The  staminate  inflorescences  at  maturity 
become  detached  from  their  short  scapes  and  rise  to  the  surface;  upon  the  opening 
of  the  bract  (spathe),  the  individual  flowers  also  become  detached  and  float  about  on 
the  water  as  miniature  boats,  the  perianth  opening  and  exposing  the  stamens.  The 
floating  staminate  flowers,  like  any  small  particles,  swirl  readily  into  the  slight  de- 
pressions formed  about  the  pistillate  flowers,  as  about  other  objects  on  the  water, 
and  come  into  contact  with  the  stigma.  After  pollination  the  scape  of  the  pistillate 
flower  coils  up  into  a  spiral,  thus  withdrawing  the  ovary  below  the  surface,  where  the 
fruit  develops.  In  essential  respects  pollination  in  the  water  weed  (Eloded)  is  com- 
parable to  that  in  Vallisneria. 

General  characteristics  of  insect-pollinated  flowers.  —  Monocliny  and 
its  advantages.  —  Were  it  not  so  common,  the  symbiotic  relation  existing 
between  flowers  and  insects  would  be  regarded  as  most  marvelous. 
From  the  standpoint  of  evolution,  no  great  facts  of  nature  are  more 
remarkable  than  that  in  many  plant  species  the  flowers  remain  unpolli- 
nated  unless  they  are  visited  by  insects  in  search  of  nectar  or  pollen, 
and  that  in  a  much  greater  number  of  species  visiting  insects  are  the  chief 
agents  of  pollination.  Insect-pollinated  flowers  *  are  in  great  part  mono- 
1  Insect-pollinated  flowers  often  are  inaptly  called  entomophtious,  that  is,  insect-loving. 


REPRODUCTION   AND    DISPERSAL  839 

clinous  (figs.  1136,  1137),  though  a  few  are  diclinous;  for  example,  the 
willows  are  dioecious,  and  many  composites  are  monoecious.  Dicliny 
has  been  thought  to  be  advantageous  in  wind-pollinated  flowers  because 
it  increases  the  probability  of  cross  pollination;  however  this  may  be, 
monocliny  would  seem  to  have  a  distinct  advantage  in  insect-pollinated 
flowers  in  that  it  makes  possible  double  the  amount  of  pollination  for 
a  given  number  of  insect  visits.  Furthermore,  pollen-gathering  insects 
would  not  visit  pistillate  flowers,  and  nectar-gathering  insects  would  visit 
both  pistillate  and  staminate  flowers  only  in  case  each  were  nectar- 
bearing,  thus  involving  two  nectaries  in  one  act  of  pollination. 

Pollen.  —  The  stamens  of  insect-pollinated  flowers  rarely  are  promi- 
nently exserted  and  the  filaments  often  are  short;  also  the  inflorescences 
are  relatively  inflexible  in  the  wind.  The  pollen,  instead  of  being  dry 
and  powdery,  commonly  is  adhesive  through  the  possession  of  spines  or  of 
other  protuberances  (figs.  1156,  1157),  or  through  the  presence  of  viscid 
substances  (as  in  Oenothera),  so  that  the  grains  often  cohere  in  masses. 
The  shape  of  the  grains  is  more  likely  to  be  elliptical  than  spherical,  the 
latter  shape  being  especially  characteristic  of  the  grains  in  wind-polli- 
nated flowers.  Such  pollen  grains  are  not  easily  blown  about  by  the 
wind,  and  they  adhere  readily  to  visiting  insects  and  to  stigmatic  surfaces. 
In  species  with  wide-open  flowers,  which  therefore  are  exposed  to  insects 
of  all  kinds,  including  pollen-gathering  insects,  the  pollen  often  is  almost 
as  abundant  as  in  wind-pollinated  species;  sometimes  also  the  stamens 
in  such  flowers  are  very  numerous  (as  in  the  roses  and  buttercups).  In 
tubular  or  otherwise  partly  closed  flowers,  where  the  stamens  are  con- 
cealed, the  latter  commonly  are  few  in  number  and  the  pollen  is  relatively 
sparse  (as  in  the  phloxes  and  mints).  As  a  rule,  the  stigmas  are  smaller 
and  otherwise  less  conspicuous  than  in  wind-pollinated  flowers. 

Features  supposed,  to  be  attractive  to  insects.  —  The  most  noticeable 
single  feature  of  insect-pollinated  flowers  is  their  showiness,  which  is  due 
to  the  color  of  the  flowers,  or  to  their  size,  position,  or  arrangement. 
Many  insect-pollinated  flowers  are  fragrant,  and  many  also  possess 
nectar.  It  is  rare  that  a  flower  which  is  pollinated  regularly  by  insects 
is  neither  showy,  fragrant,  nor  nectar-producing,  and  some  insect-polli- 
nated flowers  have  all  these  features. 

Most  insect-pollinated  plants  north  of  the  tropics  are  of  low  stature,  but  in  warm 
countries  many  trees  have  insect-pollinated  flowers.  An  odd  phenomenon,  com- 
monest in  the  humid  tropics  (but  characteristic  also  of  our  northern  redbud,  Cercis 
canadensis),  is  cauliflory  (i.e.  stem  flowering),  the  tree  trunks  often  being  covered 


840 


ECOLOGY 


with  flowers ;  this  habit  is  without  obvious  advantage,  though  it  has  been  suggested 
that  trunk  flowers  are  well  protected  from  torrential  rains.  Cauliflory  appears  to 
be  stimulated  by  an  excess  of  moisture ;  it  has  been  induced  in  the  grape  also  by 
wounding  and  in  the  orange  by  defoliation.  In  some  tropical  trees  and  shrubs  (as 
in  Ficus  geocarpd)  flowers  break  through  the  soil  from  subterranean  stems.  Tran- 
sitions between  wind-pollinated  and  insect-pollinated  flowers  sometimes  are  seen, 
as  in  the  ericads,  where  the  pollen  which  commonly  is  scattered  by  insects  ultimately 
becomes  dry  and  powdery  and  thus  may  be  scattered  by  the  wind ;  chestnut  flowers 
which  usually  are  wind-pollinated  are  fragrant  and  attract  insects.  Ephedra  cam- 

pylopoda  is  interesting  as  be- 
ing  an  insect-pollinated  gym- 
nosperm,  the  flowers,  which 
are  much  frequented  by  in- 
sects, exhibiting  nectar  and 
sticky  pollen  which  coheres 
in  masses.  Before  consider- 
ing in  detail  the  features 
that  attract  insects  to  flowers, 
it  is  necessary  to  consider 
the  pollinating  organisms 
themselves. 

Pollinating  insects.  — 

General  remarks.  —  The 
vast  majority  of  efficient 
FIGS.  1166-1169.  —  Flowers  of  Salvia,  illustrating    pollinating   animals   are 

pollination  by  bees:  1 166,  a  flower  of  Salvia  glutinosa  in 
longitudinal  section,  the  arrow  indicating  the  direction 
taken  by  visiting  bees;  s,  style;  a,  anther;  1167,  a  simi- 
lar section,  showing  the  lower  arm  of  the  connective 
lever  pushed  back,  as  by  an  entering  bee,  the  pollen- 
bearing  anther  (a)  thus  being  deflexed  in  such  a  way  as 
to  rub  pollen  over  the  insect;  1168,  a  Salvia  flower  into 

which  a  bee  has  entered,  the  anther  (a)  being  in  contact     pollen  en  route. 
with  the  bee;   1169,  an  older  flower,  showing  the  stigma     ,1 
(g)  in  such  a  position  as  to  come  into  contact  with  an 
entering  bee;  1 168  and  1169  show  that  Salvia  is  protan-    that    visit    flowers    regll- 
drous.  —  1166  and  1167  from  KERNER;   1168  and  1169     }arly  for  nectar  OF  pollen 

from  AVEBURY  (LUBBOCK).  ,    . 

are  the  most  important. 

Flowers  with  exposed  nectar  and  pollen  are  visited  by  most  of  the 
flower-frequenting  species,  but  flowers  with  hidden  nectar  or  pollen, 
especially  those  with  long  corolla  tubes  or  whose  nectar  accumulates 
in  long  spurs  (fig.  1171),  are  pollinated  only  by  highly  specialized  in- 
sects with  elongated  mouth  parts. 

Bees.  —  The  most  important  pollinating  insects  belong  to  the  Hymen- 
optera,  a  group  which  includes  the  bees,  wasps,  and  ants.     The  honey- 


1169 


particularly  fly- 
ing insects,  since  those 
which  crawl  from  flower 
to  flower  are  likely  to 
brush  off  most  of  the 
Among 
flying  insects  those 


REPRODUCTION    AND   DISPERSAL 


841 


bee  (Apis)  and  the  bumblebee  (Bombus)  are  the  most  efficient  of  all 
pollinating  insects,  because  of  their  remarkable  and  continued  activity 
from  the  opening  to  the  close  of  the  flowering  season,  because  of  their 
precision,  which  insures  the  successive  and  rapid  pollination  of  many 
individuals  of  the  same  species,  and  because  they  visit  flowers  for  pollen 
as  well  as  for  nectar.  Their  hairy  legs  are  well  suited  for  carrying  pollen, 
and  their  long  probosces  enable  them  to  secure  nectar  in  partially  closed 
or  tubular  flowrers  (figs.  1166-1169).  Among  the  flowers  that  are  almost 
entirely  dependent  upon  bees  for  pollination  are  those  with  irregular 
(zygomorphic)  corollas,  as  in  the  legumes,  the  violets,  and  many  of  the 
mints;  in  certain  instances  (as  in  the  clovers  and  aconites)  the  natural 
distribution  area  is  confined  to  those  parts  of  the  world  frequented  by 
bees.  The  bees  are  diurnal  insects  and  visit  only  diurnal  flowers,  and 
it  commonly  is  thought  that  they  have  a  high  color  sense  and  a  keen  sense 
of  smell  which  aid  them  in  detecting  the  presence  of  flowers.  The  wasps 
are  of  minor  importance  as  pol- 
linating insects,  though  some 
flowers  are  pollinated  chiefly  by 
them  (as  in  the  figwort). 

Moths  and  butterflies.  —  The 
butterflies  and  certain  moths 
(classed  in  the  Lepidoptera)  are 
nectar-feeders,  and  they  possess 
greatly  elongated  and  special- 
ized mouth  parts,  known  as 
maxillary  laminae.  The  butter- 
flies, like  the  bees,  are  diurnal 
insects  and  are  able  to  get  nectar  FIG.  1 1 70.  —  A  hawk  moth  (Phlegethonius 
from  deeply  hidden  parts  of  the  ^  visi]t,ing  *e  fl™er  °flf  Petunia;A  r!ote]  the 

r  .  .        long  corolla  tube  of  the  flower  and  the  long 

flower  ;    as    a    rule,     they    visit      mouth  parts  of  the  insect.  —  After  FOLSOM. 
showy  and  fragrant  flowers  (such 

as  various  honeysuckles  and  pinks).  Most  remarkable,  perhaps,  are  the 
hawk  moths  (Sphingidae),  a  group  consisting  chiefly  of  nocturnal  insects 
with  maxillary  laminae  of  great  length  (up  to  80  mm.),  which  are  coiled 
when  not  in  use  (fig.  1170).  The  nocturnal  hawk  moths  visit  flowers 
rapidly  and  with  the  precision  of  bees,  thus  contrasting  with  the  more  lan- 
guid and  haphazard  movements  of  the  butterflies;  they  are  attracted  espe- 
cially to  heavily  scented  white  nocturnal  flowers  with  long  corolla  tubes 
(e.g.  Nicotmna  alata,  which  becomes  fragrant  as  it  opens  in  the  dark). 


842  ECOLOGY 

Flies  and  beetles.  —  As  a  class  the  flies  (Diptera)  are  not  very  important  polli- 
nating insects,  largely  because  of  their  absence  of  precision  in  making  floral  visits. 
Some,  however,  notably  the  drone  flies  (Syrphidae),  have  extended  probosces  and 
depend  largely  upon  flowers  for  food,  and  thus  are  important  pollinating  agents. 
Most  flies  pollinate  only  flowers  with  exposed  nectar  and  pollen  (as  in  Euonymus}. 
Color  seems  to  have  but  little  attractive  significance,  but  odors  (especially  those 
offensive  to  human  nostrils)  attract  numerous  flies,  particularly  carrion  flies  and 
dung  flies,  which  may  thus  be  important  pollinating  agents  in  ill-smelling  flowers 
like  Rafflesia.  The  pollination  of  Arum  and  Aristolochia  is  thought  to  be  effected 
largely  by  small  flies,  which  are  able  to  crawl  through  the  narrow  apertures.  Beetles 
are  still  less  important  than  flies,  though  some  species  with  narrow  elongated  heads 
are  of  some  significance;  as  a  class  their  floral  visits  result  in  more  harm  than  benefit. 
Pollinating  animals  other  than  insects.  —  Apart  from  insects  the  most  important 
pollinating  animals  are  birds,  especially  those  with  long  slender  bills  and  protrusile 
tongues,  such  as  the  humming  birds,  which  visit  honeysuckles,  trumpet  flowers,  and 
other  long-tubed  blossoms  containing  nectar ;  in  some  cases  birds  visit  flowers  in 
search  of  nectar-feeding  insects.  Bird  pollination  is  much  commoner  in  the  tropics 
and  in  the  southern  hemisphere  than  in  northern  latitudes;  in  parts  of  South  Amer- 
ica, humming  birds  almost  equal  insects  in  importance  as  pollinating  agents,  and 
in  South  Africa  the  sunbirds  and  their  relatives  are  even  more  important,  pollinating 
insects  being  much  less  conspicuous  than  in  the  northern  hemisphere.  The  struc- 
ture of  bird-pollinated  flowers  does  not  differ  from  that  of  flowers  which  are  polli- 
nated by  insects  with  elongated  mouth  parts.  A  few  instances  of  pollination  by 
bats  have  been  reported,  but  they  are  not  regarded  as  important.  Pollination  by 
slugs  or  snails  is  of  possible  importance  in  a  few  cases,  as  in  Calla  and  in  other  aroids 
with  numerous  blossoms  close  together  near  the  ground. 

The  food  of  pollinating  insects.  —  Pollen.  —  Pollinating  insects  visit 
flowers  to  obtain  pollen,  nectar,  or  sap,  and  sometimes  for  shelter,  and 
it  is  while  they  are  engaged  in  one  or  more  of  these  activities  that  pollina- 
tion takes  place  incidentally.  Bees  obtain  nectar,  which  they  store  for 
future  use,  and  pollen,  which  is  in  large  part  utilized  more  immediately 
by  the  larvae,  while  butterflies  and  moths  obtain  only  nectar  and  that  for 
immediate  use;  it  is  largely  because  of  this  that  the  bees  are  more  useful 
pollinators  than  are  the  more  highly  specialized  butterflies.  In  some 
flowers  there  is  little  or  no  nectar  (as  in  Papaver,  Hypericum,  and  Sola- 
num)  and  insect  visits  are  made  mainly  for  pollen,  which  usually  is  pro- 
duced in  considerable  abundance.  The  insects  presumably  get  most 
of  the  pollen,  but  some  of  it  is  pretty  certain  to  be  rubbed  off  on  the 
stigmas.  Nectarless  insect-pollinated  flowers  commonly  are  regular 
(actinomorphic)  and  wide  open,  with  the  anthers  prominently  exposed. 
Sometimes  there  are  two  kinds  of  stamens  (as  in  Cassia),  one  which  the 
insects  visit  for  pollen  and  another  which  sprinkles  pollen  over  the  insects 


REPRODUCTION   AND   DISPERSAL 


843 


in  such  a  way  that  it  is  likely  to  come  into  contact  with  the  stigmas.1 
Commonly  pollen-gathering  insects  are  relatively  non-specialized  (ex- 
cept in  the  case  of  bees),  corresponding  in  general  to  the  lack  of  speciali- 
zation in  the  flowers,  in  which  the  pollen  is  so  exposed  that  it  may  be 
taken  readily  by  any  insect  that  visits  it. 

Nectaries  and  nectar.  —  Nectar-secreting  flowers  commonly  are  more 
specialized  than  are  nectarless  flowers,  and 
the  nectar-gathering  insects  are  the  most 
specialized  of  pollinating  insects.  How- 
ever, there  are  many  simple  actinomor- 
phic  flowers  with  exposed  nectar  (notably 
among  the  umbellifers)  or  with  nectar  but 
slightly  concealed  (as  in  the  crucifers),  which 
are  frequented  by  flies  and  by  other  insects 
with  short  probosces.  From  these  simple 
nectar-producing  flowers  there  are  grada- 
tions in  the  degree  of  concealment  of  nectar 
to  the  highly  specialized  and  often  zygomor- 
phic  forms  in  which  it  is  concealed  at  a 
considerable  depth  at  the  base  of  long  co-  FIG.  1171.- A  longitudinal 

rolla  tubes  or  in  elongated  Spurs  (fig.   Iiyi),     section  through   a   nasturtium 


where  as  a  rule  only  the  most  specialized   flower 

showing  the  spurs  (5)  with  nee- 

insects  With  long  mouth  parts  Can  obtain  tar  (n)  collected  in  its  lower  por- 
it.  In  most  Cases  it  is  difficult  Or  even  tion;  this  flower  is  hypogy- 

impossiWe   for   insects    getting    the    nectar   nous  and  zygomorphic.- From 

°  BARNES  (Part  II). 

to    avoid     rubbing    against     anthers     and 

stigmas,  thus  facilitating  pollination.  As  a  rule,  the  pollen  in  nectar- 
bearing  flowers  is  not  abundant,  and  in  long-tubed-  and  zygomorphic 
flowers  it  commonly  is  concealed. 

Most  arctic  and  alpine  flowers  and  also  most  vernal  flowers  of  temperate  climates 
are  comparatively  simple  in  structure  and  have  their  nectar  supply  relatively  ex- 
posed. On  the  other  hand,  many  tropical  flowers  and  a  large  number  of  estival 
flowers  of  temperate  climates  have  more  specialized  structures,  their  nectar  supply 
being  hidden  in  spurs  or  at  the  base  of  long  corolla  tubes.  With  the  former  there 
may  be  associated  the  general  prevalence  of  insects  with  short  probosces,  charac- 
terizing climates  or  seasons  of  low  temperature,  and  with  the  latter  there  may  be 

1  The  high  degree  of  specialization  here  present  is  shown  by  the  fact  that  the  pollen 
which  is  used  for  food  does  not  readily  germinate  on  account  of  the  absence  of  the  proper 
enzym ;  when  this  is  supplied  artificially,  it  germinates  as  readily  as  does  the  pollen  from 
the  other  stamens. 


844 


ECOLOGY 


associated  the  more  specialized  estival  or  tropical  insects  with  long  probosces.  The 
attraction  of  pollinating  insects  is  not  the  only  advantage  derived  from  floral  nectar; 
there  has  been  previously  noted  the  possibility  that  nectar  associated  with  the  sta- 
mens may  withdraw  water  from  anthers,  causing  their  dehiscence.  It  has  been 
suggested  also  that  nectar  may  play  some  part  in  the  maturation  of  fruit,  and  that  it 
may  help  to  protect  flowers  from  desiccation;  the  last-named  role  seems  especially 
evident  in  the  case  of  flowers  with  water  calyxes. 


Nectar  is  secreted  by  special  structures,  known  as  nectaries,  and  there 
exist  all  gradations  between  those  which  are  composed  of  undifferen- 
tiated  nectar-secreting  tissue  and  those  which  are  specialized  glandular 
hairs  of  complex  structure.  Usually  they  are 
associated  with  the  corolla,  but  they  may  be  con- 
nected with  the  stamens  or  with  any  other  floral 
a  part,  even  with  the  involucre  (as  in  the  poin- 
settia,  Euphorbia  pulcherrima) ,  or  they  may 
occur  on  vegetative  organs,  where  they  are  called 
extrafloral  nectaries  (p.  858);  in  the  poinsettia, 
insects  may  get  the  abundant  nectar  without 
pollinating  the  flowers,  and  in  the  case  of  the 
FIG.  1172.— A  longi-  extrafloral  nectaries  the  visitors  rarely  are  effi- 
tudinai  section  through  a  cient  pollinating  agents.  The  secreting  regions 
floral  nectary  of  the  pom-  are  composed  of  epidermal  cells  rich  in  cyto- 

settia  (Euphorbia  pulcher- 
rima), showing  palisade-     plasm;  commonly  they  are  long  and  narrow  and 
like   secretory  cells    (a)    closely  packed  in  palisade-like  rows  (fig.  1172). 

which   are   rich   in   cyto-      Nectaries    di£fer    from    other    glands    chiefly    in 
plasm;  highly  magnified. 

secreting  sugar;  the  process  is  not  well  under- 
stood, although  the  presence  of  sugar  outside  the  cell  causes  the  with- 
drawal of  water  from  within  and  the  consequent  formation  of  a  drop 
of  nectar,  of  which  sixty  to  eighty-five  .per  cent  usually  is  water. 
Sometimes  the  nectar  forms  in  sufficient  quantity  to  drip  from  the 
secreting  surface,  and  in  some  such  cases  it  collects  in  protected  pouches 
or  sacs,  which  usually  are  corolla  structures  known  as  spurs  (fig.  1171). 
In  most  cases  the  secretion  of  nectar  occurs  only  at  anthesis,  though 
it  may  continue  for  some  time  after  pollination,  as  in  the  tulip  and 
the  quince.  The  secretion  of  water,  but  not  of  sugar,  is  greater  in 
humid  than  in  dry  weather,  quite  as  with  hydathodes.  Indeed, 
there  exist  all  gradations  between  nectaries  and  hydathodes.  Espe- 
cially interesting  transitional  forms  are  seen  in  certain  tropical 
flowers,  whose  glandular  hairs  secrete  but  little  sugar,  though  exuding 


REPRODUCTION   AND    DISPERSAL  845 

large  quantities  of  water,  which  accumulates  in  the  outer  floral  organs, 
giving  rise  to  the  term  water  calyx.  . 

In  a  few  cases  insects  visit  flowers  for  other  kinds  of  food  than  pollen  or  nectar, 
as  in  certain  orchids  (e.g.  M axillarid) ,  where  there  occur  on  the  lip  of  the  corolla 
fragrant  hairs  rich  in  fatty  and  albuminous  foods.  Some  flowers  and  inflorescences 
develop  a  considerable  degree  of  heat  at  anthesis,  and  it  has  been  claimed  that  cer- 
tain insects  visit  them  for  nocturnal  shelter  and  warmth.  Since  most  of  the  con- 
spicuous heat-producing  flowers  and  inflorescences  are  found  among  tropical  palms 
and  aroids,  this  view  seems  untenable. 

Floral  features  accessory  to  pollination.  —  Color.  —  The  role  of  the 
pistil  and  the  stamens  is  very  obvious;  the  protective  and  synthetic  role 
of  the  calyx  also  is  obvious  (p.  869),  but  the  role  of  the  corolla  is  far  less 
evident.  The  corollas  of  flowers,  taken  as  a  whole,  are  ephemeral 
organs  whose  evanescence  is  due  to  their  extreme  delicacy  and  conse- 
quent easy  wilting,  and  to  their  early  abscission,  much  after  the  manner 
of  deciduous  foliage  leaves.  Corollas  present  a  most  bewildering  luxu- 
riance of  form,  color,  and  marking  without  parallel  elsewhere  among 
plant  organs.  Most  colors  except  black  and  green  occur  commonly, 
and  flowers  therefore  contrast  sharply  with  the  foliage.  Reds  and  blues 
are  due  to  anthocyans  dissolved  in  the  cell  sap,  the  former  indicating 
maximum  acidity  and  the  latter  minimum  acidity;  indeed,  certain  flowers, 
as  in  Lychnis,  vary  in  color  with  the  varying  acidity  of  the  cell  sap. 
Some  yellow  flowers  owe  their  color  to  pigments  related  to  the  antho- 
cyans and  like  them  dissolved  in  the  cell  sap.  Orange  colors  and  many 
yellow  colors  are  due  to  plastids  colored  with  carotin,  xanthophyll, 
or  with  related  pigments  (fig.  755).  Brown  colors  are  due  commonly 
to  a  combination  of  plastid  and  sap  pigments.  Flower  pigments  are 
believed  to  be  oxidation  products,  and  whiteness,  which  denotes  the 
absence  of  pigment,  arises  where  the  necessary  oxidizing  ferment  (oxi- 
dase)  is  absent,  or,  if  present,  is  neutralized  by  reducing  agents.  The 
peculiar  color-like  effect  of  white  flowers  is  due  to  the  presence  of  air  in 
the  petals  or  to  unequal  reflection  and  refraction.  Nocturnal  flowers 
especially  are  likely  to  be  white,  and  many  species,  whose  flowers  com- 
monly vary  from  blue  to  red,  may  produce  white  sports,  known  as  albinos. 
The  more  or  less  fundamental  distinction  between  the  anthocyan  (or 
cyanic]  flowers  and  the  yellow  (or  xanthic)  flowers  is  shown  by  the  fact 
that  species  and  even  genera  rarely  change  from  one  to  the  other;  for 
example,  hepaticas  and  asters,  with  all  their  variations,  are  not  yellow, 
or  goldenrods  and  sunflowers  cyanic.  The  cyanic  colors  would  seem  to 


846 


ECOLOGY 


FIG.  1173. — A  flowering 
shoot  of  the  yarrow  (Achillea 
Millefolium),  illustrating  the 
massing  of  flowers  into  heads 
(h),  and  the  massing  of  heads 
into  a  compact  corymb;  r,  ray 
flowers;  i,  involucre. 


be  the  more  specialized,  since  they  contrast 
more  sharply  with  the  foliage,  not  only  in 
aspect  but  fundamentally,  inasmuch  as  most 
yellow  petals  resemble  foliage  leaves  in  hav- 
ing plastids. 

In  many  flowers  showiness  is  increased  by  the 
presence  of  party-colored  effects.  Sometimes  the 
two  halves  or  lips  have  different  colors  (as  in  Col- 
linsia  and  Viola  pedata  bicolor),  but  more  commonly 
the  variegation  is  due  to  spots  or  lines  on  a  back- 
ground of  another  color.  In  some  plants  with  vernal 
flowers  (as  Hepalica)  a  group  of  individuals  may 
exhibit  a  number  of  colors,  varying  from  white 
through  pink  to  blue,  thus  greatly  increasing  the 
showiness  of  the  plant  group  as  a  whole.  Often 
flowers  that  are  inconspicuous  individually  are  so 

cL 


massed  into  compact  inflorescences  as  to  produce  a  showy 
effect;  such  a  condition  is  seen  in  the  umbellifers  and  even 
more  in  the  composites,  where  the  inconspicuous  central  or 
disk  flowers  often  are  surrounded  by  showy  outer  or  ray 
flowers,  giving  the  effect  of  a  large  simple  flower  (fig.  1173). 
The  inflorescences  of  Hydrangea  consist  similarly  of  incon- 
spicuous central  and  of  showy  outer  flowers,  the  latter  being 
sterile.  In  some  plants  the  calyx  is  the  showy  organ  (as  in 
Abronia  and  Mirabilis),  and  in  some  species  of  Castilleja, 
Euphorbia,  and  Monarda  the  bracts,  or  even  the  upper 
leaves,  are  much  showier  than  are  the  relatively  insignificant 
flowers.  In  some  dogwoods  the  involucre  is  much  showier 
than  are  the  flowers,  and  in  the  willows  where  there  is  no 
perianth,  the  staminate  catkins  often  are  showy  by  reason 
of  the  conspicuous  stamens. 

Zygomorphy.  —  Zygomorphy  or  irregularity  in  the  corolla 
often  adds  to  the  conspicuousness  of  flowers.  Many  flowers 
are  labiate  or  lipped  (as  in  the  mints  and  the  legumes),  the 
lower  lip  commonly  protruding  farther  than  the  upper  (fig. 
1174);  the  culmination  of  lip  development  and  zygomorphy 
is  found  in  the  orchids,  whose  flowers  are  noted  for  their 
bizarre  shapes.  The  projecting  lower  lip  is  of  obvious 
advantage  as  a  landing  place  for  pollinating  insects,  notably 
the  bees.  Often,  as  in  the  flowers  of  many  legumes,  the 
weight  of  the  insect  presses  down  the  lip  sufficiently  to 
expose  the  anthers  and  the  stigma.  Floral  lips  are  of  no 
advantage  for  the  hawk  moths  and  for  similar  insects,  which 
hover  before  the  flowers  without  alighting.  As  a  class,  acti- 
nomorphic  flowers  are  erect,  contrasting  with  the  generally 


FIG  1174. — Flow- 
ers of  Coleus,  illus- 
trating zygomorphy; 
the  calyx  (c)  and  the 
sympetalous  corolla 
(cf)  are  bilabiate,  the 
latter  having  an  as- 
cending upper  lobe  (6) 
and  a  descending  boat- 
shaped  lower  lobe  (a), 
from  which  the  sta- 
mens and  the  style 
are  partially  exserted ; 
the  lower  lobe  of  one 
flower  (/)  is  held  to 
one  side,  so  as  to  show 
more  clearly  the  up- 
turned style  (0  with 
its  two-lobed  stigma 
(g)  and  the  four  sta- 
mens (s). 


REPRODUCTION    AND    DISPERSAL  847 

lateral  display  of  zygomorphic  flowers  (fig.  1174),  which  thus  are  well  suited  for  in- 
sects that  alight  on  lips  or  hover  before  the  flowers. 

Odor.  —  The  attractiveness  of  flowers  to  insects  is  in  large  part  due  to 
their  fragrance.  Most  fragrant  flowers  are  also  showy  (as  in  the  lilacs, 
roses,  crabs,  and  water-lilies),  but  some  very  fragrant  flowers  are  incon- 
spicuous (as  in  the  grape  and  mignonette),  just  as  some  very  showy 
flowers  are  without  appreciable  odor  (as  in  the  poppy).  In  many  plants 
(as  in  Smilax  herbacea  and  Trillium  erectum)  the  odor,  though  offensive 
to  human  nostrils,  attracts  certain  insects.  Some  flowers  that  are  rela- 
tively odorless  by  day  are  very  fragrant  at  night  (as  in  species  of  Silene). 
Flower  fragrance  commonly  is  due  to  the  escape  of  volatile  oils  into  the 
atmosphere.  A  remarkable  case  of  floral  dimorphism  is  seen  in  Renan- 
thera,  a  tropical  orchid;  most  of  the  flowers  are  white  and  inodorous,  but 
at  the  base  of  the  inflorescence  are  two  fragrant  yellow  flowers  which 
bloom  first  and  remain  fresh  and  fragrant  until  all  the  other  flowers 
have  gone. 

The  sensitiveness  of  pollinating  insects  to  color  and  to  odor.  —  It  is 
believed  commonly  that  odors  and  bright  colors  in  flowers  are  of  great 
importance  as  indicators  (or  "  signals  ")  to  insects  of  the  presence  of 
nectar  or  pollen,  and  some  observers  even  go  so  far  as  to  suppose  that 
these  features  have  arisen  through  natural  selection,  the  insects  preferring 
the  more  fragrant  and  showy  flowers,  while  others  go  unpollinated,  so 
that  the  plants  bearing  them  have  no  progeny.  There  is  no  evidence 
whatever  for  the  selection  theory  of  the  prevalence  of  showiness  and  odor, 
and  even  the  theory  that  insects  are  attracted  by  color  and  by  fragrance 
rests  too  little  on  experiment  and  too  much  on  the  untenable  assumption 
that  the  theory  must  be  true,  because  nobody  knows  any  other  role  for 
these  floral  features.  It  is  a  tenable  hypothesis  that  such  features  are 
without  value  to  the  flowers  possessing  them,  and  the  "  signal  "  theory 
deserves  support  only  as  it  is  proven  experimentally. 

It  is  not  certain  that  insect  attraction  is  the  only  possible  role  of  colored  corollas; 
it  has  been  suggested  that  they  may  play  an  important  part  in  the  chemistry  of  fruit 
maturation.  Pigmented  plastids  may  be  important  in  food  making,  and  pigmented 
cell  sap  may  indicate  the  formation  of  useless  by-products.  It  is  to  be  noted  that 
some  wind-pollinated  flowers  are  very  showy,  as  in  the  larch  and  the  red  maple. 
Corollas  also  are  of  some  importance  as  protective  organs  for  the  pollen  and  stigmas, 
especially  in  flowers  whose  corollas  close  at  night  and  in  stormy  weather. 

The  possession  of  a  keen  sense  of  odor  by  pollinating  insects  is  un- 
doubted, inconspicuous  fragrant  flowers  being  visited  much  more  than 


848  ECOLOGY 

are  showy  odorless  flowers.  The  readiness  with  which  flies  are  drawn 
to  sources  of  nauseous  odors  is  well  known,  and  they  frequent  ill-smelling 
flowers  in  a  similar  fashion.  Hawk  moths  have  been  found  to  be  able 
to  detect  at  a  distance  of  several  meters  the  presence  of  fragrant  but  in- 
visible nocturnal  flowers,  and  bees  have  been  seen  to  fly  directly  toward 
honey  artificially  hidden.  Indeed,  there  are  reasons  for  believing  that 
many  insects  are  able  to  detect  odors  that  are  inappreciable  to  human 
nostrils. 

The  possession  of  a  keen  sense  of  color  is  much  less  certain.  Even  the 
ardent  supporters  of  the  "  signal  "  theory  hardly  postulate  it  except  for 
the  more  specialized  insects,  such  as  butterflies  and  bees.  The  best  ex- 
periments indicate  that  insects  are  very  short-sighted,  none  being  able  to 
see  distinctly  for  more  than  sixty  centimeters,  and  bees  very  much  less 
than  that.  Objects  in  strong  contrast  (such  as  large  light  and  dark 
bodies  in  juxtaposition,  or  bodies  in  motion)  appear  to  be  seen  much 
farther  than  are  other  objects,  certain  Lepidoptera  seeming  to  be  able 
to  see  thus  vaguely  for  a  meter  and  a  half,  and  bees  for  a  half  meter. 
The 'only  insects  in  which  color  perception  has  been  definitely  demon- 
strated are  the  honeybees  (Apis).  These  highly  organized  insects  often 
have  been  seen  to  visit  gaudy  but  nectarless  artificial  flowers,  and  some- 
times they  attempt  to  get  at  showy  natural  flowers  that  are  under  glass. 
Frequently  they  visit  colored,  unopened  buds  and  wilted  flowers,  the 
latter  being  at  times  approached,  even  after  they  have  fallen  to  the  ground. 
Apiarists  rather  generally  believe  that  honeybees  are  at>le  to  perceive 
color  differences,  and  hence  they  sometimes  paint  their  hives  in  different 
colors,  so  as  to  aid  the  bees  in  recognizing  their  abode.  To  the  extent 
that  color  is  perceived  by  insects,  it  is  a  much  more  reliable  "  signal  " 
than  odor,  since  the  latter  often  is  affected  by  the  wind  or  masked  by 
other  odors.  Probably  the  characteristic  forms  of  flowers  serve  as  in- 
dices to  nectar,  especially  in  the  case  of  flowers  that  are  conspicuous 
by  their  shape  or  by  their  size;  some  observers  think  that  form  is 
even  more  important  than  color  as  an  insect  "  signal." 


Some  investigators  believe  that  honeybees  not  only  perceive  colors,  but  that  they 
have  marked  color  preferences.  Experiments  with  honey  on  colored  papers  seem 
to  show  that  bees  tend  to  visit  a  particular  color,  even  if  others  are  more  conveniently 
situated,  and  elaborate  theories  have  been  worked  out  on  the  assumption  that  bees 
dislike  yellow  and  prefer  blue,  whence  it  seems  to  some  observers  an  easy  postulate 
that  the  day  of  yellow  flowers  is  waning  and  that  of  blue  flowers  is  in  the  ascendant. 
Such  conclusions  certainly  are  unwarranted.  The  constancy  of  the  honeybee  to 


REPRODUCTION   AND   DISPERSAL 


849 


a  given  color,  such  as  blue,  does  not  mean  a  preference  for  blue  as  such,  but  the  asso- 
ciation of  nectar  or  pollen  with  that  color.  If  a  bee  commences  its  activities  on  a 
red  flower,  or  on  honey  placed  on  a  red  paper,  it  is  constant  to  red.  In  visiting 
flowers,  bees  are  constant  not  only  to  color,  but  also  to  form,  flying  from  flower  to 
flower  of  the  same  species.  This  constancy  to  a  given  plant  species  for  a  certain 
period  is  of  great  advantage  to  the  plant,  since  it  means  a  minimum  waste  of  pollen. 
Ft  is  equally  of  advantage  to  the  bees,  since  the  nectar  or  pollen  is  all  of  the  same 
quality,  and  since  time  and  energy  are  saved  in  that  exactly  the  same  process  is 
repeated  in  each  flower  that  is  visited.  The  collapse  of  the  color  preference  theory 
is  well  shown  in  those  cases  in  which  different  individuals  of  a  given  plant  species 
have  flowers  of  different  colors.  In  such  species  bees  soon  learn  the  essential  like- 
ness of  the  differently  colored  flowers,  going  from  one  color  to  another  indifferently. 
In  other  words,  bees  learn  to  ignore  differences  in  color  that  are  unaccompanied  by 
differences  in  nectar  or  pollen.  Even  if  bees  prove  to  be  the  only  insects  with  a 


FIG.  1175.  —  A  colony  of  morning  glories  (Calystegia  Soldanella)  in  dune  sand;  note 
the  striking  contrast  in  tone  between  the  flowers  and  the  foliage,  illustrating  the  possibility 
of  floral  showiness  even  for  color-blind  insects;  New  Zealand.  —  From  COCKAYNE. 

color  sense,  other  insects  certainly  are  able  to  appreciate  differences  in  tone,  as  they 
appear  in  a  photographic  print  (fig.  1175),  where  whites  and  various  colors  come 
into  sharp  contrast  with  the  darkness  of  the  foliage.  Similarly,  the  prevalent  white- 
ness of  nocturnal  flowers  makes  them  more  conspicuous  than  would  any  pigment 
color. 

Memory  and  instinct.  —  When  bees  are  taken  to  a  new  feeding  ground, 
their  first  flights  are  more  or  less  misdirected  and  haphazard,  resembling 


850  ECOLOGY 

the  habitual  movements  of  such  insects  as  the  flies.  Soon  they  appear 
to  be  attracted  by  various  odors  or  colors,  and  after  some  days  they 
show  their  accustomed  rapid  and  precise  movements.  That  is,  memory 
appears  to  replace  both  odor  and  color  as  the  directive  stimulus  of  first 
importance.  Probably  many  discordant  results  of  various  observers 
can  be  harmonized  if  the  memory  factor  is  taken  into  account.  There 
are  some  cases  where  instinct  seems  to  be  the  controlling  factor,  as  in 
the  pollinating  insects  of  the  figs  and  the  yuccas  (pp.  860,  864). 

Many  experiments  with  bees  show  the  importance  of  the  memory  factor.  When 
showy  flowers  are  deprived  of  their  corollas,  the  number  of  visiting  bees  at  first  is 
small,  but  after  a  time  the  insects  become  accustomed  to  the  new  conditions  and 
visits  become  numerous.  In  some  such  experiments  the  seed  production  is  less  than 
in  flowers  with  corollas,  but  this  may  be  due  to  lessened  protection  of  the  ovary  or 
to  a  less  effective  dusting  of  the  stigmas  with  pollen.  If  flowers  are  artificially 
hidden  by  leaves,  bees  soon  learn  the  new  conditions,  and  the  visits  which  at  first 
are  few  soon  become  frequent.  Similarly  bees  soon  learn  to  visit  wind-pollinated 
flowers  if  there  is  placed  on  them  honey  and  water,  or  sugar  and  a  fragrant  volatile 
oil.  The  ability  of  bumblebees  to  learn  is  shown  by  Bombus  terrestris,  which  has 
a  proboscis  too  short  to  get  honey  from  Aquilegia  vulgar  is;  after  vain  attempts  to 
reach  the  nectar  in  the  ordinary  way,  it  has  been  seen  to  bite  a  hole  in  the  spur  and 
suck  it  out,  repeating  the  process  thenceforth.  Similar  holes  are  bitten  in  the  spurs 
of  Tropaeolum  by  Bombus  hortorum. 

Concluding  remarks.  —  As  a  directive  stimulus,  insuring  the  visitation 
of  flowers  by  insects,  odor  seems  to  be  more  important  than  color,  be- 
cause it  is  distinguished  from  a  much  greater  distance  and  by  a  much 
larger  number  of  insects;  in  the  higher  insects,  notably  among  the  bees, 
which  do  most  of  the  pollinating,  memory  seems  to  be  a  still  more  im- 
portant factor.  In  the  majority  of  flies  and  in  most  lower  insects  it  is 
doubtful  if  either  color  or  memory  plays  a  very  conspicuous  part,  the 
odor  sense  here  being  regarded  as  the  most  important.  Odor  is  of  par- 
ticular significance  where  flowers  grow  in  masses.  So  far  as  color  and 
form  play  a  part,  it  is  only  in  the  immediate  vicinity  of  the  flower  and  in 
the  most  general  way.  The  elaborate  theories  which  assign  a  distinct 
role  for  each  floral  form  and  for  each  shade  of  color,  which  regard  the 
lines  and  spots  on  the  corolla  as  guides  to  the  nectar,1  and  which  relate 
the  showiness  of  alpine  flowers  to  the  paucity  of  insects  have  no  support 
from  exact  observation  and  experiment. 

Features  favoring  the  sprinkling  of  insects  with  pollen.  —  In  most 
flowers,  especially  in  those  that  are  open  and  actinomorphic,  the  anthers 

1  Striking  spots  or  lines  may  occur  in  such  nectarless  flowers  as  that  of  the  poppy. 


REPRODUCTION   AND   DISPERSAL 


851 


are  exposed  so  conspicuously  and  the  pollen  is  so  abundant  that  visiting 

insects  scarcely  can  avoid  getting  more  or  less  pollen  on  their  bodies, 

even  if  they  are  searching  only  for  nectar;  of  course,  much  pollen  must 

adhere  to  all  pollen-gathering  insects.     In  the  composites  the  dense 

massing  of  flowers  into  heads  greatly  facilitates  pollen  removal,  since  the 

visiting  insects  necessarily  crawl  over  numerous  flowers  with  their  ex- 

serted  stamens.     In   many  flowers,  especially  in 

those  which   are  zygomorphic  or  which  contain 

but  little  pollen,  there  often  are  specialized  features 

that  facilitate  pollen  removal.     Certain  parts  of 

the  body  (chiefly  about  the  head)  may  receive  the 

pollen  somewhat  locally;  in  certain  flowers  exserted 

stamens  often  are  grasped  by  the  alighting  insect  in 

such  a  way  that  the  under  parts  of  the  body  receive 

the  pollen. 


Some  flowers  with  introrse  anthers  (i.e.  opening  in- 
wards), as  in  the  gentians,  have  nectar  to  the  interior  of 
the  stamens,  while  some  flowers  with  extrorse  anthers  (i.e. 
opening  outwards),  as  in  Iris,  have  nectar  to  the  exterior  of 
the  stamens.  Often  the  stamens  grow  rapidly  just  before 
dehiscence  (as  in  Parnassia),  assuming  a  position  corre- 
sponding to  that  of  the  stigma.  In  a  number  of  instances 
the  insect  occasions  the  release  of  the  pollen,  as  in  the 
legumes,  where  the  alighting  of  a  bee  causes  the  anthers  to 
protrude  suddenly  from  the  enclosing  petal  and  to  sprinkle 
pollen  over  the  visitor.  In  Pyrola  and  Kalmia  the  anthers 
are  held  in  unstable  equilibrium,  and  the  sudden  release 
coming  with  the  insect  visit  causes  the  pollen  to  be  shaken 
out.  In  various  ericads  with  pendulous  flowers  the  sta- 
mens have  appendages,  which  are  likely  to  be  struck  by 
visiting  insects  in  such  a  way  as  to  result  in  the  scattering 
of  the  pollen.  Sensitive  mechanisms  occur  also  in  Lopezia, 
where  a  petal-like  structure  holds  the  single  stamen  in  un- 
stable equilibrium,  in  Berberis,  where  the  stamen  itself  is 
sensitive  to  contact  (fig.  1176),  in  Galeopsis,  where  contact 
causes  the  anther  lids  to  fly  open,  and  in  Crucianella, 
where  the  style  is  held  in  unstable  equilibrium  until  the 
flower  is  touched,  whereupon  the  style  is  suddenly  released,  bringing  out  with  it  a 
shower  of  pollen.  In  Salvia,  in  which  there  is  a  swinging  anther,  an  entering  bee  so 
presses  against  the  lower  arm  of  the  lever  as  to  dust  himself  with  pollen  from  the 
upper  arm  (figs.  1166-1169).  In  orchids  the  pollen  masses  (pollinia)  have  an  ex- 
posed adhesive  disk,  which  sticks  to  the  head  parts  of  a  visiting  insect.  The  some- 
what similar  pollinia  of  milkweeds  have  clips  that  fasten  about  the  feet  of  the  insect. 


FIG,  1176.  —  A  bar- 
berry flower  (Berberis 
Thunbergii)  with  the 
calyx  and  corolla  re- 
moved, so  as  to  show 
the  pistil  (p)  and  the 
hypogynous  stamens, 
which  at  maturity  lie 
back  upon  the  inner 
surface  of  the  petals; 
when  an  insect  comes 
in  contact  with  the  base 
(6)  of  the  filament  (/), 
the  latter  flies  forward, 
assuming  the  position 
of  the  stamen  at  the 
right,  and  pollen  is 
dusted  on  the  insect  and 
on  the  stigma  (g) ;  note 
that  the  filaments  (/) 
broaden  toward  the 
apex,  and  that  the  an- 
ther valves  (a)  open 
upwards,  being  hinged 
at  the  filament  apex. 


852  ECOLOGY 

Features  favoring  the  deposition  of  pollen  on  stigmas.  —  Stigmas,  as 
previously  noted,  secrete  sticky  substances,  and  their  hairy  or  papillate 
surfaces  still  further  favor  pollen  reception  (fig.  1158).  In  many  plants 
the  stigmas  at  maturity  have  essentially  the  same  position  as  that  of  the 
mature  anthers  (as  in  the  figwort,  figs.  1178,  1179),  so  that  the  part  of 
the  insect  which  is  covered  by  pollen  is  likely  to  touch  the  stigma.  In 
many  other  cases  (as  in  the  violets)  the  stigma  projects  beyond  the  an- 
thers, so  that  it  is  likely  to  receive  pollen  from  the  entering  proboscis. 

In  Centaurea  mechanical  irritation  (as  from  a  visiting  insect)  causes  the  fila- 
ments to  contract,  thus  exposing  the  stigma  to  pollination  by  the  visitor.  The  most 
remarkable  situation  is  in  the  orchids,  where  the  pollinia  above  noted,  after  removal 
from  the  flower,  move  into  such  a  position  that  they  are  likely  to  come  into  contact 
with  the  stigma  of  the  next  flower  visited.  The  orchid  stigmas  remain  receptive  a 
remarkably  long  time  if  potent  pollen  fails  to  come  in  contact  with  them,  though 
they  wither  soon  after  the  proper  pollen  begins  to  germinate.  The  corollas  also 
remain  fresh  on  unpollinated  flowers  some  days  or  even  weeks  longer  than  on  polli- 
nated flowers. 

Features  which  impede  close  pollination  and  facilitate  cross  pollina- 
tion. —  Mechanical  features  impeding  close  pollination.  —  In  a  vast 
number  of  flowers  close  pollination  is  difficult  or  even  impossible.  Fre- 
quently the  stigma  projects  beyond  the  anthers  (as  in  certain  lilies  and 
evening  primroses),  so  that  pollen  cannot  fall  upon  the  stigma  of  the  same 
flower  (fig.  1174);  in  pendulous  flowers,  of  course,  the  stamens  would 
have  to  project  beyond  the  stigma  to  have  a  like  result.  Sometimes,  as 
in  Iris,  the  receptive  surface  of  the  stigma  is  so  oriented  that  the  insect 
rubs  against  it  upon  entering,  but  not  upon  leaving  the  flower,  thus 
facilitating  cross  pollination  and  preventing  close  pollination.  Close 
pollination  is  difficult  in  flowers  with  extrorse  anthers.  In  orchids  it  is 
almost  impossible  for  the  pollinia  to  come  into  contact  with  the  stigma 
of  the  same  flower;  in  some  lady's  slippers  (as  in  Cypripedium  Cal- 
ceolus)  the  insect  enters  and  leaves  the  flowers  by  different  routes,  brush- 
ing the  stigma  upon  entering  and  the  anthers  before  leaving. 

Dichogamy.  —  The  commonest  floral  feature  that  facilitates  cross 
pollination  and  makes  close  pollination  difficult  is  dichogamy,  or  the 
consecutive  maturity  of  anthers  and  stigmas,  contrasting  with  simulta- 
neous maturity  or  homogamy.  Dichogamy  may  be  complete,  that  is, 
the  pollen  may  be  shed  before  the  stigma  matures,  or  the  stigma  may 
wither  before  the  pollen  sheds;  more  commonly  it  is  incomplete,  that  is, 
there  is  a  partial  overlapping  of  the  periods  of  stigmatic  receptiveness 


REPRODUCTION    AND    DISPERSAL 


8S3 


and  of  the  shedding  of  pollen.     Stigmas  commonly  remain  receptive 

(especially  when  unpollinated)  for  a  longer  time  than  that  required  for 

the  shedding  of  the  pollen,  hence  cross  pollination 

is  more  likely  to  result  when  flowers  are  protan- 

drous  (i.e.  with  the  anthers  maturing  first)   than 

when  they  are  protogynous  (i.e.  with  the  stigmas 

maturing    first);    however,    pollination    of    some 

kind,  either  cross  or  close,  is  more  likely  to  result 

when  the    flowers   are  protogynous,   because  of 

the  greater  likelihood  of  overlap  in  the  latter  case. 

Probably  the  number  of  protandrous  and  protogynous 
species  is  about  equal,  though  there  are  a  greater  number 
of  conspicuously  protandrous  forms,  such  as  the  saxifrage 
(fig.  1177),  the  evening  primrose 
(in  which  the  anthers  may  shed 
before  the  corolla  opens),  the 
composites,  and  the  umbclli- 
fcrs,  than  there  are  of  conspic- 
uously protogynous  forms,  such 
as  the  figwort  (figs.  1178,  1179) 
and  the  crucifers.  A  striking 
case  of  protogyny  occurs  in 
Aristolochia  Clematitis,  where 
the  interior  of  the  narrow  calyx 
tube  is  lined  with  reflexed  hairs. 
Insects  enter  easily  and  crawl 
over  the  mature  stigma,  but  on 
account  of  the  stiff  hairs  they 
cannot  leave  until  the  anthers 

mature,  when  they  become  dusted  with  pollen;  the  subse- 
quent withering  of  the  calyx  hairs  permits  their  exit,  and 
upon  their  entering  another  flower,  cross  pollination  takes 
place. 

In  most  dichogamous  flowers  the  stigmas  and  the  an- 
thers, though  usually  occupying  the  same  position  consecu- 
tively, nevertheless  are  out  of  the  way  of  the  one,  when  the 
other  is  mature.  In  the  mallows  the  anthers  at  first  hide 
the  stigmas,  but  later  bend  back  and  expose  them,  while 
in  Salvia  the  style  which  at  first  is  short  grows  out  after 
the  pollen  is  shed,  assuming  a  favorable  position  for  pollen  reception  by  the  stigma. 
In  the  figwort,  which  has  a  protogynous  flower,  the  style  bends  back  over  the  lip 
after  maturity  (fig.  1179).  In  Parnassia  one  stamen  after  another  assumes  a  posi- 
tion where  visiting  insects  are  likely  to  come  into  contact  with  them.  In  most 
dichogamous  flowers  (but  not  in  Aristolochia)  two  insect  visits  are  necessary  if  both 


FIGS.  1178,  1179.— 
Flowers  of  the  figwort 
(Scrophidaria,  marilan- 
dica),  illustrating  pro- 
togyny: 1178,  a  young 
flower  with  a  promi- 
nently exserted  style  (t) 
and  a  receptive  stig- 
matic  surface  (g);  1179, 
the  same  flower  a  day  or 
two  later,  with  its  style 
(t~)  declined  and  out  of 
the  way  of  visiting  in- 
sects, the  stamens  hav- 
ing grown  sufficiently  to 
expose  the  anthers  (a) 
to  such  pollinating 
agents;  note  that  the 
sympetalous  corolla  (c?) 
is  bilabiate;  c,  calyx. 


FIG  .1177.  —  Flowers 
of  a  saxifrage  (Saxi- 
fraga  sarmentosa),  illus- 
trating protandry;  in 
the  younger  (upper) 
flower,  the  anthers  (a) 
are  mature  and  the  pis- 
tils (s)  immature;  in  the 
older  (lower)  flower,  the 
anthers  (a')  have  shed 
their  pollen  and  the  pis- 
tils (sr)  have  become 
mature;  note  that  the 
corolla  is  zygomorphic, 
three  of  the  petals  (p) 
being  short  and  two 
long  (/>'). 


$54 


ECOLOGY 


pollen  and  stigma  are  to  play  a  part  in  pollination;  this  is  an  apparent  disadvan- 
tage as  compared  with  homogamous  flowers. 

Heterostyly.  —  A  highly  specialized  condition  that  opposes  close 
pollination  and  favors  cross  pollination  is  that  known  as  heterostyly,  in 
which  the  stigmas  and  the  anthers  in  different  flowers  occupy  different 
positions.  Most  such  flowers  are  actinomorphic,  and  they  are  illustrated 
by  the  primrose,  flax,  forget-me-not,  and  bluets.  In  the  primrose  some 
plants  have  flowers  with  long  styles,  the  stamens  being  attached  toward 
the  base  of  the  corolla  tube,  while  other  plants  have  flowers  with  short 

styles  and  with  the  sta- 
mens attached  toward  the 
upper  part  of  the  corolla 
tube  (figs.  1 1 80,  1181). 
In  LythrumSalicaria  there 
are  three  kinds  of  flowers, 
one  with  long  styles  and  in- 
termediate and  short  sta- 
mens, another  with  short 
styles  and  intermediate 
and  long  stamens,  and  a 
third  with  intermediate 
styles  and  long  and  short 
stamens.  The  same  part 


FIGS.  1180,  1181.  —  Longitudinal  sections  through 
flowers  of  the  Chinese  primrose  (Primula  sinensis), 
illustrating  heterostyly:  1180,  a  flower  with  a  long 
style  (/)  and  with  stamens  (s)  inserted  near  the  base 
of  the  corolla  tube  (</);  "81,  a  flower  with  a  short 
style  (t)  and  with  stamens  (s)  inserted  at  the  median 
part  of  the  corolla  tube  (c7);  that  part  of  a  visiting 
insect  which  strikes  the  anthers  of  one  flower  will  be 
likely  to  strike  the  stigma  of  the  other,  thus  effecting 
cross  pollination;  note  that  the  corolla  tube  of  1181 
is  dilated  where  the  stamens  are  inserted;  these 
flowers  illustrate  perigyny;  c,  calyx;  o,  ovary. 


of  the  insect  that  comes  in 
contact  with  the  lower  sta- 
mens will  touch  the  stigma  of  a  short-styled  flower,  while  pollen  from 
the  upper  stamens  will  come  in  contact  with  the  stigma  of  a  long-styled 
flower,  thus  insuring  cross  pollination. 

Commonly  the  upper  stamens  of  heterostyled  flowers  have  large  pollen  grains 
corresponding  to  the  large  long-haired  stigmas  of  the  long-styled  flowers,  while 
the  lower  stamens  have  small  grains  corresponding  to  the  small  smooth  stigmas  of 
the  short-styled  flowers;  the  corollas  and  other  organs  also  may  differ  considerably. 
Some  investigators  regard  the  large  size  of  the  pollen  grains  of  the  upper  stamens 
as  advantageous,  since  their  pollen  tubes  have  to  traverse  a  greater  distance  upon 
germination;  this  view,  which  is  doubtful  a  priori  on  account  of  their  parasitic 
nourishment,  has  been  experimentally  disproven. 

Impotent  and  prepotent  pollen. . —  So  far  as  the  prevention  of  close 
pollination  is  concerned,  the  most  specialized  flowers  are  those  in  which 
the  pollen  of  a  given  flower  is  impotent  (i.e.  unable  to  initiate  seed  produc- 


REPRODUCTION   AND   DISPERSAL  855 

tion)  on  the  stigma  of  the  same  flower.  Complete  impotence  is  compara- 
tively rare,  well-known  cases  being  found  in  Corydalis  cava,  Hemerocallis 
fulva  (day  lily),  Fagopyrum  esculentum  (buckwheat),  Secale  cereale 
(rye),1  and  also  in  several  of  the  Leguminosae,  but  there  are  many  plants 
in  which  foreign  pollen  (i.e.  pollen  from  other  flowers)  is  prepotent 
(i.e.  more  or  earlier  effective)  on  a  given  stigma  than  own  pollen 2  (i.e. 
pollen  from  the  same  flower);  foreign  pollen  that  is  sown  on  a  stigma 
several  hours  after  own  pollen  often  gains  the  ascendency  in  a  very  short 
time.  The  acme  of  impotence  is  found  in  various  orchids,  in  which  own 
pollen  actually  is  prejudicial  to  the  stigmas  (or  vice  versa),  appearing 
to  behave  like  a  poison.  In  the  Leguminosae  own  pollen  is  much  more 
potent  in  the  annual  species  than  in  the  perennials.  In  some  legumes 
(as  Cytisus  Laburnum)  the  usual  impotence  of  own  pollen  is  due  to  the 
fact  that  the  pollen  tube  cannot  penetrate  the  cuticle  of  the  stigma; 
when  this  is  ruptured  artificially,  own  pollen  is  potent.  In  Corydalis 
cava  own  pollen  frequently  germinates,  but  the  pollen  tube  is  unable  to 
penetrate  to  the  ovules. 

In  nearly  every  case  pollen  from  a  given  flower  is  no  more  potent  on 
other  flowers  of  the  same  plant  than  on  the  stigma  of  the  flower  that  pro- 
duced it,  thus  showing  in  a  most  striking  way  that  geitonogamy  is  essen- 
tially the  same  as  autogamy  and  should  not  be  classed  with  xenogamy. 
In  a  number  of  cases  own  pollen  appears  sometimes  to  be  impotent,  and 
sometimes  variously  potent  (as  in  Eschscholtzia  and  in  Brassica  Rapa), 
possibly  by  reason  of  varying  external  conditions.  From  the  viewpoint 
of  pollen  potency,  therefore,  there  are  three  classes  of  plants:  (i)  those 
in  which  own  pollen  is  as  potent  as  foreign  pollen,  forming  a  class  with 
numerous  representatives  (as  Oenothera  and  most  crucifers) ;  (2)  those 
in  which  foreign  pollen  is  prepotent,  also  forming  a  class  of  large  size; 
and  (3)  those  in  which  own  pollen  is  impotent,  forming  a  comparatively 
small  class. 

Among  the  most  remarkable  examples  of  impotence  are  those  afforded 
by  heterostyled  flowers,  own  pollen  being  completely  impotent  in  Linum, 
and  slightly  potent  in  Primula.3  The  most  extraordinary  feature  of 
these  plants,  however,  is  that  cross  pollination  between  the  anthers  and 


1  Even  in  rye  geitonogamy  may  occur. 

2  However,  the  pollen  must  not  be  too  foreign,  as  from  another  genus  or  family.     Thus 
impotence  is  found  at  the  extremes  of  relationship,  that  is,  where  pollination  occurs  be- 
tween anthers  and  stigma  in  the  same  flower  or  in  flowers  of  distantly  related  plants. 

3  Some  observers  report  the  complete  potency  of  own  pollen  in  some  species  of  Primula. 


856  ECOLOGY 

stigmas  of  different  position  in  separate  plants  is  quite  as  ineffective  as 
is  close  pollination,  thus  showing  clearly  that  the  cause  of  impotence  is 
not  closeness  of  relationship,  but  something  as  yet  unknown.  A  possible 
advantage  of  this  peculiar  phenomenon  is  seen  in  the  fact  that  the  prog- 
eny of  individuals  which  are  close  pollinated,  or  of  individuals  where 
there  is  cross  pollination  between  anthers  and  stigmas  of  different  posi- 
tion, usually  is  made  up  of  plants  with  but  one  kind  of  flower.  If  cross 
pollination  is  advantageous  (see  p.  866),  the  combination  of  heterostyly 
and  impotence  in  own  pollen  would  seem  to  be  particularly  advantageous, 
since  own  pollen  is  scarcely  likely  to  be  deposited  on  a  stigma,  and  if  it 
should  chance  to  lodge  there,  it  would  not  initiate  seed  production. 

Dicliny.  —  Dicliny,  which  commonly  is  regarded  as  a  primitive  floral 
feature,  is  more  characteristic  of  wind-pollinated  than  of  insect-pollinated 
plants,  but  it  is  far  more  common  in  the  latter  than  formerly  was  supposed, 
and  there  is  almost  certain  proof  of  a  strong  evolutionary  tendency  from 
monocliny  to  dicliny,  as  in  the  figs  and  in  many  composites.  Among 
the  diclinous  insect-pollinated  species  that  probably  are  primitive,  the 
best  known  are  the  willows,  which  are  dioecious.  Some  investigators 
doubt  whether  as  many  as  half  of  the  plants  that  appear  to  be  monoclinous 
are  so  in  fact.  A  large  number  of  species  have  both  monoclinous  and 
diclinous  flowers  on  the  same  or  on  different  plants;  the  maples  illus- 
trate this  condition,  some  of  them  (as  the  box  elder)  appearing  to  have 
become  completely  dioecious.  Certain  cultivated  varieties  of  the  straw- 
berry exhibit  similar  features. 

Asparagus  appears  to  have  become  essentially  dioecious,  since  the  stamens  of 
some  plants  and  the  pistils  of  others  appear  to  play  no  part  in  pollination.  Many 
species  (as  in  the  grape  and  the  horse  chestnut)  have  been  found  to  possess  impotent 
pollen  in  some  flowers,  and  non-receptive  stigmas  in  others.  Rhamnus  lanceolata,  a 
heterostyled  species,  seems  to  be  approaching  dioecism,  since  the  short-styled  flowers 
produce  the  most  seed,  while  the  long-styled  flowers  have  but  little  pollen  and  that 
small-grained.  Many  plants  have  organs  occupying  the  position  of  stamens,  which 
now  play  no  direct  part  in  pollination,  whatever  may  have  been  the  case  formerly ; 
notable  illustrations  are  the  so-called  sterile  stamens  of  Parnassia  (now  nectar- 
secreting  organs)  and  of  Pentstemon. 

Much  the  most  significant  tendency  toward  dicliny  is  seen  in  the  composites, 
which  commonly  are  regarded  as  the  highest  family  of  plants.  In  this  family  there 
are  three  common  floral  conditions,  that  in  which  all  the  flowers  are  actinomorphic 
and  inconspicuous  (as  in  Eupatorium),  that  in  which  all  the  flowers  have  conspicu- 
ous strap-shaped  (ligulate),  zygomorphic  corollas  (as  in  the  dandelion,  fig.  1182), 
and  that  in  which  there  are  actinomorphic  and  inconspicuous  disk  flowers,  sur- 
rounded by  petal-like  zygomorphic  ray  flowers;  the  third  group  is  much  the  largest 


REPRODUCTION   AND   DISPERSAL 


857 


and  includes  the  asters,  goldenrods,  and  sunflowers  (fig.  1173).  Perfect  monocliny 
is  confined  essentially  to  the  second  and  to  a  part  of  the  first  group,  so  that  a  greater 
or  less  amount  of  dicliny  characterizes  the  majority  of  this  great  family.  In  the 
third  or  ray-flowered  group  there  are  three  common  conditions  :  (i)  that  in  which 
all  flowers  are  seed-producing,  the  disk  flowers  being  monoclinous  and  the  ray 
flowers  pistillate  (as  in  Aster);  (2)  that  in  which  the  disk 
flowers  are  monoclinous  and  the  ray  flowers  reduced  essen- 
tially to  corollas  (as  in  Helianthus);  and  (3)  that  in  which 
the  disk  flowers  are  monoclinous  but  with  the  pistils  sterile, 
while  the  ray  flowers  are  pistillate  (as  in  Polymnia);  Sil- 
phium  belongs  in  the  last  group,  but  it  seems  to  have  pro- 
gressed still  more  toward  dicliny,  since  the  styles  of  the  disk 
flowers  do  not  even  fork  into  stigmas. 

Composites  without  ray  flowers  show  as  much  diversity 
as  do  the  ray-flowered  forms,  though  the  latter  are  much 
more  numerous.  Some  forms  have  all  flowers  alike  and 
monoclinous  (as  in  Eupatorium).  In  Artemisia  the  heads 
in  some  species  consist  of  monoclinous  and  pistillate  flowers, 
while  in  other  species  they  consist  of  monoclinous  but  sterile 
disk  flowers  and  of  pistillate  marginal  flowers.  Iva  exhibits 
monoecious  dicliny,  the  inner  flowers  being  staminate  and  the 
outer  pistillate.  -Ambrosia  and  Xanthium  also  are  monoe- 
cious, but  the  two  kinds  of  flowers  are  in  separate  heads. 
The  evolution  of  dioecism  from  monoecism  appears  to  be 
illustrated  by  Petasites,  for  though  all  heads  have  monocli- 
nous but  sterile  central  flowers  and  pistillate  marginal 
flowers,  some  plants  have  heads  with  many  staminate  and 
few  pistillate  flowers,  while  other  plants  exhibit  the  reverse 
condition.  Gnaphalium  alpinum  is  essentially  dioecious, 
since  in  some  plants  the  stamens  do  not  shed  pollen,  while  in 
others  the  pistils  are  sterile.  Complete  dioecism  is  illus- 
trated by  the  related  Antennaria.  Some  composites,  notably 
Ambrosia,  are  wind-pollinated,  as  well  as  diclinous.  The 
possible  significance  of  the  remarkable  floral  diversity  of 
the  Compositae  will  be  considered  elsewhere  (p.  877). 

The  protection  of  flowers  from  deleterious  insects. — Deleterious  kinds  of  insects. 
—  Crawling  insects,  such  as  the  ants,  are  disadvantageous  floral  visitors,  since  the 
pollen  they  carry  is  likely  to  be  brushed  off  as  they  crawl  from  flower  to  flower. 
Even  among  the  flying  insects,  where  such  pollen  losses  are  reduced  to  a  minimum, 
those  that  fly  about  in  a  haphazard  manner,  visiting  various  plant  species  in  succes- 
sion, are  far  less  beneficial  than  are  such  insects  as  the  bees  which  on  any  given 
day  visit  individuals  of  the  same  plant  species  with  consistency.  It  has  often  been 
supposed  that  various  floral  features  are  highly  advantageous  because  they  exclude 
certain  insects,  but  the  evidence  for  this  view  in  many  cases  is  more  imaginary  than 
real.  In  any  case,  it  is  not  to  be  supposed  that  the  development  of  such  features 
has  had  any  relation  to  deleterious  insects;  so  far  as  they  have  value  in  this  connec- 
tion, it  must  be  regarded  as  purely  incidental. 


FIG.  1182.  —  A 
flower  from  a  dande- 
lion head  (Taraxacum 
ojfficinale),  illustrating 
epigyny;  note  the 
achene  (o),  the  capil- 
lary pappus  (/>)  rep- 
resenting the  calyx, 
the  strap-shaped,  five- 
toothed,  sympetalous 
corolla  (c),  the  tubular 
column  of  syngene- 
sious  anthers  (a)  sur- 
rounding the  basal 
portion  of  the  style 
(0,  and  the  two  re- 
curved stigmas  (#). 


858 


ECOLOGY 


Hairs  and  glandular  surfaces.  —  Stiff  bristly  hairs  have  been  thought  to  serve 
as  barriers  against  various  crawling  animals,  especially  snails.  Glandular  hairs 
doubtless  are  still  more  effective,  and  it  is  noteworthy  that  they  abound  on  floral 
stems  more  than  elsewhere.  Perhaps  the  most  undoubted  instance  of  such  protec- 
tion is  in  Silene,  some  species  of  which  (as  S.  antirrhina)  develop  just  at  anthesis 
an  extensive  glandular  surface  on  the  upper  stem  internodes;  insects  are  caught  by 
these  plants  so  frequently  as  to  have  led  to  the  common  name  of  catchflies. 

Extrafloral  nectaries.  —  On  many  plants  there  occur  extrafioral  nectaries  (i.e. 
nectar-secreting  organs  apart  from  inflorescences),  as  in  various  legumes,  and  in 
Ricinus  and  Passiflora  (figs.  1183,  1184).  Usually  they  are  most  abundant  on  the 
upper  side  of  the  petioles  and  on  the  under  side  of  the  leaf  blades.  Ants  frequently 


1184 


FIGS.  1183,  1184.  —  Extrafloral  nectaries  on  the  leaf  of  a  passion  flower  (Passiflora): 

1183,  a  palmately  five-lobed  leaf  with  nectaries  (n)  on  the  petiole  and  also  on  the  blade; 

1184,  a  single  nectary  (n)  with  a  large  drop  of  nectar  (d);  considerably  magnified. 

visit  these  nectaries  for  food,  and  it  commonly  has  been  supposed  that  the  organs 
thus  are  advantageous  to  plants,  the  view  being  that  the  insects  are  satisfied  with 
what  they  obtain  from  the  extrafloral  nectaries  and  thus  keep  away  from  the  flowers, 
where  the  rifling  of  the  floral  nectaries  might  endanger  cross  pollination.  It  has 
even  been  held  that  nectar-feeding  ants  are  combative  and  keep  off  leaf-cutting 
ants  and  other  Harmful  insects.  There  is  no  valid  evidence  for  these  fanciful 
theories,  and  recent  careful  experiments  in  which  plants  have  been  deprived  of  extra- 
floral  nectaries  without  affecting  seed  production  or  other  plant  activities  would 
seem  to  make  them  untenable.  Indeed,  the  greater  frequency  of  the  visits  of  ants 
to  the  nectary-bearing  individuals  has  been  shown  to  lead  to  more  flower-rifling  than 
in  the  plants  deprived  of  nectaries.  In  some  cases,  as  in  Vicia,  bees  have  been  ob- 
served to  visit  extrafloral  nectaries  in  preference  to  floral  nectaries;  in  such  a  case 
also  extrafloral  nectaries  are  a  positive  disadvantage  to  plants.  Furthermore,  in 


REPRODUCTION   AND   DISPERSAL 


859 


many  plants  the  chief  secretion  of  nectar  occurs  before  and  in  others  after  anthesis; 
rarely,  if  ever,  is  there  any  exact  correlation  with  this  period,  as  in  the  case  of  floral 
nectaries.  The  theory  that  these  nectaries  have  no  r61e  of  importance  is  more  ten- 
able than  the  theory  of  protection  from  ants.  This  view  of  the  case  is  supported  by 
the  fact  that  extrafloral  nectaries  occur  in  flowerless  plants,  as  in  Pteris  and  in  various 
other  ferns. 

Flower  structure.  —  Many  flowers  are  so  constructed  that  certain  flying  insects,  as 
well  as  ants,  are  unable  to  disturb  the  pollen  or  nectar ;  this  is  most  obvious  in  flowers 
with  long  corolla  tubes  and  in  zygomorphic  flowers. 
In  flowers  with  long  corolla  tubes  (such  as  the  petunia, 
fig.  1185),  or  with  long  spurs,  only  such  insects  as 
various  Lepidoptera,  which  have  corresponding  elon- 
gated mouth  parts,  can  reach  the  nectar;  in  some 
cases  the  corolla  tubes  are  lined  with  bristly  hairs 
which  still  further  tend  to  keep  out  small  insects, 
though  they  offer  practically  no  obstruction  to  long 
probosces.  In  a  number  of  zygomorphic  flowers  (as 
in  the  snapdragon  and  in  various  legumes)  it  is  diffi- 
cult for  small  and  weak  insects  to  force  their  way  to 
the  nectar  or  pollen.  Among  the  features  which  tend 
to  exclude  undesirable  insects,  floral  zygomorphy,  long 
corolla  tubes,  and  spurs  are  much  the  most  impor- 
tant. Since  insects  with  long  mouth  parts  can  get 
freely  exposed  nectar,  the  chief  value  of  zygomorphy 
and  tubular  corollas  would  seem  to  be  the  exclusion 
of  undesirable  insects.  While  such  structures  may 
have  some  evolutionary  connection  with  insect  visita- 
tion, the  connection  is  too  complex  to  be  understood 
at  present.  It  would  seem  much  better  for  a  flower  to  be  pollinated  in  any  manner 
than  to  run  the  chance  of  no  pollination  if  the  proper  insect  were  not  present.  The 
significance  of  flower  structure,  here  as  elsewhere,  is  an  unsolved  enigma. 

Some  instances  of  specialized  cross  pollination.  —  General  remarks.  —  The  con- 
sideration of  cross  pollination  cannot  be  concluded  without  a  short  account  of  some 
of  the  more  striking  instances  of  extreme  specialization.  In  some  of  the  cases  to  be 
mentioned  the  dependence  of  the  flower  upon  the  insect  (and  often  of  the  insect 
upon  the  flower)  is  absolute,  and  therefore  to  be  regarded  as  illustrating  obligate 
symbiosis.  When  such  specialized  forms  are  taken  to  other  countries  for  cultiva- 
tion, they  may  not  produce  seed,  unless  the  insects  also  are  transported.1 

Silene  and  the  orchids.  —  Some  of  the  night-blooming  catchflies  (Silene)  are 
visited  by  nocturnal  Lepidoptera,  especially  Dianthoecia,  whose  movements  in 
getting  nectar  incidentally  effect  pollination;  later  the  moth  deposits  eggs  in  the 
ovary  with  its  long  ovipositor,  and  the  developing  larvae  feed  upon  the  ovules.  In 

1  An  excellent  illustration  of  this  is  afforded  by  the  orchid,  Vanilla,  whose  fruits  fur- 
nish commercial  vanilla;  the  absence  of  the  proper  pollinating  insect  in  certain  regions 
makes  artificial  pollination  a  necessity  for  profitable  cultivation.  The  day  lily  (Hemero- 
callisfulva)  never  fruits  in  Europe,  probably  because  of  the  absence  of  the  proper  insect. 


FIG.  1185.  —  A  flower  of 

Petunia  ;  note  the  long  tube 
(0  of  the  sympetalous  corolla 
(Oi  well-fitted  for  pollina- 
tion by  moths  with  long 
mouth  parts;  c,  calyx  of 
five  sepals. 


86o  ECOLOGY 

such  an  instance  symbiosis  is  more  obligate  for  the  insect  than  for  the  flower.  The 
orchids,  as  a  class,  show  the  most  extreme  floral  specialization,  dependence  upon 
some  particular  insect  often  being  obligate. 

Arum.  —  In  Arum,  although  entirely  different  structures  are  involved,  there  is 
much  to  recall  the  cross  pollination  of  Aristolochia  (p.  853).  The  inflorescences 
are  composed  of  staminate  flowers  above  and  of  pistillate  flowers  below,  which  are 
arranged  on  a  club-shaped  central  axis,  the  spadix,  and  enclosed  within  a  large 
bract,  the  spathe,  which,  though  enlarged  below,  is  considerably  constricted  above. 
At  anthesis  the  flowers  give  forth  a  nauseous  odor  that  attracts  numerous  small 
flies,  whose  exit  for  a  time  is  said  to  be  barred  by  reflexed  hairs  between  the  pistillate 
and  staminate  flowers  and  in  the  narrow  passageway  above.  The  pistillate  flowers 
mature  first,  and  when  the  staminate  flowers  mature,  the  lower  ring  of  hairs  dies, 
permitting  the  insects  to  crawl  over  the  stamens,  where  they  become  covered  with 
pollen.  Very  soon  the  upper  ring  of  hairs  withers  also,  permitting  escape  to  the 
exterior.  If  the  insects  visit  another  inflorescence  at  once,  it  is  evident  that  the 
mature  pistillate  flowers  are  likely  to  be  cross-pollinated.  Since  Arum,  as  well  as 
most  other  aroids  with  similar  features,  is  monoecious,  it  is  obvious  that  a  highly 
specialized  mechanism  of  this  sort  prevents  not  autogamy,  but  geitonogamy.  Re- 
cent observations  call  most  of  this  familiar  account  in  question,  especially  as  to 
Arum  maculatum.  In  this  species  it  is  claimed  that  the  exit  for  the  visiting  flies  is 
not  barred,  since  the  hairs  are  not  stiff  enough  to  impede  the  insects  and  often  are 
not  even  long  enough  to  fill  the  passageway.  So  far  as  the  insects  are  held  in  the 
spathe,  it  is  said  to  be  due  to  drugging  by  the  plant,  and  it  is  claimed  that  there 
is  a  sufficient  amount  of  overlapping  in  the  periods  of  maturity  of  the  staminate  and 
pistillate  flowers  to  result  in  geitonogamy.  Furthermore,  there  is  no  adequate  proof 
that  the  flies  which  manage  to  escape  visit  other  inflorescences  sufficiently  soon  to 
effect  cross  pollination. 

The  Jig.  —  The  most  remarkable  of  the  known  cases  of  cross  pollination  whose 
evolutionary  development  cannot  even  be  imagined,  is  that  of  the  figs,  a  group  of 
plants  which,  like  the  aroids,  are  diclinous  and  commonly  placed  low  in  the  scale 
of  seed  plants.  The  inflorescence,  known  as  a  synconium,  is  unique;  the  numerous 
flowers  line  the  walls  of  a  chamber  (representing  the  receptacle)  and  are  entirely 
hidden  (figs.  1186,  1187).  Entrance  to  the  flowers  is  possible  only  through  a  small 
apical  orifice  (as  in  the  India-rubber  tree,  Ficus  elastica),  which  is  lined  with  scales. 
All  species  of  figs  are  diclinous,  some  having  the  two  kinds  of  flowers  somewhat 
indiscriminately  mixed;  in  most  species,  however,  the  staminate  flowers  are  toward 
the  top  and  the  pistillate  flowers  toward  the  base  of  the  synconium.  Some  species 
approach  dioecism,  certain  trees  having  pistillate  and  others  monoecious  synconia. 
The  fig  of  commerce  (Ficus  Carica)  is  essentially  dioecious,  the  pistillate  flowers 
of  apparently  monoecious  synconia  being  sterile;  in  rare  cases,  however,  the  stam- 
inate trees  bear  some  pistillate  flowers  and  even  ripen  seeds. 

The  commercial  fig  is  pollinated  by  a  small  wasp,  Blastophaga  glossorum,  whose 
life  cycle  is  most  extraordinary.  The  females  force  their  way  through  the  synconial 
orifice,  and  this  is  so  difficult  of  accomplishment  that  usually  they  lose  their  wings 
in  the  process;  after  laying  their  eggs  within  the  young  ovules,  they  die  within  the 
growing  fig.  Those  females  that  chance  to  enter  the  pistillate  synconia  (which 
become  the  figs  of  commerce)  have  no  progeny,  since  the  flowers  have  styles  of  such 


REPRODUCTION   AND   DISPERSAL 


861 


length  that  the  insects  are  unable  to  lay  their  eggs  in  the  right  spot  (fig.  1188).  But 
in  the  staminate  figs,  known  as  caprifigs,  there  are  short-styled  rudiments  of 
pistillate  flowers  (often  called  gall  flowers,  fig.  1189),  in  which  eggs  may  be  placed 
properly,  later  hatching  into  wasps.  Some  stimulus  exerted  by  the  insect  causes 
the  ovary  primordia  to  develop  into  seedless  galls.  After  a  time  the  males  hatch, 
eating  their  way  out 
of  the  galls  in  which 
they  developed  and 
into  the  galls  occu- 
pied by  developing 
females ;  copulation  is 
followed  by  the  death 
of  the  males  within 
the  caprifig.  The  fe- 
males thereupon 
escape  (fig.  1190), 
crawling  over  the 
staminate  flowers  of 
the  caprifig  and  be- 
coming dusted  with  i*n89  1137  u  JJ88  1186  1190 
pollen;  those  that  FlGS.  Il86-n9o.  —  Pollination  of  the  fig  (Ficus  Carica}: 
1 1 86,  a  synconium  cut  longitudinally,  showing  gall  flowers  pro- 
duced by  the  fig  wasp  (Blastophaga  grossorum) ;  near  the  mouth 
of  the  cavity  is  a  female  fig  wasp,  which  has  escaped  from  one 
of  the  galls;  1187,  a  similar  synconium  with  seed -producing 
pistillate  flowers;  near  the  mouth  of  the  cavity  are  two  female 
fig  wasps,  one  of  which  has  already  crept  inside;  1188,  a  long- 
styled  seed-producing  flower;  1189,  a  short-styled  gall  flower; 
1190,  a  fig  wasp  escaping  from  a  gall  flower.  — From  KERNER. 


chance  to  visit  figs 
incidentally  pollinate 
the  stigmas  therein, 
but  have  no  progeny, 
while  those  that  go 
to  caprifigs  have  prog- 
eny, but  are  of  no 
service  in  pollination. 

One  of  the  strangest  features  of  a  process  strange  throughout  is  that  the  pistil- 
late flowers  mature  two  months  before  the  staminate  flowers;  however,  by  the  time 
the  latter  are  mature,  another  crop  of  synconia  has  developed  with  stigmas  ready  for 
pollination,  so  that  stigmas  of  a  given  generation  are  pollinated  from  inflorescences 
of  the  preceding  generation.  In  southern  Italy  there  are  three  such  crops  of  figs 
and  caprifigs  each  year  (viz.,  in  April,  June,  and  August),  and  three  corresponding 
generations  of  wasps.  This  symbiosis  between  Ficus  and  Blastophaga  has  been 
denominated  mutualism,  but  surely  it  is  a  somewhat  destructive  form  of  mutualism, 
where  death  without  progeny  comes  to  such  a  large  proportion  of  the  symbionts  on 
each  side,  namely,  to  the  female  insects  that  enter  the  figs  and  to  the  pistillate  flowers 
of  the  caprifigs. 

Centuries  before  the  process  of  pollination  was  discovered,  the  ancients  cultivated 
the  commercially  valueless  caprifigs,  and  placed  branches  with  maturing  synconia 
on  fertile  fig  trees ;  this  process,  known  as  caprification,  makes  it  easy  for  the  female 
wasps  (which  fly  weakly,  though  possessing  wings)  upon  emergence  from  the  capri- 
figs to  enter  and  pollinate  the  figs.  Caprification  and  pollination  are  quite  unneces- 
sary for  reproduction  commercially,  since  figs  always  are  propagated  from  cuttings. 
Caprification  is  not  always  necessary,  even  for  the  production  of  commercial  figs, 


862  J;  ECOLOGY 

there  being  some  varieties  which  mature  edible  fruits  (though  without  viable  seeds) 
without  caprification  or  pollination.  Other  varieties,  notably  the  Smyrna  fig,  re-^ 
quire  pollination  for  their  best  development,  the  caprified  fruits  surpassing  those 
that  are;  not  caprified  in. plumpness,  juiciness,  and  flavor. 

Geitonogamy.  —  General  remarks.  —  In  spite  of  the  numerous  and 
remarkable  features  which  facilitate  cross  pollination,  geitonogamy  (or 
pollination  between  different  flowers  of  the  same  plant)  and  close  polli- 
nation are  very  common  and,  taken  together,  perhaps  are  more  common 
in  monoclinoUs  flowers  than  is  cross  pollination.  Furthermore,  there  are 
cases  in  which  the  features  facilitating  these  kinds  of  pollination  are  about 
as  specialized  as  any  that  have  been  previously  noted  in  connection  with 
cross  pollination.  In  most  species  a  number  of  flowers  bloom  at  once  on 
the  same  plant,  so  that  dichogamy  does  not  prevent  geitonogamy,  es- 
pecially because  insects  usually  visit  all  the  flowers  on  a  given  plant  before 
flying  to  another.  The  first  flower  on  each  plant  visited  may  get  only 
foreign  pollen,  but  the  chance  of  geitonogamy  increases  with  the  number 
of  flowers  visited,  though  it  should  be  remembered  that  the  general 
prepotency  of  foreign  pollen  greatly  favors  the  latter  in  initiating  seed 
production.  Geitonogamy  is  commonest  in  plants  with  compact  in- 
florescences, especially  where  these  are  umbels,  spikes,  or  heads,  as  in 
the  umbellifers  (the  highest  of  the  polypetalous  dicotyls)  and  in  the 
composites  (the  highest  of  plants).  While  such  floral  massing  does  not 
exclude  cross  pollination,  it  so  greatly  facilitates  geitonogamy  that  the 
latter  probably  is  the  chief  method  of  pollination.  Although  the  almost 
habitual  dichogamy  of  these  plants  prevents  autogamy  and  peculiarly 
facilitates  geitonogamy,  there  is  no  adequate  evidence  that  the  latter  is 
perceptibly  more  advantageous  than  the  former. 

Illustrations  of  geitonogamy  in  the  composites  and  umbellifers.  —  The  culmination 
of  conditions  favorable  to  geitonogamy  occurs  in  the  composites,  a  group  that  is 
notably  protandrous.  The  outer  flowers  bloom  first,  but  the  stigmas  remain  recep- 
tive until  the  flowers  next  within  shed  pollen.  After  the  stamens  mature,  the  style 
elongates,  pushing  up  through  the  surrounding  tube  of  united  anthers  and  swabbing 
out  some  of  the  pollen,  which  adheres  to  the  style  bristles.  Later  the  style  forks, 
exposing  the  receptive  stigmatic  surfaces  (fig.  1182),  which  with  the  adherent 
pollen  commonly  come  into  contact  with  the  stigmatic  surfaces  of  adjoining 
flowers.  Entangled  style  branches  of  this  sort  are  especially  conspicuous  in  Eupa- 
torium.  In  contrast  to  pollination  by  wind  or  insects,  this  may  be  termed  contact 
pollination.  Geitonogamy  by  contact  is  especially  characteristic  of  the  milky- 
juiced  composites  (such  as  the  dandelion,  figs.  1193,  1194),  since  the  heads  open  by 
day  and  close  at  night  and  in  rainy  weather,  insuring  to  an  unusual  degree  the 
contact  of  stigmas  and  pollen-covered  organs  of  adjoining  flowers. 


REPRODUCTION  AND   DISPERSAL  863 

In  composites  with  conical  or  columnar  receptacles  (such  as  Rudbeckia  and 
Lepachys),  there  occurs  what  may  be  termed  gravity  pollination,  pollen  from  the 
upper  flowers  dropping  upon  the  stigmas  of  the  lower  and  older  flowers.  In  An- 
themis,  as  the  flowers  develop,  the  disk  elongates  in  such  a  way  that  the  stigmas  of 
the  older  flowers  are  exactly  under  the  shedding  stamens  of  the  younger  flowers.  It 
will  be  recalled  that  in  those  composites  that  tend  toward  dicliny,  it  is  the  outer 
(older)  flowers  that  ordinarily  are  pistillate,  and  the  inner  (younger)  flowers  that 
ordinarily  are  staminate.  So  far  as  geitonogamy  is  concerned,  such  a  condition  is 
economical,  since  stamens  would  be  useless  in  the  outer  flowers  and  pistils  in  the 
inner  flowers,  though  they  might  be  of  value  in  case  of  cross  pollination  by  insects. 

The  structural  facilitation  of  geitonogamy  in  the  umbellifers  is  almost  as  marked 
as  in  the  composites.  In  Eryngium  the  flowers  are  in  dense  heads  that  facilitate 
contact  pollination  between  adjoining  flowers.  In  Sanicula  there  are  monoclinous 
and  staminate  flowers,  the  long  styles  of  the  former  bending  over  and  bringing  the 
stigmatic  surfaces  into  contact  with  the  latter.  In  such  wind-pollinated  composites 
as  Ambrosia,  geitonogamy  is  likely  to  take  place  between  flowers  of  different  heads, 
as  the  staminate  heads  are  uppermost. 

Close  pollination  or  autogamy.  —  General  remarks.  —  Autogamy, 
that  is,  pollination  between  anthers  and  stigmas  of  the  same  flower,  once 
thought  to  be  relatively  rare,  is  now  known  to  be  extremely  common.  In 
some  cases  the  features  that  facilitate  autogamy  are  quite  as  striking 
as  are  those  previously  mentioned  features  that  impede  or  prevent  it. 
Autogamy  by  contact  may  be  called  self  pollination,  a  term  often  incor- 
rectly made  synonymous  with  autogamy  in  general ;  as  with  geitonogamy, 
close  pollination  occurs  chiefly  by  contact  or  through  the  agency  of  grav- 
ity, though  wind  and  insects  also  may  be  operative.  In  many  cases, 
especially  where  foreign  pollen  is  prepotent  over  own  pollen,  autogamy 
probably  is  effective  only  in  the  absence  of  cross  pollination;  in  many 
other  cases  both  appear  to  be  equally  effective,  and  in  a  number  of  in- 
stances autogamy  is  the  only  form  of  pollination  possible.  Careful 
study  has  shown  that  in  alpine  and  arctic  regions  autogamy  habitually 
exceeds  xenogamy. 

Illustrations  of  autogamy.  —  Simple  cases  of  autogamy  occur  in  Trillium  and  in 
Geranium,  the  anthers  and  stigmas  being  in  close  juxtaposition;  most  such  flowers 
are  slightly  protogynous,  so  that  cross  pollination  may  occur  before  there  is  a  chance 
for  contact  pollination.  Somewhat  more  complex  are  the  many  cases  in  which 
anthers  and  stigmas  come  into  contact  through  growth  movements,  as  in  the  stamen 
movements  of  Circaea  and  of  many  crucifers,  and  in  the  style  movements  of  Epilo- 
bium  and  of  many  other  plants;  such  flowers  also  are  as  a  rule  slightly  dichogamous. 
In  the  composites  autogamy  as  well  as  geitonogamy  may  take  place  through  style 
inflections  and  through  the  closing  and  the  opening  of  heads;  similarly,  many  flowers 
that  open  and  close  daily  may  exhibit  autogamy  (as  in  Gentiana).  Pollen  drops 
directly  on  the  stigma,  illustrating  gravity  pollination  in  many  erect  flowers  with 


864  ECOLOGY 

styles  shorter  than  the  stamens  (as  in  the  lilac),  and  in  many  pendulous  flowers  with 
styles  longer  than  the  stamens  (as  in  Dodecatheori).  In  Pedicularis  the  growing 
corolla  rubs  over  the  anthers,  so  that  the  pollen  falls  upon  the  stigma  underneath. 
In  Cyclamen  and  Moneses  the  growth  curvatures  of  the  flower  stalk  at  the  time  of 
anther  maturity  cause  the  pollen  to  drop  out  upon  the  stigma. 

Yucca.  — The  flowers  of  Yucca  commonly  are  pendent  (fig.  1192),  and  though 
the  stigmas  extend  beyond  the  anthers,  pollination  by  gravity  is  unlikely  if  not  im- 
possible. The  flowers  are  nocturnal,  blossoming  but  once,  and  are  pollinated  by 
a  small  moth,  Pronuba.  The  females  pierce  the  ovaries  with  their  ovipositors  and 
lay  eggs  among  the  ovules,  before  and  after  which  they  deliberately  take  pollen 
from  the  anthers,  holding  it  in  their  specially  constructed  maxillary  palps,  and  ram 
it  into  the  stigma.  As  a  result  of  this  astounding  process  the  ovules  develop;  each 
larva  eats  about  twenty,  and  the  rest  develop  into  seeds.  This  symbiosis  is  be- 
lieved to  be  obligate  for  each  symbiont;  in  any  event,  Yucca  is  seedless  in  the  ab- 
sence of  Pronuba.  The  mode  of  evolution  of  an  instinct  that  impels  an  insect  to 
stuff  a  stigma  with  pollen  cannot  even  be  imagined.1 

Cleistogamy.  —  The  culmination  of  structures  facilitating  close  pol- 
lination is  found  in  flowers  that  never  open,  since  in  these  with  rare  ex- 
ceptions autogamy  alone  is  possible.  Flowers  that  regularly  open,  such 
as  those  heretofore  considered,  are  termed  chasmogamous,  while  flowers 
that  never  open  are  called  deistogamous.  Cleistogamy  may  be  habitual 
(as  in  the  subterranean  flowers  of  Viola  and  Poly  gala),  or  it  may  be 
facultative,  depending  upon  definite  external  factors.  Conspicuous  cases 
of  facultative  cleistogamy  are  found  in  Oxalis,  Specularia,  Impatiens, 
and  Lamium.  Subjection  to  low  temperature  is  believed  to  be  the  chief 
cause  of  such  cleistogamy;  in  Lamium,  for  example,  the  spring  and 
autumn  flowers  are  cleistogamous,  while  the  summer  flowers  are  chas- 
mogamous or  sometimes  cleistogamous  in  cold  or  rainy  weather.  In 
Viola  sepincola  the  aerial  flowers  are  chasmogamous  in  the  sunshine  and 
cleistogamous  in  the  shade,  indicating  that  light  as  well  as  heat  may  be 
a  factor.  The  submersed  flowers  of  Alisma  also  are  cleistogamous. 

Habitual  cleistogamy  is  well  illustrated  by  the  subterranean  and 
generally  colorless  flowers  of  Viola  cucullata  (and  many  other  violets), 
Amphicarpaea,  and  Poly  gala  polygama  (fig.  1191);  in  Viola  cucullata 
they  appear  some  weeks  or  months  after  the  showy  aerial  flowers,  but 
in  Polygala  the  two  kinds  of  flowers  are  nearly  synchronous.  In  all 
three  cases  they  are  much  more  productive  than  are  the  showy  and  pre- 
sumably cross-pollinated  aerial  flowers.  In  the  rock  rose  (Helianthe- 

1  While  Yucca  is  here  considered  as  illustrating  close  pollination,  the  moths,  after 
gathering  pollen,  often  fly  to  other  flowers  or  even  to  other  plants,  so  that  they  may  effect 
autogamy,  geitonogamy,  or  xenogamy. 


REPRODUCTION   AND   DISPERSAL 


865 


mum)  both  the  open  and  the  closed  flowers  are  aerial,  the  former  being 
large  and  showy,  while  the  latter  appear  later  and  are  much  smaller. 
In  Leersia  oryzoides,  the  conspicuous  open  monoclinous  flowers  rarely 
fruit,  though  the  cleistogamous  flowers  hidden  within  the  leaf  sheaths 
are  fertile.  Few  if  any  species  have  exclusively  cleistogamous  flowers, 

though  this  condition  has  been  reported 
for  Myrmecodia  echinata,  Salvia  cleis- 
togama,  Ophrys  apifera,  Ammannia 
latifolia,  and  several  grasses. 

The    cleistogamous   and  chasmoga- 
mous  flowers  of  the  same  species  differ 
widely  in  structure,  though  they  agree 
in  some  important  respects,  as  in  the 
potency  of  own  pollen.     In  general  thj 
former  are  much  the  smaller,  and  in 
some    respects    they    resemble    early 
stages  in  the  development  of  the  latter; 
this  is  especially  true  as  to  the  corolla, 
which  either  is  entirely  lacking  or  exists 
in  the  form  of  protuberances  (e.g.  in 
Specidaria),  as  in  the  bud  of  a  chas- 
mogamous  flower.     The  development 
of  the  stamen  and  pistil  is  not  arrested, 
as  is  that  of   the  corolla,  though  the 
stamens  may 
be    fewer    in 
number,  and 
usually    the 

pollen  grains  are  much  less  numerous 
than  in  chasmogamous  flowers.  In 
Helianthemum  the  closed  flowers  have 
but  three  to  ten  stamens,  as  contrasted 
with  the  numerous  stamens  of  the  open 
flowers;  sometimes  the  stamens  are 
reduced  to  a  single  anther,  and  the  pol- 
len grains  may  number  only  a  dozen 
or  even  less  in  each  chamber  (as  in 
Occasionally  the  pistil  exhibits  reduction;  for  example,  in 


FIG.  1191.  —  Fruiting  shoots  of 
Poly  gala  polygama  ;  the  aerial  stem  (a) 
bears  showy  racemed  flowers  which 
open  and  may  be  cross  pollinated;  on 
underground  stems  («)  are  borne 
numerous  cleistogamous  flowers  (/), 
which  are  close  pollinated  and  give 
rise  to  abundant  seed  pods  (/>). 


Oxalis) . 

Helianthemum  the  open  flowers  have  many  ovules,  while  there  are  only 


866  ECOLOGY 

a  few  in  the  closed  flowers.  Perhaps  the  most  striking  case  of  reduction 
is  where,  instead  of  a  stigma,  there  is  an  open  passageway  to  the  ovules, 
recalling  gymnosperms.  Pollination,  of  course,  is  by  direct  contact,  but 
there  are  some  remarkable  cases  (as  in  Oxalis  and  Impatient}  in  which, 
strictly  speaking,  there  is  no  pollination  at  all,  since  the  pollen  grains  ger- 
minate within  the  anther,  putting  forth  their  tubes  which  grow  out  toward 
the  stigma.1  In  Lamium  and  in  Viola  odorata  the  anthers  do  not  even 
dehisce,  so  that  the  pollen  tubes  have  to  penetrate  the  anther  walls;  in 
Viola  the  anther  wall  is  devoid  of  the  usual  thickening,  and  the  pollen 
tubes  pass  readily  through  permeable  spots  of  small  plasmatic  cells.  As 
might  be  expected,  cleistogamous  flowers  do  not  exhibit  marked  dichog- 
amy. Allied  to  cleistogamy  is  bud  pollination  (as  in  Oenothera  and  in  vari- 
ous orchids),  where  autogamy  occurs  in  ordinary  flowers  before  they  open. 

The  fact  that  the  open  flowers  of  plants  which  possess  also  cleistogamous  flowers 
usually  produce  but  few  seeds  has  led  to  the  theory  that  the  failure  of  cross  polli- 
nation probably  has  resulted  in  the  evolution  of  cleistogamy.  This  theory  has  no 
evidence  in  its  favor.  In  Viola  biflora  there  are  cleistogamous  flowers,  although 
the  showy  open  flowers  fruit  abundantly.  Furthermore,  close  pollination  can  take 
place  quite  as  readily  in  the  open  as  in  the  closed  flowers  of  a  given  species,  since  own 
pollen  is  as  potent  in  one  case  as  in  the  other.  As  noted  above  (see  also  p.  901), 
cleistogamy  is  in  part  associated  with  arrested  development,  and  usually  is  due  to 
definite  external  conditions,  which  are  unfavorable  for  chasmogamy;  for  example, 
in  Lamium  amplexicaule  the  first  flowers  in  spring  and  the  last  flowers  in  autumn 
are  cleistogamous,  while  the  summer  flowers  are  open  and  showy.  Cleistogamy 
is  advantageous  in  that  closed  flowers  are  well  protected  from  rain  and  from  the 
visits  of  pollen-gathering  insects.  Subterranean  cleistogamy  is  advantageous  in 
that  the  seeds  are  self-planted  and  are  well-protected  from  many  seed-eating  ani- 
mals, such  as  birds. 

The  comparative  advantages  of  cross  pollination  and  close  pollination. 

—  Introductory  remarks.  —  Usually  it  is  believed  that  cross  pollination 
must  be  highly  advantageous  because  it  is  so  common,  and  particularly 
that  the  diverse  and  sometimes  extraordinary  features  which  impede 
or  even  prevent  close  pollination  are  prima  facie  evidence  of  the  value 
of  xenogamy.  The  usual  reason  for  regarding  cross  pollination  as  supe- 
rior to  close  pollination  is  either  that  it  facilitates  the  merging  of  diverse 
racial  characters 2  or  that  it  promotes  variability  or  racial  vigor.  The 

1  The  germination  of  pollen  grains  within  the  anther  has  been  reported  occasionally 
even  in  chasmogamous  flowers. 

2  If  this  conception  is  true,  it  still  further  emphasizes  the  essential  difference  between 
true  cross  pollination  and  geitonogamy,  since  there  is  no  such  merging  in  the  latter,  the 
flowers  of  a  single  individual  having  a  common  immediate  ancestry. 


REPRODUCTION   AND   DISPERSAL  867 

problem  of  cross  pollination  is  essentially  a  phase  of  the  problem  of 
sexuality,  which  has  been  considered  previously. 

Experimental  data.  —  It  long  has  been  believed  by  plant  breeders 
that  occasionally  crossing  is  necessary  if  the  individuals  of  a  species 
are  to  be  kept  in  a  state  of  the  highest  vigor,  inbreeding  (i.e.  breeding 
between  closely  related  forms,  as  in  close  pollination)  often  having  been 
shown  to  result  in  diminished  luxuriance  in  the  progeny.  Careful  ex- 
periments conducted  many  years  ago  on  a  number  of  species,  though 
yielding  rather  discordant  results,  tended  to  show  that  cross  pollination, 
on  the  whole,  is  more  advantageous  than  close  pollination.  In  some 
cases  diminished  vigor,  which  is  made  evident  by  smaller  size  and  les- 
sened seed  production,  is  obvious  in  the  progeny  of  the  first  generation  (as 
in  Ipomoea  purpurea  and  Mimulus  luteus) ;  in  a  much  larger  number  of 
cases  it  is  obvious  only  after  continued  inbreeding  for  several  genera- 
tions, and  in  still  other  cases  inbreeding  appears  never  to  result  in 
deterioration. 

Recent  experiments  made  on  Indian  corn  show  that  close  pollination 
results  in  the  first  generation  in  reduced  height  and  yield;  in  the  gener- 
ations following  there  is  further  reduction,  but  the  decrement  becomes 
less  each  time  until  about  the  fifth  generation,  after  which  continued 
close  pollination  makes  no  appreciable  change  in  the  offspring.  Such 
dwarfs  may  reproduce  as  such  indefinitely,  but  if  crossed  the  offspring 
of  the  first  generation  has  the  original  full  size.  In  the  earlier  experiments 
geitonogamy  usually  resulted  in  quite  as  much  weakening  as  did  autog- 
amy, showing  conclusively  that  it  is  much  more  nearly  related  to  close 
pollination  than  to  cross  pollination.  Progeny  from  crossed  individuals 
were  found  to  flower  first  and  to  suffer  less  in  crowded  cultures  than 
did  progeny  from  inbred  races;  this  fact  has  been  taken  to  be  of  great 
significance,  since  it  suggests  the  likelihood  of  the  submergence  of  inbred 
races  by  cross-pollinated  races  in  the  "  struggle  for  existence  "  in  nature. 

The  possible  advantages  of  cross  pollination.  —  The  experiments  cited 
may  mean  either  that  cross  pollination  is  in  some  way  advantageous, 
or  that  close  pollination  is  in  some  way  disadvantageous.  The  support- 
ers of  the  first  theory  have  held  that  cross  pollination  is  advantageous, 
because  it  insures  the  mingling  of  two  lines  of  ancestry  with  their  sup- 
posedly different  beneficial  characters,  or  because  it  promotes  varia- 
bility or  racial  vigor.  There  is  very  little  in  favor  of  this  view  and  very 
much  against  it.  In  the  first  place,  experiments  on  the  same  species 
at  different  times  and  places  vary  widely  in  their  results;  sometimes  own 


868  ECOLOGY 

pollen  may  be  quite  impotent,  sometimes  imperfectly  potent,  and  some- 
times as  potent  as  foreign  pollen.  Usually  these  variations  are  due  to 
external  factors,  such  as  differences  in  moisture,  light,  etc.  Many  of 
the  very  species  that  have  been  supposed  to  demonstrate  the  benefits  of 
cross  pollination  grow  under  certain  conditions  quite  as  well  when 
close  pollinated.  Similarly,  the  results  of  cross  pollination  vary  widely 
under  different  external  conditions.  In  case  of  crossing  between  un- 
related individuals  of  inbred  ancestry  grown  under  similar  external  con- 
ditions, the  progeny  if  grown  under  like  conditions  are  scarcely  more 
vigorous  than  with  close  pollination,  showing  that  the  vigor  supposed 
to  result  from  crossing  may  result  rather  from  favorable  external  con- 
ditions. Comparable  phenomena  are  known  also  in  the  animal  king- 
dom. 

The  data  from  heterostyled  plants  oppose  the  usual  theory  of  cross 
pollination,  since  weak  individuals  or  none  at  all  result  from  crossing  be- 
tween plants  with  styles  of  unequal  length,  irrespective  of  nearness  or 
remoteness  of  relationship;  in  such  cases  the  progeny  is  weak,  regardless 
of  the  mingling  of  "  diverse  racial  characters."  Dichogamy  is  cited  as 
affording  a  priori  evidence  that  cross  pollination  must  be  beneficial,  yet 
dioecious  plants  (in  which  close  pollination  is  of  course  impossible)  ex- 
hibit the  earlier  maturation  of  anthers  or  of  stigmas  almost  as  habitually 
as  do  monoclinous  plants. 

The  chief  reasons  for  disbelieving  that  occasional  cross  pollination  is 
necessary  in  order  to  prevent  the  deterioration  of  plant  species  are 
afforded  by  the  phenomena  of  autogamy  and  vegetative  reproduction. 
Usually  the  argument  for  the  value  of  cross  pollination  is  built  on  those 
cases  in  which  foreign  pollen  is  prepotent  or  own  pollen  impotent,  omit- 
ting the  equally  numerous  and  significant  cases  where  own  pollen  is  fully 
potent.  Apart  from  the  vast  number  of  plants  that  exhibit  frequent 
autogamy,  there  are  many  in  which  autogamy  is  habitual,  particularly 
the  numerous  species  with  cleistogamous  flowers;  in  these  there  is  no 
deterioration,  in  spite  of  repeated  close  pollination.  In  the  experiments 
cited  above,  autogamy  in  several  forms  (as  in  Petunia,  Eschscholtzia, 
and  Nicotiana)  resulted  in  progeny  that  was  essentially  as  vigorous 
as  when  xenogamy  was  employed .  Even  in  the  two  species  '(viz.  Ipomoea 
purpurea  and  Mimulus  luteus)  which  were  thought  most  clearly  to 
demonstrate  the  benefits  of  cross  pollination,  luxuriant  sports  arose  in  the 
autogamous  cultures,  which  had  as  vigorous  progeny  when  close  polli- 
nated, as  did  cultures  from  cross-pollinated  individuals.  Perhaps  most 


REPRODUCTION   AND   DISPERSAL  869 

significant  of  all  is  the  prevalence  of  vegetative  reproduction  in  nearly 
all  the  higher  plants.  There  is  not  a  single  case  known  in  which  the 
indefinite  continuance  of  vegetative  reproduction  causes  the  slightest 
deterioration.  Indeed,  there  are  many  plants  in  which  crossing  has 
essentially  disappeared  (as  the  duckweeds  and  the  horn  worts).  Nor 
is  there  any  evidence  of  deterioration  in  the  species  that  are  persistently 
parthenogenetic.  It  is  to  be  noted  that  the  plants  which  in  experiments 
have  shown  benefit  from  crossing  are  chiefly  garden  varieties  that  have 
been  much  hybridized.  No  benefit  from  crossing  has  been  shown  in 
natural  species  that  have  long  been  pure.  Nor  is  there  good  evidence 
that  crossing  promotes  variability.  Among  the  most  variable  of  plants 
are  the  cleistogamous  violets  and  such  parthenogenetic  genera  as  Hiera- 
cium  and  Taraxacum. 

The  possible  disadvantages  of  close  pollination.  —  Even  if  there  are  no 
conspicuous  advantages  in  cross  pollination,  there  may  be  disadvantages 
in  close  pollination.  Such  a  view  seems  particularly  plausible  in  the 
case  of  those  orchids  in  which  own  pollen  seems  to  be  prejudicial  and 
possibly  even  poisonous  to  the  stigma  (or  the  stigma  to  the  germinating 
own  pollen),  perhaps  in  a  way  analogous  to  the  excreta  of  root  hairs, 
except  that  here  the  deleterious  effects  concern  only  individuals  of  a 
species,  and  not  the  species  as  a  whole.  If  this  conception  is  valid,  it 
may  account  for  the  occasional  impotence  of  own  pollen;  in  species 
where  foreign  pollen  is  prepotent,  the  deleterious  influence  of  own  pollen 
may  be  considered  to  be  less  marked.  In  the  large  number  of  species 
with  potent  own  pollen  it  may  be  supposed  that  such  deleterious  effects 
are  wanting.  The  benefits  of  cross  pollination  and  the  disadvantages 
of  close  pollination  have  been  too  much  emphasized.  Close  pollination 
and  its  essential  equivalent,  geitonogamy,  are  extremely  common  in 
nature,  nor  must  it  be  forgotten,  also,  that  many  of  the  important  plant 
and  animal  races  utilized  by  man  have  reached  their  present  state  of 
commercial  perfection  by  the  most  careful  inbreeding. 

The  protective  features  of  flowers.  —  The  calyx.  —  Flowers  are  among 
the  most  delicate  of  plant  organs,  the  ephemeral  petals  and  the  stamens 
and  pistils  with  their  gametophytes  and  embryos  being  particularly 
sensitive.  Foremost  among  the  protective  organs  is  the  calyx  (fig.  1136), 
which,  during  development,  often  is  the  only  exposed  floral  organ,  and 
commonly  is  much  less  delicate  than  are  the  structures  it  encloses. 
Among  the  dangers  to  which  the  developing  corolla  and  the  essential 
organs  might  otherwise  be  exposed  are  those  arising  from  rain,  drought, 


870  ECOLOGY 

heat,  cold,  and  wind.1  Since  the  calyx  commonly  is  composed  of  green 
and  leaflike  sepals,  it  probably  plays  an  important  part  in  food  manufac- 
ture, as  well  as  in  protection,  and  occasionally  it  replaces  the  corolla  in 
the  matter  of  showiness.  Sometimes  bracts  supplement  or  replace  the 
calyx  as  protective  structures,  as  in  Desmodium.  In  the  composites 
a  calyx-like  involucre  is  the  chief  protective  organ  (fig.  1194).  In  the 
aroids  the  large  spathe  protects  the  entire  inflorescence  in  flower  as  well 
as  in  bud.  Even  the  corolla  may  be  a  protective  organ,  as  in  the  grape, 
where  it  falls  as  soon  as  the  bud  opens,  and  in  flowers  which  close  at 
night  and  in  stormy  weather. 

The  duration  of  flowers.  —  From  the  standpoint  of  protection, 
flowers  may  be  divided  into  those  that  remain  open  throughout  anthesis 
and  those  that  close  once  or  more  during  that  period.  Of  those  which 
remain  open,  many  are  ephemeral,  and  hence  need  but  little  protection, 
particularly  as  opening  usually  comes  only  in  favorable  (i.e.  warm  and 
sunny)  weather.  Among  the  latter  are  the  flower-of-an-hour  (Hibiscus 
Trionum),  which  has  the  most  ephemeral  of  flowers,  rarely  lasting  for 
rnqre  than  three  hours,  the  day  lily  (Hemerocallis  fulva) ,  and  the  night- 
blooming  cereus.  In  striking  contrast  with  these  are  certain  orchid 
flowers  which  may  remain  open  for  fifty  to  eighty  days  if  unpollinated 
(as  in  Odontoglossum)?  On  the  whole,  plants  with  ephemeral  flowers 
are  more  likely  to  have  a  large  number  of  blossoms  in  a  season  than  are 
plants  with  long-lived  flowers. 

The  protection  of  non-closing  flowers.  —  Long-lived,  non-closing 
flowers  would  seem  to  need  some  protection,  the  greatest  dangers,  per- 
haps, being  the  waste  of  pollen  through  rain,  the  drying  of  the  stigmatic 
surface  through  drought,  and  injury  from  low  temperature  or  frost. 
Pollen  is  not  readily  wetted,  which  is  itself  a  matter  of  considerable  pro- 
tective importance;  also  it  will  be  recalled  that  some  pollen,  such  as  that 
of  exposed  vernal  flowers,  is  not  readily  injured  by  wetting  or  by  low 
temperature.  Many  plants  have  nodding  flowers,  notably  the  ericads, 
and  also  Bryophyllum  and  Yucca  (fig.  1192),  and  thus  are  well  protected 
from  injury  by  rain.  Many  of  the  features  that  protect  flowers  from 
crawling  insects  also  protect  them  from  rain;  among  these  are  contracted 
corolla  throats  (-as  in  Arctostaphylos  and  in  various  borages),  and  zygo- 

1  An  illustration  of  calyx  protection  is  afforded  by  the  peach  and  strawberry,  in  which 
unopened  buds  are  much  less  subject  to  injury  from  frost  than  are  buds  that  are  partly 
or  fully  opened. 

2  The  early  withering  of  orchid  corollas  may  be  induced  not  only  by  pollination,'  but 
also  by  the  mechanical  irritation  of  the  stigma. 


REPRODUCTION   AND   DISPERSAL 


87I 


morphic  corollas,  which  not  only  are  more  or  less  closed,  but  commonly 
are  swung  laterally,  as  in  the  labiates  (fig.  1174)  and  in  the  orchids. 

The  protection  of  flowers  by  closing.  —  Closure  during  anthesis  usually 
involves  a  single  flower,  but  sometimes  it  involves  an  entire  inflorescence, 


FIG.  1192.  —  Yucca  flowers:  A,  an  inflorescence  of  Yucca  patens  with  numerous 
pendulous  flowers ;  B,  flowers  of  Yucca  Treculeana  ;  note  the  perianth,  the  stamens  with 
recurved  anthers,  and  the  stigmas.  —  From  TRELEASE. 

as  in  the  heads  of  the  milky-juiced  composites  (figs.  1193,  1194).  Most 
such  flowers  or  inflorescences  open  in  sunshine  and  close  at  night  and  in 
cloudy  weather,  though  in  a  few  cases  this  order  is  reversed,  the  flowers 
opening  at  night  and  in  cloudy  weather  and  closing  in  the  sunshine. 
Some  flowers  open  for  two  successive  days  (as  in  the  poppy),  others 
for  three  to  five  days  (as  in  Eschscholtzia) ;  the  flowers  of  Crocus  and 
Hepatica  may  open  daily  for  more  than  a  week.  Usually  nocturnal 


872 


ECOLOGY 


flowers  are  more  ephemeral  than  are  diurnal  flowers  (as  in  the  night- 
blooming  cere  us),  but  the  flowers  of  evening  primroses,  although  opening 
but  once,  remain  open  for  some  hours  after  sunrise;  the  flowers  of  some 
species  of  Silene  open  for  at  least  three  successive  nights.  Flowers 

often  have  a  longer  period  of  anthesis  in 
spring  and  autumn  than  in  summer;  even 
such  ephemeral  flowers  as  those  of  Hibiscus 
Trionum  and  Hemerocattis  fulva  may  open 
toward  the  beginning  or  the  end  of  the 
flowering  period  on  three  successive  days. 

Flowers  cannot  be  classed  simply  as  diurnal  or 
nocturnal,  since  most  hours  of  the  day  and  night 
are  marked  by  the  opening  or  closing  of  the  flowers 
of  some  species.  The  opening  and  the  closing 
hours  of  a  given  species  vary  widely  with  the  lati- 
tude and  the  season,  but  in  general  the  earlier 
diurnal  flowers  open  about  sunrise  (as  in  the 
chicory  and  the  morning  glory)  and  the  later  about 
noon  (as  in  Mesembrianthemum).  The  nocturnal 
series  begins  about  dusk  (as  in  Silene)  and  closes 
about  ten  (as  in  Cereus).  The  times  of  closing 
are  much  less  definite  than  are  those  of  opening, 
and  they  extend  over  most  of  the  twenty-four 
hours;  this  is  partly  because  flowers  open  much 
more  rapidly  than  they  close,  the  former  process 
sometimes  being  sudden,  as  in  Oenothera.  Begin- 
ning with  the  early  forenoon  (as  in.  salsify  and 
chicory)  each  hour  until  after  sunset  is  marked  by 
the  closing  of  some  diurnal  flowers.  Similarly 

nocturnal  flowers  may  close  at  any  time  from  midnight  (as  in  Cereus)  to  sunrise,  or 
even  during  the  following  forenoon  (as  in  the  evening  primrose).  The  entire 
scheme,  as  above  outlined,  may  be  disarranged  in  cloudy  weather. 

The  factors  involved  in  the  opening  and  the  closing  of  flowers.  —  The 
mechanism  of  opening  and  closing  and  the  factors  controlling  this  meclv 
anism  are  in  part  unknown.  It  has  been  shown  in  a  few  cases,  and  it  is 
believed  to  be  true  in  others,  that  these  movements  are  true  movements  of 
growth,  opening  being  due  to  epinasty,  and  closing  to  hyponasty,  in  the 
segments  of  the  perianth  or  involucre.  Probably  the  chief  single  factor 
causing  epinasty,  and  hence  opening,  is  an  increase  of  temperature. 
Opening  as  a  result  of  an  increase  of  temperature  has  been  proven  ex- 
perimentally in  a  number  of  cases,  notably  in  the  tulip  and  the  crocus; 


FIGS.  1193,  1194-  —  Floral 
opening  and  closing  in  the  dan- 
delion (Taraxacum  officinale)'. 
1193,  an  open  head  as  seen  in 
full  sunlight;  1194,  the  same 
head  as  seen  at  night;  the  invo- 
lucre (i)  is  double,  consisting  of 
short  outer  bracts  (6)  and  a 
single  row  of  long  inner  bracts 
(V) ;  opening  and  closing  are  due 
chiefly  to  the  movements  of  the 
inner  bracts,  the  position  of  the 
outer  bracts  shifting  but  slightly. 


REPRODUCTION   AND    DISPERSAL  873 

a  crocus  flower  opens  in  two  to  four  minutes,  when  the  temperature  is 
raised  suddenly  fifteen  or  twenty  degrees,  and  it  is  sensitive  to  a  change 
of  half  a  degree.  Successive  alternations  of  cold  and  warmth  may  induce 
several  successive  closings  and  openings  within  an  hour.  It  is  believed 
by  many  investigators  that  changes  in  turgor  are  responsible  for  some 
cases  of  opening  and  closing,  as  in  the  tulip.  Possibly  some  flowers  open 
and  close  autonomously;  the  flowers  of  Calendula  arvensis,  for  example, 
open  in  the  dark  without  any  change  in  temperature. 

In  some  cases  light,  independently  of  temperature,  has  been  shown  to  cause 
opening  (as  in  the  day  lily  and  in  the  gentians).  In  nature  both  light  and  heat  prob- 
ably cooperate,  especially  in  opening;  the  early  closing  of  far  northern  flowers,  in 
spite  of  long  daylight,  probably  is  a  matter  of  temperature  alone.  In  a  few  cases 
opening  and  closing  are  due  to  moisture  changes;  so  sensitive  is  the  head  of  Carlina 
to  such  changes  that  it  has  been  used  as  a  hygroscope,  closing  in  moist  air  and  open- 
ing in  dry  air.  Obviously  the  explanation  of  the  movements  of  flowers  that  do  not 
open  rather  promptly  at  sunrise  or  close  at  sunset  presents  certain  difficulties.  While 
these  have  not  as  yet  been  solved  experimentally,  it  is  likely  that  certain  species 
require  a  greater  amount  of  heat  or  light  for  opening  or  a  less  amount  for  closing 
than  do  others,  or  that  they  are  less  quickly  sensitive.  Most  remarkable  are  the 
nocturnal  flowers,  in  which  opening  is  caused  by  decreased  temperature  and  light 
instead  of  by  increased  temperature  and  light,  as  in  diurnal  flowers. 

The  advantages  of  flower  closing.  —  In  general,  flowers  that  close 
during  anthesis  are  open  at  the  time  when  their  special  pollinating  in- 
sects are  most  active.  Most  of  the  latter  (especially  the  bees  and  butter- 
flies) are  diurnal,  and  are  affected  by  the  same  factors  that  influence 
flower  opening,  such  as  increased  heat  and  light  and  low  atmospheric 
humidity  (or  at  least  absence  of  rain) .  Nocturnal  flowers  are  pollinated 
almost  exclusively  by  nocturnal  insects,  such  as  certain  moths.  How 
such  a  remarkable  correlation  of  flower  opening  and  insect  activity  may 
have  arisen  is  altogether  unknown.  It  is  probable  that  closure  when 
the  insects  are  not  active  is  likely  to  be  of  considerable  protective  impor- 
tance; in  diurnal  flowers  pollen  probably  is  conserved  by  closure  in 
rainy  weather,  and  in  all  cases  closure  for  a  part  of  the  time  would  seem 
to  favor  the  lengthening  of  the  period  of  stigmatic  receptivity,  particu- 
larly in  nocturnal  flowers,  which  are  freed  from  the  desiccating  influence 
of  sunlight.  In  addition,  as  previously  noted,  opening  and  closing  favor 
close  pollination  and  even  geitonogamy  in  the  composites. 

Protective  movements  other  than  floral  opening  and  closing.  —  In  some 
flowers  the  anther  valves  are  hygroscopic  (as  in  Alchemilla  and  Laurus), 
closing  in  moist  weather ;  usually  they  are  closed  much  more  quickly 


874 


ECOLOGY 


than  the  closure  of  perianth  segments  is  effected  by  low  temperature, 
a  few  seconds  commonly  sufficing.  Such  closure  is  of  direct  value  in 
protecting  pollen  from  rain.  It  may  be  recalled,  also,  that  in  anthers, 
dehiscence  generally  is  caused  by  desiccation,  so  that  the  first  opening 
is  unlikely  to  occur  in  wet  weather.  Many  plants  show  movements  of 

the  flower  axes  (pedi- 
cels) or  inflorescence 
axes  (peduncles).  In 
certain  instances  the 
buds  are  erect  and 
the  flowers  pendent  (as 
in  Aquilegia).  More 
closely  related  to  floral 
movements  are  those 
cases  in  which  the 
flower  is  erect  by  day 
and  pendent  by  night 
and  in  rainy  weather 
(as  in  Geranium  and 
Anemone}',  in  the  um- 
bel lifers  there  is  a  simi- 
lar movement  of  the 
entire  inflorescence. 
Such  movements  are 
due  to  growth  inequali- 
ties in  the  pedicels  or 
the  peduncles,  and  the 
advantages  therefrom 
would  appear  to  be  in 
exposure  to  pollinating 


1197 


FIGS;  1195-1197.  —  Growth  movements  accompany- 
ing flower  and  fruit  development  in  a  wild  onion  (Allium 
cernuum):  1195,  a  flower  bud,  showing  the  nodding  posi- 
tion of  the  young  peduncle  (p);  b,  spathe;  1196,  a  shoot 
in  full  bloom ;  note  that  the  peduncle  (p)  has  straightened 
out  except  at  the  tip  (/);  the  flowers  are  arranged  in  an 
umbel,  the  pedicels  (£')  being  oriented  in  various  direc- 
tions; note  the  exserted  stamens  (s);  1197,  a  shoot  in 
fruit,  showing  the  conspicuous  capsules  (c) ;  note  that  the 
pedicels  (/>')  have  become  erect  or  ascending. 


insects  in  sunshine,  and  in  protection  from  rain  and  cold  during  the 
night  or  in  rainy  weather.  In  the  poppy  the  buds  are  nodding,  but  the 
flowers  are  erect,  the  pedicels  becoming  apogeotropic.  Movements  of 
much  complexity,  but  without  obvious  advantages,  are  seen  in  Allium 
cernuum  (figs.  1195-1197),  where  the  nodding  bud  becomes  erect  by 
differential  growth  in  the  peduncle,  while  later  the  fruits  become  erect 
by  similar  growth  in  the  pedicels. 

,  Protection  during  fruit  development.  —  Usually  the  developing  ovules 
are  delicate.  Ultimately  the  enveloping  ovary  walls  and  seed  coats 


REPRODUCTION   AND   DISPERSAL 


875 


become  thick  protective  structures  of  considerable  value  to  the  embryos, 
but  often  during  the  earlier  stages  of  growth  some  protection  from  desic- 
cation and  other  dangers  is  afforded  by  special  structures  or  habits. 
After  anthesis  the  calyx  may  close  about  the  developing  fruits  (as  in 
Phy 'sails) ,  appearing  much  as  in  the  bud,  thus  once  more  serving  as  an 
organ  of  protection,  as  well  as  continuing  to  manufacture  food.  In 
the  composites  the  protective  and  synthetic  activity  of  the  calyx  is  re- 
placed by  that  of  the  involucre.  Many  fruits  which 
are  edible  when  ripe,  during  immaturity  are  more 
or  less  protected  from  predatory  animals  by  un- 
pleasant flavors,  hardness,  and  spinescence  (as  in 
Opuntia  and  Ribes),  and  perhaps  by  inconspicu- 
ousness,  since  their  green  color  is  similar  to  that  of 
the  leaves. 

In  many  plants,  especially  in  hydrophytes,  the 
developing  fruit-stalks  exhibit  striking  growth  cur- 
vatures. In  most  cases  the  previously  erect  stalk 
recurves  (as  in  Peltandra  and  Nymphaea),  causing 
the  downward  orientation  and  consequent  submer- 
gence of  the  fruit.  .  Some  land  plants  show  similar 
reactions,  notably  Phryma  (fig.  1198),  whose  fruits 
become  strongly  reflexed,  whence  the  common 
name,  lopseed;  in  the  peanut,  stalks  that  are  erect 
or  ascending  until  anthesis,  later  recurve  and  force 
the  developing  nut  into  the  ground.  Probably  in 
all  of  the  above  cases  pedicels  or  peduncles  orig- 
inally apogeotropic  become  progeotropic  after  an- 
thesis, but  no  explanation  of  such  peculiar  behavior 
has  been  given,  nor  is  there  any  obvious  advantage,  except  perhaps  a 
slight  one,  in  connection  with  planting  (p.  929). 

The  origin  of  floral  structures.  —  The  flower  is  the  most  complicated 
of  plant  structures,  and  the  organs  concerned  directly  or  indirectly  with 
pollination  form  the  most  complicated  part  of  the  flower.  An  adequate 
theory  of  flower  structure,  however,  must  explain  not  alone  this  complex- 
ity, but  also  the  evolution  of  the  mouth  parts  of  the  flower-visiting  in- 
sects (notably  those  of  the  bees,  butterflies,  and  moths),  which  appear 
to  be  so  obviously  related  to  the  flowers.  Capping  all,  and  most  diffi- 
cult of  all  to  explain,  are  the  cases  of  obligate  reciprocal  symbiosis,  of 
which  the  fig  and  the  yucca  are  the  most  remarkable. 


FiG.  1 198. — A  flow- 
ering spike  of  the  lop- 
seed  (Phryma  Lepto- 
stachya) ;  at  anthesis 
the  flowers  (/)  with 
their  bilabiate  corollas 
(c)  are  horizontal,  but 
subsequent  epinastic 
growth  causes  the 
fruits  (/')  to  become 
strongly  reflexed. 


876  ECOLOGY 

Since  the  abandonment  of  the  theory  of  special  creation,  a  common 
hypothesis  has  been  that  floral  structures  and  specialized  mouth  parts 
have  arisen  pari  passu  by  reciprocal  natural  selection.  This  theory 
implies  that  those  flowers  and  insects  of  each  generation  that  happen  to 
exhibit  the  greatest  reciprocal  specialization  will  be  the  ones  to  have 
progeny,  while  the  more  generalized  forms  will  be  so  handicapped  that 
they  will  be  submerged  in  the  "  struggle  for  existence."  Upon  analysis 
this  theory  seems  almost  inconceivable;  furthermore,  there  are  few  if  any 
facts  to  support  it,  and  many  facts  to  controvert  it. 

In  the  first  place,  many  floral  features,  such  as  the  kind  of  inflorescence 
the  position  of  the  various  organs,  the  forms  and  markings  of  the  corolla, 
and  the  association  of  dichogamy  with  dioecism,  have  no  known  ad- 
vantage, much  less  an  advantage  great  enough  to  make  their  possessors 
better  adapted  than  are  their  neighbors.1  Nor  is  there  the  remotest 
evidence  that  generalized  flowers  are  less  successful  than  those  that  are 
specialized.  Indeed,  the  orchids,  which  have  the  most  specialized  of 
flowers,  appear  to  be  on  the  way  toward  extinction,  because  of  this 
very  specialization;  they  represent  a  case  of  "  over-adaptation,"  and 
therefore  present  a  condition  that  is  contrary  to  the  fundamental 
postulates  of  natural  selection.  In  some  other  groups  of  plants  the 
flowers  are  so  strongly  protandrous  that  pollination  rarely  takes  place, 
because  insect  visits  occur  either  after  the  pollen  is  shed  or  before 
the  stigmas  mature.  In  contrast  to  the  orchids,  the  grasses  and  the 
catkin-bearing  trees  are  dominant  and  widely  successful  groups  of 
plants,  although  they  possess  generalized  flowers,  which  are  diclinous 
and  wind-pollinated. 

Actinomorphic  flowers  with  exposed  pollen  and  nectar  often  are 
visited  in  preference  to  long-tubed  or  zygomorphic  flowers,  even  by 
such  specialized  insects  as  the  bees,  and  it  has  been  noted  that  the  former 
usually  set  seed  more  regularly  than  do  the  latter.  That  dichogamy 
is  not  due  to  natural  selection  seems  to  be  indicated  by  the  fact  that  often 
it  is  modified  by  external  factors  ;  for  example,  in  the  sunshine  the 
flowers  of  Parnassia  are  protandrous  and  those  of  Biscutella  and  Thlaspi 
are  protogynous,  but  all  alike  become  homogamous  in  the  shade  or  in 
cloudy  weather.  In  many  cases  the  postulated  intimate  and  exact 
relation  between  a  specific  insect  and  a  specific  flower  may  well  be 
doubted  for  a  number  of  reasons:  the  same  flowers  are  pollinated  by 

1  The  association  of  protandry  and  geitonogamy,  which  is  very  common  in  the  com- 
posites, would  also  seem  to  be  without  advantage. 


REPRODUCTION   AND   DISPERSAL  877 

very  different  insects  in  different  countries;  naturalized  flowers  seem 
to  be  pollinated  by  the  insects  of  the  new  country  quite  as  successfully 
as  their  congeners  are  pollinated  by  the  insects  of  their  native  country  ; 
insects  sometimes  are  held  captive  and  even  are  killed  by  specialized 
floral  structures,  as  in  Asdepias  and  Hedychium;  bees  that  are  unable 
to  reach  the  honey  in  long  floral  spurs  frequently  bite  holes  at  the  side, 
thus  getting  the  nectar  without  effecting  pollination. 

Probably  the  chief  reason  for  not  holding  to  natural  selection  as  a 
factor  of  prominence  in  the  origin  of  floral  structures  is  that  flowers, 
though  they  are  the  most  diversified  and  specialized  of  plant  organs, 
probably  have  played  a  comparatively  minor  role  in  determining  the 
success  of  plant  groups.  It  is  likely  that  the  success  of  the  grasses  and 
the  catkin-bearing  trees  is  due  less  to  the  floral  features  above  noted 
than  to  vegetative  reproduction  in  the  former  and  to  the  tree  habit  in 
the  latter. 

Perhaps  the  best  evidence  in  support  of  the  view  that  flowers  contribute 
largely  to  the  success  of  plants  is  found  in  the  composites,  though  even 
here  it  is  likely  that  vegetative  reproduction  and  the  wind  dispersal  of 
seeds  play  a  larger  part.  Even  the  composite  flower  owes  its  advantage 
not  so  much  to  floral  specialization  as  to  the  massing  of  inconspicuous 
and  relatively  non-specialized  flowers  into  compact  heads,  which  greatly 
facilitates  pollination.  Furthermore,  it  is  to  be  remembered  that  the 
composites,  forming  supposedly  the  highest  of  plant  groups  and  certainly 
the  largest  in  number  of  species  and  one  of  the  largest  in  display  of  indi- 
viduals, are  notable  for  their  geitonogamy  and  autogamy,  for  their  rela- 
tively actinomorphic  flowers  (the  disk  flowers  being  strictly  actino- 
morphic)  with  their  pollen  exposed  for  any  insects  that  may  visit  them, 
and  for  their  tendency  toward  dicliny;  it  may  be  significant,  also,  that 
the  greatest  display  of  parthenogenesis  among  seed  plants  is  among  the 
composites. 

The  preceding  paragraphs  appear  to  show  that  the  fundamental  pos- 
tulate of  natural  selection,  namely,  that  the  trend  of  evolution  is  along 
the  line  of  maximum  advantage,  is  untenable,  at  least  so  far  as  flowers 
are  concerned.  The  evolution  of  the  orchids  beyond  the  point  of  maxi- 
mum advantage,  the  phenomenal  success  of  the  groups  with  generalized 
flowers,  and  the  probable  dominance  of  the  vegetative  over  the  repro- 
ductive factors  in  determining  success  in  the  majority  of  groups,  all 
appear  to  indicate  that  some  other  factor  than  natural  selection  has 
determined  the  diversity  of  floral  structures.  Though  the  theory  of 


878  ECOLOGY 

natural  selection  seems  to  explain  such  structures  quite  as  inadequately 
as  did  the  old  and  discredited  theory  of  special  creation,  it  is  not  possi- 
ble as  yet  to  put  one  which  is  adequate  in  its  place.  Perhaps  the  most 
tenable  theory  is  that  of  orthogenesis.  This  theory  postulates  a  definite 
trend  in  the  course  of  evolution,  regardless  of  the  influence  of  selection. 
It  would  assume  that  the  specialized  features  of  flowers  and  also  of 
insects  are  organization  characters  that  are  more  or  less  inherent  in  the 
species.  According  to  this  conception  the  insects  and  flowers  are  not 
adapted  to  each  other,  but  insects  in  their  floral  visits  select  those  flowers 
whose  structures  happen  to  be  suited  to  their  mouth  parts.  It  is  obvious 
that  this  still  leaves  unanswered  the  most  fundamental  question  of  all, 
namely,  the  cause  of  floral  structures.  In  the  present  state  of  knowledge, 
it  is  not  possible  to  say  whether  the  evolution  of  floral  structures  has  been 
determined  chiefly  by  external  factors  or  by  factors  that  we  call  internal. 
This  subject,  in  so  far  as  it  has  to  do  with  external  factors,  belongs 
properly  to  the  following  section. 

3.    THE    INFLUENCE    OF   EXTERNAL   FACTORS 

UPON   THE 
DEVELOPMENT   AND   FORM    OF   REPRODUCTIVE    ORGANS 

Introductory  remarks.  —  Variations  in  the  development  and  form 
of  reproductive  organs  are  less  common  than  are  similar  variations  in 
vegetative  organs,  but  they  are  much  more  common  than  has  been  sup- 
posed. Their  relative  invariability  has  long  made  differences  in  repro- 
ductive structures  the  chief  basis  of  classification.  For  this  very  reason 
variation  is  nowhere  more  significant,  since,  if  the  present  theories  of 
classification  are  correct,  the  study  of  reproductive  variations,  however 
few  or  inevident  they  prove  to  be,  may  lead  to  the  interpretation  of 
evolution.  The  possibilities  of  experimentation  in  this  field  are  well 
shown  by  a  recent  study  of  the  fungus,  Saprolegnia;  from  a  single  my- 
celium there  have  been  derived  by  appropriate  changes  in  the  media 
the  forms  of  asexual  reproduction  that  have  been  held  to  be  charac- 
teristic of  six  different  genera. 

Reproductive  variation  in  the  seedless  plants.  —  Experimental  data 
from  the  algae  and  the  fungi.  —  In  the  seed  plants  it  is  common  to  speak 
of  two  contrasting  states,  namely,  the  vegetative  and  the  reproductive, 
but  in  many  algae  there  are  three  such  states,  characterized  respectively 
by  vegetative  activity,  by  asexual  reproduction,  and  by  sexual  reproduc- 


REPRODUCTION   AND   DISPERSAL  879 

tion.  It  is  believed  that  the  inception  of  each  of  these  states  depends 
upon  definite  external  factors,  though  vegetative  activity  necessarily  must 
antedate  the  others,  since  it  is  the  stage  of  food  accumulation. 

Vegetative  activity  may  be  prolonged  indefinitely,  being  favored  by 
the  continued  uniformity  of  optimum  vegetative  conditions.  The  most 
important  single  factor  favoring  such  activity  appears  to  be  the  constant 
presence  of  sufficient  water  to  keep  the  cell  sap  dilute,  and  to  facilitate 
active  growth.  Another  important  factor  seems  to  be  a  uniform  and 
moderately  high  temperature,  chiefly,  perhaps,  because  of  its  effect  upon 
the  absorption  of  water.  Under  uniformly  high  temperatures,  Bacillus 
anthracis  and  other  bacteria  have  been  kept  in  a  state  of  continued  vege- 
tative activity,  with  no  tendency  to  develop  resting  cells  ("  spores  "). 
Saprolegnia  has  been  kept  for  six  years  in  a  purely  vegetative  condition, 
and  brewers'  yeast  probably  has  been  kept  essentially  vegetative  for 
centuries. 

Both  sexual  reproduction  and  asexual  reproduction  are  induced  by 
changes  in  external  conditions,  and  particularly  by  changes  that  are 
detrimental  to  optimum  vegetative  activity.  Although  species  differ 
quantitatively  and  qualitatively  as  to  the  precise  external  factors  that 
are  involved  in  the  initiation  of  reproductive  activity,  it  has  been  shown 
in  many  cases  that  the  development  of  reproductive  structures  is  induced 
by  desiccation,  by  increased  concentration  of  the  medium,  by  very  high 
and  by  very  low  temperatures,  by  intense  illumination,  by  decreased 
food  supply,  and  by  the  presence  or  absence  of  specific  chemical  sub- 
stances. It  is  scarcely  possible  as  yet  to  distinguish  sets  of  factors  which 
commonly  initiate  sexual  reproduction  as  opposed  to  asexual  reproduc- 
tion, although  in  some  cases  (notably  in  the  molds)  the  development  of 
asexual  spores  is  favored  by  those  factors  which  are  most  opposed  to 
vegetative  activity,  namely,  desiccation,  food  impoverishment,  low  tem- 
perature, high  concentration  of  the  medium,  and  strong  illumination; 
zygospore  formation,  on  the  other  hand,  is  favored  in  the  molds  by  con- 
ditions which  more  closely  resemble  those  favoring  vegetative  activity, 
namely,  moisture,  rich  food  supply,  high  temperature,  low  concentration 
of  the  medium,  and  darkness.  However,  sexual  reproduction  is  favored 
by  strong  illumination  in  Vaucheria,  by  low  temperature,  and  by  food 
impoverishment  in  Saprolegnia,  and  by  desiccation  in  Spirogyra.  In 
some  cases  it  seems  as  if  almost  any  alteration  of  previous  conditions 
serves  to  initiate  reproductive  activity,  and  in  other  cases  there  seem  to 
be  certain  individuals  or  strains  predisposed  to  continued  vegetative 


88o  ECOLOGY 

activity,  while  other  individuals  or  strains  appear  to  develop  reproduc- 
tive organs,  almost  regardless  of  external  conditions.  From  the  repro- 
ductive standpoint  one  of  the  most  plastic  of  plants  is  Vaucheria  (figs. 
94-100),  which  in  poorly  illuminated  running  water  may  be  kept  in  a 
vegetative  condition  for  several  years,  while  in  standing  water  it  pro- 
duces zoospores  if  weakly  illuminated,  and  sex  organs  if  well-illuminated 
or  if  grown  in  media  poor  in  food  l ;  zoospores  may  be  formed  also  when 
the  food  is  scanty,  and  desiccation  may  result  in  the  formation  of  non- 
motile,  thick- walled  resting  spores  (aplanospores) . 

In  the  lichens  shade  and  moisture  favor  the  formation  of  the  soredia,  while  light 
and  desiccation  favor  the  development  of  the  organs  concerned  in  asexual  repro- 
duction (apothecia).  In  Botrydium,  zoospores  develop  in  water,  but  when  the  plants 
are  desiccated,  there  develop  aplanospores  comparable  to  those  of  Vaucheria, 
Saprolegnia  is  quite  as  plastic  as  is  Vaucheria,  vegetating  indefinitely  if  well  nour- 
ished, but  developing  zoospores  if  grown  in  distilled  water ;  the  development  of 
sex  organs  is  favored  by  growth  on  solid  substrata,  by  low  temperatures,  by  food 
impoverishment,  and  by  the  addition  of  specific  salts  to  the  media.  In  Spirogyra 
zygospore  formation  is  facilitated  by  high  temperature  as  well  as  by  desiccation; 
there  is  a  striking  contrast  between  the  dark  green  vegetative  filaments  of  dilute 
fresh  water  and  the  yellowish  reproductive  filaments  of  ponds  that  are  drying  up. 
In  Oedogonium,  zoospore  production  is  favored  by  depriving  the  media  of  nitrates 
and  phosphates,  by  growth  in  darkened  distilled  water,  and  by  transfer  from  a  rich 
to  a  poor  nutrient  solution.  In  Botrytis  there  is  a  reciprocal  relation  between  the 
sclerotia  and  the  conidia,  the  former  being  favored  by  good  vegetative  conditions, 
while  spore  formation  is  favored  by  desiccation,  by  poor  nutrition,  and  by  high 
concentration  of  the  medium.  Species  differ  widely  as  to  the  effect  of  increased 
concentration  of  the  medium;  in  Sligeoclonium,  and  perhaps  in  most  forms,  low 
concentrations  favor  zoospore  production,  but  in  Tetraspora,  zoospores  continue  to 
develop  at  high  concentrations,  and  in  Vaucheria,  concentration  seems  to  make 
but  little  difference.  In  Basidiobolus  low  concentrations  favor  zygospore  production, 
and  high  concentrations  facilitate  the  development  of  thick-walled  resting  spores. 
A  reduced  supply  of  oxygen  appears  to  induce  reproduction  in  Ulothrix.  Monas, 
one  of  the  infusorians,  reproduces  vegetatively  or  sexually  at  20°  C.,  but  by  asexual 
spores  at  temperatures  between  i°  C.  and  4°  C. 

Comparatively  little  is  known  concerning  reproductive  reactions  to  external  con- 
ditions among  the  higher  fungi,  though  in  Coprinus,  Stereum,  and  Xylaria  reproduc- 
tive activity  is  favored  by  illumination,  by  poor  nutrition,  and  by  partial  desiccation 
In  the  rusts  the  development  of  teleutospores  is  hastened  by  refrigeration,  as  in  alpine 
cultures.  It  has  been  found  also  that  in  Uromyces  Veratri,  similar  aecidiospores 
produce  the  uredo  generation  if  sown  on  young  leaves,  and  the  teleuto  generation 
if  sown  on  old  or  wounded  leaves,  suggesting  that  the  kind  of  spore  that  is  formed 
may  be  related  to  nutrition.  In  some  cases  external  factors  not  only  initiate  periods 

1  In  Hydrodictyon  intense  light  favors  zoospore  production,  and  in  Ulothrix  light  seems 
to  be  without  influence  in  this  connection. 


REPRODUCTION   AND   DISPERSAL  88 1 

of  reproductive  activity,  but  they  influence  the  character  of  the  reproductive  struc- 
tures ;  for  example,  in  some  of  the  rusts  the  spore  walls  are  thicker  in  xerophytic 
situations  than  elsewhere,  and  in  Bornetina  the  size,  shape,  and  sculpturing  of  the 
spores  vary  with  the  culture  media  and  with  the  illumination. 

In  certain  marine  algae,  as  Dictyota  dichotoma,  there  is  a  remarkable  periodicity, 
which  seems  to  be  related  to  external  conditions.  In  England  the  sex  organs  de- 
velop at  fortnightly  periods,  the  gametes  being  liberated  at  a  fixed  interval  after  the 
highest  spring  tide.  In  North  Carolina  there  also  is  a  relation  to  the  tides,  but  the 
production  of  sex  organs  occurs  monthly  rather  than  fortnightly.  Similar  phenom- 
ena have  been  observed  at  Naples,  and  in  Japan  a  fortnightly  period  of  gamete 
liberation  has  been  discovered  for  Sargassum.  The  most  probable  causative  stimu- 
lus of  reproductive  periodicity  is  the  increased  illumination  that  is  associated  with 
the  fortnightly  recurrence  of  extreme  low  water  ;  at  Naples  the  liberation  of  gametes 
appears  to  be  on  the  day  when  low  water  occurs  nearest  to  midday.  Factors  which 
modify  the  tides,  such  as  wind  or  change  of  atmospheric  pressure,  also  affect  the  time 
of  gamete  liberation. 

The  influence  of  external  factors  upon  reproductive  activity  in  animals  appears 
to  be  much  less  obvious  than  in  plants.  However,  in  Paramoecium  and  in  other 
infusorians  the  continuance  of  favorable  nutritive  conditions  seems  to  cause  con- 
tfnued  vegetative  activity,  whereas  conjugation  is  due  chiefly  to  changes  in  the 
media.  In  the  water-fleas  ( Daphnid)  there  are  two  kinds  of  generations,  one  being 
composed  of  males  and  females,  and  the  other  being  composed  solely  of  partheno- 
genetic  females.  It  has  been  ascertained  that  parthenogenetic  generations  result 
when  the  conditions  for  nutrition  are  favorable,  whereas  bisexual  generations  result 
from  conditions  unfavorable  for  nutrition,  such  as  increased  concentration  of  the 
medium,  desiccation,  high  or  low  temperature,  the  accumulation  of  excreta,  and 
starvation.  In  nature  the  bisexual  generation  is  especially  to  be  seen  in  shallow 
pools,  and  in  autumn  in  ordinary  ponds.  The  conditions  for  the  development  of 
parthenogenetic  and  bisexual  generations  are  very  similar  in  certain  other  animals, 
such  as  rotifers  (Hydatina),  plant  lice  (aphids),  and  the  grape-louse  (Phylloxera). 
Under  favorable  nutrient  conditions  there  may  be  many  successive  parthenogenetic 
generations  without  any  intervening  bisexual  generations. 

The  origin  of  sexuality.  —  There  is  little  experimental  evidence 
bearing  upon  the  origin  of  sexuality,  although  there  exist  a  number  of 
forms  with  facultative  gametes,  and  even  with  facultative  gamete-produc- 
ing organs  (gametangia) .  In  Ulothrix  (figs.  1133,  1134)  there  are 
intergradations  (e.g.  zoospores  of  intermediate  size  with  two  or  four  cilia) 
between  the  large  quadriciliate  zoospores  and  the  small  biciliate  gametes, 
suggesting  the  possible  origin  of  gametes  from  zoospores;  indeed,  it  is 
known  that  without  fusion  gametes  sometimes  develop  into  plants,  quite 
as  do  zoospores.  In  Hydrodictyon  similar  primordia  produce  gametes 
in  some  media  and  zoospores  in  others.  In  Zygnema  stellinum  there  are 
intergradations  between  the  isogamous,  the  heterogamous,  and  the 
parthenogenetic  gametes,  and  in  the  sea  lettuces,  Ulva  and  Entero- 


882  ECOLOGY 

morpha,  there  are  small  conjugating  gametes  and  large  parthenogenetic 
gametes.  The  auxospores  of  diatoms  may  develop  vegetatively,  may 
form  asexual  spores,  or  may  conjugate  sexually.  In  Ectocarpus  there 
are  transitions  between  sporangia  and  gametangia,  the  same  structures 
producing  either  zoospores  or  isogamous  or  heterogamous  gametes, 
thus  suggesting  the  possible  origin  of  sex  as  well  as  of  sexuality.  In  all 
of  these  cases  the  exact  determinative  factors  remain  to  .be  discovered, 
although  it  has  been  suggested  from  their  small  size  in  comparison  with 
zoospores  and  parthenogenetic  gametes  that  conjugating  gametes 
represent  poorly  nourished  spores. 

Artificial  parthenogenesis.  —  The  most  important  experimental  evi- 
dence concerning  parthenogenesis  is  derived  from  animals,  and  in  view 
of  its  great  significance,  it  must  be  cited  here.  Parthenogenesis  is  ob- 
served somewhat  frequently  in  a  number  of  animals,  such  as  bees,  wasps, 
and  plant  lice ;  in  the  latter  it  occurs  especially  at  high  temperatures  or 
when  the  host  plant  is  very  watery.  It  has  been  demonstrated  that  the 
eggs  of  the  sea  urchin  (Arbacia)  develop  into  larvae  in  the  absence  of 
sperms,  if  they  are  placed  in  somewhat  concentrated  solutions  of  mag- 
nesium chlorid  and  sea  water.  Comparable  results  were  obtained  with 
other  salts,  and  all  were  at  first  referred  to  the  increased  osmotic  pressure 
occasioned  by  their  presence.  Hence  it  was  suggested  that  the  stimulus 
necessary  for  egg  development  is  the  extraction  of  water.  Later  ex- 
periments have  demonstrated  that  artificial  parthenogenesis  can  be 
brought  about  in  many  other  ways  than  by  exposing  eggs  to  increased 
osmotic  pressure,  and  it  is  becoming  evident  that  the  explanation  of  the 
process  is  by  no  means  simple;  a  feature  of  recent  experiments  is  the 
emphasis  that  has  been  placed  upon  chemical  factors. 

Experiments  with  similar  results  have  been  made  upon  the  eggs  of  other  echino- 
derms  than  the  sea  urchin  (e.g.  those  of  the  starfish,  Asterias),  and  also  those  of 
certain  annelids  (as  Chaetopterus  and  Polynoe)  and  mollusks  (as  Sottia).  In  the 
starfish,  eggs  develop  parthenogenetically  when  they  are  exposed  for  several  hours 
to  temperatures  below  7°  C.  It  was  found  some  time  ago  that  the  eggs  of  Chae- 
topterus develop  parthenogenetically  by  the  addition  to  the  medium  of  potassium 
ions  in  an  amount  too  small  to  produce  an  osmotic  effect,  and  more  recently  various 
acids  and  alkalis  have  been  seen  to  act  similarly.  Treatment  with  a  fatty  acid 
(as  acetic  acid)  before  placing  in  a  concentrated  solution  greatly  stimulates  develop- 
ment, because  it  causes  the  formation  of  a  membrane,  just  as  when  a  sperm  fuses 
with  the  egg.  The  mechanical  agitation  of  eggs  sometimes  causes  their  partheno- 
genetic development.  Indeed  it  would  seem  that  almost  any  disturbance  may  serve 
to  stimulate  the  development  of  certain  eggs.  Probably  a  large  factor  in  the  case 
is  the  permeability  of  the  egg  to  the  substances  it  needs  for  its  development,  and  it 


REPRODUCTION   AND   DISPERSAL  883 

may  be  that  the  various  stimulating  influences  have  as  their  chief  r61e  the  establish- 
ment of  increased  permeability.  In  most  cases  artificially  parthenogenetic  animals 
die  in  an  early  stage  of  development,  but  in  at  least  one  instance  mature  sea  urchins 
have  been  secured  by  this  means. 

Few  similar  experiments  have  been  performed  with  plant  eggs,  though 
parthenogenesis  has  been  induced  in  Spirogyra  and  in  Chlamydomonas 
by  growing  plants  in  concentrated  (6  per  cent)  solutions  of  cane  sugar; 
in  these  experiments  plasmolysis  occurred,  indicating  the  extraction  of 
water,  as  in  Arbacia.  Artificial  parthenogenesis  has  been  reported  for 
other  plants,  such  as  Protosiphon  and  Marsilea,  high  temperature  seem- 
ing to  be  the  stimulating  factor.  The  experiments  on  artificial  partheno- 
genesis seem  to  suggest  that  the  role  of  the  sperm  is  less  that  of  a  carrier 
of  necessary  hereditary  substance  than  that  of  a  growth  excitant,  which 
by  physical  or  chemical  means  makes  the  egg  permeable  to  the  sub- 
stances which  bring  about  development. 

Sexuality  in  ike  fungi.  —  The  sexual  relations  of  the  fungi  are  very 
suggestive  of  modifications  resulting  from  saprophytic  or  parasitic  modes 
of  life,  although  confirmatory  experimental  evidence  is  largely  lacking. 
In  Saprolegnia  and  Achlya  (figs.  155-157)  there  are  all  gradations  be- 
tween completely  developed  male  sexual  organs  and  the  absence  of  such 
organs.  Some  forms  have  apparently  complete  sexual  organs  but  the 
eggs  develop  parthenogenetically;  other  forms  have  antheridial  tubes 
which  reach  the  egg  but  remain  closed  or  merely  pierce Tlie  oogonium 
wall  without  reaching  the  egg;  still  other  forms  have  no  antheridial  tube, 
and  some  forms  have  no  antheridium.  There  may  be  considerable 
variation  also  within  a  given  species;  for  example,  antheridia  are  rarely 
present  in  Saprolegnia  Thureti,  as  often  absent  as  present  in  6".  mixta, 
and  usually  present  in  S.  hypogyna;  they  are  always  present  in  S. 
monoica.  In  no  case  are  the  female  organs  absent,  so  that  Saprolegnia 
forms  a  striking  instance  of  parthenogenesis  by  reduction.1  In  the 
zygomycetes  there  are  gradations  between  heterogamy  and  isogamy, 
suggesting  the  evolution  of  the  latter  from  the  former  by  reduction,  and 
in  the  ascomycetes  there  appear  to  exist  many  stages  in  the  reduction  of 
sexuality.  In  comparatively  few  fungi  does  there  appear  to  be  a  fusion 
of  ordinary  gametes,  though  a  number  of  apparently  modified  forms  of 

1  Cases  of  reduction  are  known  also  in  animals;  for  example,  some  rotifers  have  small 
and  functionless  males  or  none  at  all,  and  in  some  crustaceans  (as  Limnadia  Hermanni) 
and  ostracods  (as  Cypris  reptans)  only  females  are  known,  though  they  still  retain  the 
sperm  sac. 


884  ECOLOGY 

sexuality  have  recently  been  discovered  in  the  rusts  and  smuts  and  in 
certain  other  groups.  Even  the  formation  of  asexual  spores  appears  to 
have  ceased  in  some  fungi,  as  in  the  internal  fungus  of  Lolium,  which 
probably  is  a  smut,  and  it  is  rare  in  others,  as  in  most  mycorhiza  fungi. 
It  commonly  has  been  supposed  that  the  reduced  or  modified  sexuality 
of  the  fungi  is  in  some  way  associated  with  their  saprophytic  or  para- 
sitic mode  of  life;  since  well- nourished  plants  reproduce  sexually  less 
than  do  poorly  nourished  plants,  it  is  possible  that  the  good  nutritive 
conditions  of  the  group  in  part  account  for  the  character  of  its  sexual 
development. 

Reproductive  variations  in  the  bryophytes  and  pteridophytes.  —  Light 
favors  the  development  of  sex  organs  in  liverworts,  mosses,  and  ferns. 
In  Marchantia,  weak  light  or  an  excess  of  moisture  favors  ordinary  veg- 
etative reproduction;  an  increase  of  illumination  favors  the  development 
of  gemmae,  and  strong  illumination  favors  the  development  of  sex  organs. 
In  weak  light,  fern  gametophytes  develop  into  filaments  resembling 
moss  protonemata  instead  of  producing  sex  organs.  If  in  Salvinia  the 
sperms  and  eggs  fail  to  fuse,  the  female  gametophyte,  whose  growth 
commonly  is  checked  at  such  fusion,  continues  to  grow  vegetatively, 
producing  new  female  organs;  thus  embryo  development  seems  in  some 
way  to  check  gametophytic  vegetative  activity.  Similarly,  the  gameto- 
phytes of  Osmunda  are  long-lived,  if  fusion  does  not  take  place.  While 
most  fern  gametophytes  are  monoecious,  producing  first  male  organs 
and  then  female  organs,  gametophytes  that  are  poorly  nourished  (having, 
for  example,  a  small  supply  of  nitrogen)  or  are  exposed  to  strong  illumina- 
tion, may  prbduce  male  organs  only,  as  though  the  food  supply  were 
insufficient  for  complete  development;  in  rare  instances,  vigorous,  well- 
nourished  gametophytes  bear  only  archegonia.  In  the  ostrich  fern 
(Onoclea  Struthiopteris)  the  gametophytes  commonly  are  dioecious,  the 
larger  plants  being  female,  and  the  smaller  plants  being  male;  in  or- 
dinary cultures  one  to  twelve  per  cent  of  the  plants  are  monoecious. 
Under  certain  culture  conditions,  as  when  female  plants  are  transferred 
to  rich  nutrient  media,  at  least  fifty  per  cent  of  the  plants  may  become 
monoecious.  Similar  phenomena  occur  in  some  monoecious  mosses, 
antheridia  being  the  only  sex  organs  developed  when  the  nutritive  con- 
ditions are  poor  ;  in  some  dioecious  forms  (as  in  Dicranum)  the  male 
plants  are  smaller  than  the  female  plants.  In  the  ferns  it  often  is  easy 
to  induce  apogamy  and  apospory,  especially  where  the  illumination  is 
weak,  or  where  the  soil  is  dry  or  poor  in  food  materials. 


REPRODUCTION   AND   DISPERSAL  885 

A  somewhat  remarkable  situation  occurs  in  Equisetum;  though  it  is  a 
homosporous  genus,  some  of  its  ancestral  relatives  were  heterosporous, 
and  even  now  the  gametophytes  usually  are  dioecious,  though  arising 
from  approximately  similar  spores.  However,  the  smaller  and  more 
poorly  nourished  gametophytes  usually  bear  male  organs  and  the  larger 
gametophytes,  female  organs.  Furthermore,  the  smaller  spores  are 
likely  to  give  rise  to  male  gametophytes  and  the  larger  spores  to  female 
gametophytes  ;  in  the  true  ferns,  however,  there  appears  to  be  no  re- 
lation between  the  size  of  the  spore  and  the  sex  of  the  gametophyte  that 
comes  from  it.  Occasionally,  the  gametophytes  are  monoecious,  the 
female  organs  appearing  last,  as  in  ordinary  ferns.  In  Marsilea,  one  of 
the  heterosporous  pteridophytes,  the  development  of  fruiting  organs 
(sporocarps)  may  be  incited  by  partial  desiccation  (as  in  the  drying  up 
of  a  pond),  by  increased  illumination,  or  by  high  temperature  ;  on  the 
other  hand,  fruiting  may  be  retarded  or  prevented  by  placing  the  plants 
under  water  in  weak  light,  at  low  temperatures,  or  in  crowded  cultures. 
In  the  microsporangia  there  are  sixty-four  primordia  which  develop 
commonly  into  microspores,  but  of  the  sixty-four  megaspore  primordia, 
only  one  develops,  and  that  at  the  expense  of  the  others  nutritively.  It 
has  been  shown  that  by  subjecting  developing  Marsilea  sporocarps  to 
spraying  by  cold  water,  no  megaspore  primordia  develop,  but  that  struc- 
tures resembling  megaspores  may  be  made  to  develop  from  microspore 
primordia  under  optimum  nutritive  conditions,  growth  being  at  the  ex- 
pense of  other  primordia,  as  in  the  development  of  ordinary  mega- 
sporangia;  sometimes  such  spores  are  sixteen  times  as  large  as  ordinary 
microspores.  Thus  it  is  suggested  that  heterospory  may  have  arisen 
from  homospory  through  the  influence  of  optimum  nutrition  on  develop- 
ing sporangia. 

Some  ferns  show  interesting  transformations  of  reproductive  primordia  into 
vegetative  organs;  for  example,  in  Osmunda  and  Botrychium  there  often  are  leafy 
organs  in  the  reproductive  region,  and  in  Onoclea  the  removal  of  the  foliage  leaf  is 
followed  by  the  development  of  another  foliage  leaf  from  the  primordium  of  the 
reproductive  shoot. 

Reproductive  variations  in  the  seed  plants.  —  Vegetative  and  repro- 
ductive periods.  —  In  the  seed  plants  it  is  convenient  to  distinguish  as 
the  reproductive  phase  all  of  the  complex  phenomena,  both  sporophytic 
and  gametophytic,  from  the  inception  of  the  flower  to  the  maturation 
of  the  seed,  contrasting  this  with  the  vegetative  phase  of  the  sporophyte. 
As  in  the  lower  plants,  a  vegetative  period  always  antedates  the  period  of 


886  ECOLOGY 

reproduction.  The  length  of  this  initial  vegetative  period  differs  widely, 
varying  from  a  few  weeks  in  certain  xerophytic  annuals  to  a  number  of 
years  in  the  century  plant  and  in  most  trees.  There  may  be  but  one 
reproductive  period  following  this  initial  vegetative  period,  as  in  annuals 
and  biennials,  and  in  such  perennials  as  the  century  plant ;  in  these 
forms,  which  are  known  as  monocarpic  plants,  death  follows  fruit  matura- 
tion. In  most  perennials,  however,  reproduction  either  continues  in- 
definitely after  its  inception  or  more  often  recurs  at  certain  periods, 
vegetative  activity  also  continuing  indefinitely  or  periodically  ;  such 
forms  are  termed  polycarpic  plants.  The  most  representative  poly- 
carpic  plants  are  trees  and  shrubs,  in  which  the  shoots  do  not  die  down 
after  flowering;  in  rhizomatous  and  bulbous  plants,  however,  each  shoot 
dies  soon  after  flowering,  much  as  in  annuals,  and  new  shoots  arise  by 
vegetative  reproduction. 

Probably  in  the  majority  of  the  perennials  of  temperate  climates,  the 
vegetative  and  reproductive  periods,  to  some  extent  at  least,  alternate 
with  one  another,  the  flowering  period  being  rather  sharply  defined,  and 
often  of  short  duration.  Excellent  cases  of  such  alternating  periods  occur 
among  plants  with  vernal  flowers,  as  in  the  willows  and  poplars,  and  in 
such  herbs  as  Hepatica  and  Sanguinaria,  which  bloom  before  vegeta- 
tive activity  begins.  In  such  plants  reproductive  activity  merely  ap- 
pears to  antedate  vegetative  activity,  early  flowering  being  possible 
only  because  of  the  food  accumulated  during  the  previous  season;  for 
that  matter,  the  reproductive  period  in  such  plants  begins  in  the  spring 
or  in  the  summer  previous  to  flowering,  and  in  some  cases  (as  in  the 
alder  and  hazel)  the  buds  are  fully  formed  before  winter  begins  (fig. 
1234).  In  many  plants  the  spring  and  summer  are  periods  of  vegeta- 
tive activity  (as  in  the  goldenrods,  asters,  gentians,  and  witch-hazel), 
while  the  reproductive  period  falls  in  the  late  summer  or  in  autumn. 
In  some  plants  (as  Satureja  and  Lechea)  there  are  strongly  marked  vege- 
tative periods  in  spring  and  in  autumn,  separated  by  the  summer  repro- 
ductive period.  An  unusually  sharp  contrast  between  vegetative  and 
reproductive  activity  is  afforded  by  the  wild  leek  (Allium  tricoccum) 
and  by  the  meadow  saffron  (Colchicum  autumnale),  in  which  the  leaves 
appear  in  spring  and  soon  die  down,  while  the  flowers  do  not  appear 
until  summer  or  autumn. 

There  are  many  species  in  which  the  vegetative  and  reproductive 
periods  are  essentially  coincident.  In  a  few  instances  (as  in  Dicentra 
and  Claytonia)  the  two  periods  not  only  are  more  or  less  coincident,  but 


REPRODUCTION   AND   DISPERSAL  887 

they  are  of  short  duration;  in  most  cases,  however,  as  in  the  chick  weed 
(Stellaria  media),  the  periods  of  vegetative  and  of  reproductive  activity 
are  coincident  and  of  long  duration;  such  plants  may  be  called  ever- 
bloomers.  As  might  be  expected,  everbloomers  flourish  particularly 
in  uniform  tropical  climates.  While  in  temperate  climates  each  month 
or  even  each  week  from  spring  to  autumn  is  characterized  by  the  anthesis 
of  particular  species,  in  uniform  tropical  climates  almost  any  species 
may  bloom  at  almost  any  time,  and  a  large  number  of  species  are  true 
everbloomers,  being  in  flower  at  all  times.  Even  those  species  which 
are  strictly  periodic  in  temperate  climates  may  be  everbloomers  in  the 
tropics  (as  in  the  grape  and  the  Virginia  creeper).  In  many  species  of 
tropical  everbloomers  there  is  a  suggestion  of  periodicity,  since  some 
branches  bloom  at  one  time  and  some  at  another;  for  example,  in  the 
grape  one  shoot  on  a  given  vine  may  be  putting  forth  leaves  and  another 
flowers,  while  still  another  is  bearing  ripe  fruit.  In  such  species  the 
phenomena  exhibited  by  an  individual  branch  are  periodic,  but  taking 
the  plant  as  a  whole  the  phenomena  may  be  termed  spasmodic.  The 
most  representative  everbloomers  are  plants  with  unbranched  trunks, 
such  as  Cocos  or  Carica,  for  in  them  there  is  essential  continuity  in  both 
vegetative  and  reproductive  activity  in  a  given  shoot;  new  leaves  are 
found  at  all  times,  as  well  as  flowers  and  fruits  in  all  stages  of  develop- 
ment. In  many  tropical  plants,  on  the  other  hand,  flowering  is  of  rela- 
tively rare  occurrence,  several  years  or  even  many  years  elapsing  between 
periods  of  anthesis.  The  most  remarkable  case  of  this  sort  is  afforded 
by  a  bamboo,  Dendrocalamus  strictus,  which  is  said  to  flower  regularly 
at  thirty-year  intervals.  Some  tropical  plants  and  even  some  plants 
of  high  latitudes  (as  the  duckweed)  are  almost  never  seen  in  blossom, 
their  reproduction  being  essentially  vegetative. 

The  relation  of  anthesis  to  meteorological  factors. —  While  the  occurrence 
and  the  duration  of  flowering  periods  often  have  been  regarded  as  due 
to  inherent  causes,  it  always  has  been  known  in  a  general  way  that 
climatic  factors  may  hasten  or  retard  such  periods  and  modify  their 
length.  If  vernal  plants  bloom  sooner  than  usual,  the  season  is  called 
"early,"  while  delayed  anthesis  causes  a  season  to  be  called  "late." 
The  observation  of  meteorological  phenomena  in  connection  with  the 
periodic  activities  of  plants,  and  particularly  of  temperature  in  relation 
to  anthesis,  is  known  as  phenology.  In  a  general  way  it  is  known  that 
temperature,  among  other  factors,  bears  an  important  relation  to  flower- 
ing, which  is  facilitated  by  high  temperatures  and  retarded  by  low 


888  ECOLOGY 

temperatures.  Phenological  observers,  however,  often  have  regarded 
temperature  as  of  such  controlling  importance  that  they  have  prepared 
tables  showing  the  total  amount  of  heat  necessary  for  flowering  in  the 
various  species.  Such  tables  are  almost  worthless,  since  they  fail  to 
include  the  many  other  factors  involved,  some  of  which,  as  soil  moisture 
or  atmospheric  moisture,  equal  or  surpass  temperature  in  importance. 
Further,  in  preparing  tables,  temperatures  below  o°  C.  commonly  are 
ignored,  although  they  are  certainly  of  considerable  significance  in  some 
plants  (as  in  those  of  arctic  regions),  while  the  temperatures  just  above 
o°  C.  may  be  without  significance  in  other  plants  (as  in  palms). 

The  difficulties  involved  in  discovering  the  factors  that  determine  the 
inception  of  anthesis  are  best  illustrated  in  those  species  which  form 
flower  buds  early  in  the  season  previous  to  flowering.  Some  buds,  as 
in  the  lilac  and  the  white  birch,  begin  to  develop  a  year  before  they  come 
into  bloom,  and  in  most  vernal  species  the  flower  buds  are  in  evidence  by 
midsummer.  The  insufficiency  of  the  phenological  method  in  the  case 
of  such  plants  is  most  striking,  since  certain  buds  (as  in  the  alder  and 
the  hazel)  that  withstand  days  and  even  weeks  of  warm  weather  in  the 
autumn  without  blooming  require  but  a  few  days  of  warm  weather 
in  spring  to  induce  anthesis.1  Years  ago  it  was  shown  that  summer  and 
autumn  temperatures  have  little  or  no  influence  upon  the  flower  buds  of 
the  cherry  (Prunus  Avium),  though  the  buds  are  evident  as  early  as 
July;  however,  shoots  taken  into  a  hothouse  in  the  middle  of  December 
bloomed  in  twenty-seven  days,  whereas  those  taken  in  the  middle  of 
January,  in  early  March,  and  in  early  April,  bloomed,  respectively,  in 
eighteen  days,  in  twelve  days,  and  in  five  days.  In  some  recent  com- 
prehensive experiments  with  nearly  three  hundred  species  of  woody 
plants,  more  than  half  of  the  twigs  which  were  brought  into  a  greenhouse 
in  November  started  to  grow  within  two  weeks;  the  twigs  of  seventy 
species  began  to  grow  in  February,  and  those  of  thirty-six  species  did 
not  become  active  until  March.  These  results  make  it  very  obvious  that 
the  influence  of  temperature  or  of  other  external  factors  upon  anthesis 
depends  entirely  upon  the  condition  of  the  buds  at  the  inception  of  the 
experiment.  While  buds  in  February  look  much  as  they  do  in  Decem- 
ber, in  reality  they  are  different,  one  determinable  change  being  that  in 

1  Occasionally  vernal  species  flower  in  autumn  (as  in  the  violet,  strawberry,  and  apple), 
particularly  if  favorable  temperature  and  moisture  conditions  are  long  continued.  The 
wonder  is  that  such  phenomena  are  relatively  rare,  especially  since  some  buds  seem  to 
be  fully  formed  by  early  autumn  (as  in  the  alder  and  the  hazel). 


REPRODUCTION   AND    DISPERSAL  889 

winter  there  is  a  gradual  increase  of  available  food  in  the  embryonic 
organs;  probably  this  relative  absence  of  available  food  is  one  of  the  chief 
reasons  why  autumnal  buds  open  so  tardily  or  remain  closed,  when  they 
are  exposed  to  favorable  temperatures. 

It  is  probable  that  buds  undergo  progressive  changes  other  than  those 
related  to  the  food  supply,  though  the  nature  of  such  changes  is  unknown. 
Recently  it  has  been  shown  that  the  development  of  buds  can  be  greatly 
stimulated  by  various  methods  of  treatment  during  the  early  part  of  the 
resting  period.  For  example,  the  subjection  of  resting  buds  to  anes- 
thetics, to  freezing  temperatures,  to  warm  water  baths,  or  to  various 
methods  of  chemical  treatment,  results  in  a  material  shortening  of  the 
rest  period,  provided  the  plants  are  brought  subsequently  into  conditions 
suitable  for  bud  development;  as  might  be  expected,  this  artificial 
hastening  of  development  has  proven  to  be  of  great  commercial  advan- 
tage in  the  "  forcing  "  of  bulbs,  and  of  lilacs  and  other  ornamental 
shrubs.  The  exact  effect  of  these  methods  is  unknown,  although  it  is 
believed  that  the  stimulation  of  development  in  potato  tubers  that  have 
been  subjected  to  low  temperatures  is  due  to  the  fact  that  at  such 
temperatures  there  is  a  rapid  accumulation  of  diastase,  which  results 
in  the  transformation  of  starch  into  sugar,  and  also  to  the  probability 
that  the  cell  membranes  are  more  permeable  than  at  higher  tem- 
peratures. 

Flowering  periods  in  arid  and  in  frigid  climates.  —  In  uniform  trop- 
ical climates,  the  flowering  of  plants  does  not  characterize  any  one  season 
more  than  another,  many  species  even  being  everbloomers.  In  most  tem- 
perate climates,  flowers  appear  at  all  seasons  that  are  in  any  way  favor- 
able; estival  flowering  occurs  chiefly  at  the  expense  of  food  accumulated 
in  spring,  but  the  earlier  vernal  flowers  utilize  the  food  accumulated 
during  the  previous  vegetative  period.  In  respect  to  anthesis,  arid  and 
frigid  climates  present  certain  features  of  marked  contrast  to  temperate 
climates  and  to  uniform  tropical  climates.  In  arid  climates  the  incep- 
tion of  the  rainy  period  is  marked  by  vegetative  activity,  but  this  is 
checked  at  the  beginning  of  the  next  dry  period.  Flowering,  however, 
is  to  a  large  extent  associated  with  the  dry  period;  indeed,  in  many  cases, 
anthesis  is  as  definitely  associated  with  the  dry  season  as  is  vegetative 
activity  with  the  rainy  season.  In  the  monsoon  district  of  eastern  Java, 
where  the  year  is  about  equally  divided  into  two  periods,  one  of  con- 
siderable rain  and  the  other  of  drought,  more  than  60  per  cent  of  the 
species  bloom  solely  in  the  dry  period,  while  only  8  per  cent  bloom 


890  ECOLOGY 

solely  in  the  wet  period;  the  remaining  30  per  cent  are  either  everbloomers 
or  forms  which  overlap  the  two  periods. 

In  alpine  and  in  arctic  climates  the  flowering  period  is  very  short,  often 
not  lasting  more  than  two  or  three  months.  Plants  that  bloom  in  the 
lowlands  in  April  (as  Erythronium  and  Claytonia)  may  not  bloom  until 
June  in  alpine  meadows,  because  of  the  long-continued  cold  and  the 
tardy  melting  of  the  snow  at  high  altitudes.  Strangely  enough,  however, 
the  alpine  season  soon  catches  up  with  that  of  the  lowlands,  so  that  by 
July  similar  forms  may  be  blooming  at  all  altitudes,  and  in  August  the 
alpine  season  actually  is  ahead  of  that  of  the  lowlands;  for  example, 
goldenrods  and  gentians  commonly  blossom  sooner  in  the  mountains 
than  at  lower  altitudes.  Similarly,  spring  is  much  later  and  autumn 
much  earlier  in  high  than  in  low  latitudes;  the  farther  grain  grows  from 
the  equator,  the  shorter  is  its  maturation  period,  barley,  for  example, 
ripening  in  ninety  days  in  northern  Norway,  but  requiring  one  hundred 
days  in  southern  Sweden.  In  part  this  surprising  phenomenon  may  be 
due  to  the  fact  that  alpine  and  arctic  species  are  different  from  the  low- 
land species,  and  therefore,  perhaps,  inherently  characterized  by  shorter 
periods.  That  this  is  a  minor  matter  in  the  explanation,  however,  is 
shown  by  the  fact  that  some  of  the  species  are  common  to  high  and  to 
low  altitudes  (as  the  yarrow  and  the  harebell),  but  particularly  by  the 
fact  that  alpine  plants  grown  in  the  lowlands,  or  lowland  plants  grown 
in  alpine  districts,  behave  in  each  case  precisely  like  the  indigenous 
plants.  Obviously,  also,  the  usual  phenological  assumption  that  low 
temperatures  retard  anthesis  is  the  very  reverse  of  the  fact,  for  the  heat 
sums  are  much  greater  in  low  than  in  high  altitudes  and  latitudes. 

It  has  been  suggested  that  the  greater  intensity  of  alpine  light  and  the 
greater  duration  of  arctic  light,  respectively,  account  for  the  "  hurrying 
up  "  of  the  seasons  at  high  altitudes  and  high  latitudes,  enabling  plants 
to  make  the  food  necessary  for  anthesis  in  a  shorter  time.  The  experi- 
ments about  to  be  cited  give  another  suggestion,  namely,  that  those  fac- 
tors that  are  detrimental  to  vegetative  activity  and  which,  therefore, 
cause  its  early  cessation,  are  at  the  same  time  favorable  to  reproductive 
activity.  Among  the  factors  in  alpine  habitats  that  tend  to  check  op- 
timum vegetative  activity  are  low  nocturnal  temperatures,  great  tem- 
perature differences  between  day  and  night,  high  transpiration  in  pro- 
portion to  absorption,  and,  perhaps,  intense  light. 

The  experimental  determination  of  vegetative  and  of  reproductive 
periods.  —  Adequate  experimental  study  has  shown  that  the  length  of 


REPRODUCTION   AND   DISPERSAL  891 

the  initial  vegetative  period  (i.e.  the  period  from  germination  to  anthesis) 
and  the  length  of  the  reproductive  period  are  subject  to  wide  modifica- 
tion through  the  operation  of  external  factors,  and  it  has  been  found 
possible  also  to  extend  the  initial  vegetative  period  indefinitely  by  the 
inhibition  of  reproduction.  These  experiments  shed  much  light  upon  the 
phenomena  cited  in  the  preceding  pages.  Many  interesting  facts  con- 
cerning reproductive  periods  have  long  been  recognized,  because  of  their 
important  practical  bearing.  For  example,  crops  like  peas,  tomatoes,  and 
sweet  corn  may  mature  some  days  or  even  some  weeks  sooner  on  dry, 
well-lighted  slopes  than  in  rich,  moist  lowlands,  so  that  the  profit  from 
the  former  is  often  much  the  greater;  but  in  crops  where  the  vegetative 
organs  are  marketed,  the  rich,  moist  habitat  often  is  preferable,  because 
of  the  greater  luxuriance  of  the  foliage.  Comparable  phenomena  are 
exhibited  by  trees,  Pinus  silvestris  maturing  fruit  in  fifteen  years  if  stand- 
ing alone  in  dry  soil,  but  requiring  thirty  to  forty  years  in  a  grove  ;  simi- 
lar differences  are  seen  in  the  beech  and  in  many  other  trees. 

Early  reproduction,  which  often  is  of  great  practical  benefit,  frequently 
is  brought  about  by  various  mechanical  means.  Picea  excelsa,  which 
commonly  flowers  in  thirty  to  forty  years,  may  be  induced  to  flower  in 
four  to  ten  years  by  transplanting,  especially  if  the  roots  are  injured. 
Orchard  trees  often  fruit  much  better,  if  some  of  the  roots  are  removed. 
Girdling  sometimes  induces  flowering  in  apple  trees  that  otherwise 
exhibit  only  vegetative  activity.  Shoots  of  a  young  tree  grafted  on 
an  old  tree  bloom  much  sooner  than  those  that  are  left  on  the  young  tree.1 
Cuttings  bloom  long  before  seedlings,  a  matter  of  the  highest  economic 
importance.  Of  much  interest  is  the  fact  that  a  cutting  from  an  old  plant 
blooms  much  sooner  than  one  from  a  young  plant,  though  the  cuttings 
may  be  of  equal  size  and  similar  aspect.  This  phenomenon  is  most 
strikingly  displayed  in  leaf  cuttings  (as  in  Begonia  or  Achimenes),  in 
which  the  young  shoot  flowers  almost  at  once  if  the  leaf  is  taken  from 
a  flowering  plant,  but  only  after  a  long  time  if  taken  from  a  young  plant. 
This  phenomenon  has  been  explained  by  postulating  the  accumulation 
of  flower-forming  substances  in  plants  approaching  maturity,  but  this 
assumption  needs  explanation  as  much  as  do  the  facts  which  it  attempts 
to  explain.  Furthermore,  there  are  some  cases,  as  in  Toreniat  where 
leaf  cuttings  flower  at  once  almost  regardless  of  the  age  of  the  part  of  the 

1  For  example,  when  a  twig  from  an  apple  sapling  is  grafted  on  an  old  stock,  It  may 
fruit  in  a  year  or  two  instead  of  in  ten  or  fifteen  years,  while  a  twig  from  an  old  stock 
grafted  on  a  sapling  does  not  fruit  for  many  years. 


892  ECOLOGY 

plant  from  which  the  cutting  is  taken.  At  any  rate,  the  maturity 
of  the  flowering  plant  seems  to  be  in  some  way  transmitted  to  the 
propagule. 

It  has  been  proved  conclusively  that  plants  may  be  kept  in  a  vegetative 
state  indefinitely,  and  that  the  usual  successive  stages  in  a  plant's  life 
history  are  reversible.  For  example,  when  the  ground  ivy  (Nepeta 
hederacea),  which  commonly  has  a  reproductive  period  intercalated 
between  the  vegetative  periods  of  spring  and  autumn,  is  grown  in  a  green- 
house under  uniform  conditions  of  moderate  temperature  and  consid- 
erable moisture,  vegetative  activity  continues  uninterruptedly.  How- 
ever, flowering  may  be  induced  at  any  time  by  transferring  the  plants  to 
a  dry,  well-lighted  situation.  Similarly,  by  exposure  to  proper  external 
conditions,  "  winter  buds  "  have  been  induced  in  Utricularia  at  any 
season,  hyacinths  have  been  induced  to  flower  twice  without  an  inter- 
vening period  of  rest,  and  Parietaria  has  been  kept  in  constant  bloom. 
Annuals  have  been  transformed  into  biennials  or  perennials  by  keeping 
them  under  constant  conditions  favorable  to  vegetative  activity,  and 
Echium,  which  usually  is  a  biennial,  has  been  known  to  grow  in  the 
tropics  ten  years  without  flowering. 

When  the  annuals,  Poa  annua  and  Senecio  vulgaris,  are  transferred  to  alpine 
habitats,  the  season  is  too  short  for  fruit  maturation  and  the  plants  become  biennial. 
Many  garden  annuals  may  be  transformed  into  biennials  by  removing  the  flower 
buds  as  they  appear  ;  in  this  manner  the  mignonette  may  be  transformed  even  into 
a  woody  perennial.  In  monocarpic  species  life  may  be  shortened  by  hastening 
reproduction,  as  well  as  lengthened  by  promoting  vegetative  activity  ;  thus  the  castor 
bean,  a  tropical  perennial,  has  been  transformed  into  an  annual  in  temperate  cli- 
mates, where  the  conditions  facilitate  early  reproduction.  The  closeness  with  which 
death  follows  reproduction  in  monocarpic  species  is  well  illustrated  in  hemp,  a  dioe- 
cious annual;  the  staminate  plants  die  immediately  after  anthesis,  while  the  pistil- 
late plants  live  until  the  fruit  has  matured,  several  weeks  later.  A  number  of  plants 
which  display  vigorous  vegetative  reproduction  (as  the  yam,  the  potato,  and  the 
sweet  potato)  rarely  produce  seeds,  hence  it  has  been  suggested  that  seed  production 
and  vegetative  production  may  be  more  or  less  mutually  exclusive;  however,  there 
are  many  plants  in  which  both  kinds  of  reproduction  are  vigorous  (as  in  the  dahlia, 
the  strawberry,  and  the  willow). 

The  usual  succession  of  events  from  the  inception  of  vegetative  ac- 
tivity to  the  maturation  of  fruit  is  so  familiar  that  it  has  often  been  mis- 
takenly referred  to  as  normal,  thereby  implying  that  any  change  in  the 
order  of  events  is  abnormal.  It  has  been  shown  that  the  order  is  re- 
versible at  almost  any  point.  In  certain  species  of  Veronica,  for  example, 


I 


REPRODUCTION   AND   DISPERSAL 


893 


if  an  inflorescence  is  cut  off  and  allowed  to  strike  root  in  a  moist  chamber 
the  tip  grows  into  a  vegetative  shoot  (figs.  1199,  1200).  The  oldest 
buds  develop  into  the  usual 
flowers,  while  younger  buds  de- 
velop into  cleistogamous  flowers 
without  prominent  corollas;  still 
younger  buds  develop  only  the 
calyx,  and  the  very  youngest 
lateral  buds,  as  well  as  the  ter- 
minal bud,  develop  vegetative 
shoots.  If  a  flowering  shoot  of 
Myriophyllum  helerophyllum  is 
transferred  from  a  pond  to  a 
covered  aquarium,  it  becomes 
transformed  into  a  vegetative 
shoot.  Ajuga  reptans  has  three 
phases,  a  rosette,  a  flowering 
shoot,  and  a  stolon,  and  any  of 
the  three  can  be  induced  at  any 
time  by  supplying  the  requisite  ex- 
ternal conditions.  If  the  flowers 
of  Opuntia  are  removed  from  the  FlGS  I  lg9 ^  200>  _  variation  in  the  flower- 

plant  and  placed  in  the  soil,  they     ing  shoots  of  Veronica  Chamaedrys:   1199,  an 

soon  strike  root  and  give  rise  to    ordinary  flowering  shoot  with  buds,  flowers, 

and  young  fruits;  1200,  a  similar  shoot  that 
was  placed  in  moist  air  at  the  inception  of 
anthesis ;  note  the  metamorphosis  of  the  upper 
part  into  a  leafy  shoot.  —  From  KLEBS. 

which  also  has  three  phases,  comparable  to  those  of  Ajuga;  vegetative 
activity  may  be  made  to  continue  indefinitely,  stolon  formation  may  be 
eliminated,  and  phenomena  unusual  in  nature  (such  as  rosette  for- 
mation at  the  stem  apex,  and  the  transformation  of  the  inflorescence 
into  a  vegetative  shoot)  may  be  induced  at  will. 

Reversibility  of  stages  is  not  confined  to  the  seed  plants;  if  a  fruiting  shoot  of 
Selaginella  lepidophylla  is  cut  off  and  placed  in  the  soil  of  a  moist  hothouse,  it  be- 
comes transformed  into  a  vegetative  shoot,  even  developing  rhizophores.  Reversi- 
bility can  be  induced  also  in  animals,  for  if  a  polyp  of  the  hydroid.  Campanularia, 
is  brought  into  contact  with  a  solid  body,  it  gradually  becomes  undifferentiated  and 
finally  develops  into  a  stolon,  whereas  removal  to  the  original  habitat  soon  results  in 
a  transformation  back  to  a  polyp.  It  may  be  noted  finally  that  reversibility  is  the 
usual  thing  in  the  pineapple,  a  vegetative  shoot  developing  at  the  apex  of  the  fruit. 


f199 


1200 


vegetative  shoots.  Most  striking 
of  all,  perhaps,  are  the  reactions 
of  the  xerophyte,  Sempervivum, 


894  ECOLOGY 

The  above  experimental  evidence  compels  the  abandonment  of  the 
notion  that  only  the  usual  succession  of  events  in  a  plant  in  nature  is 
to  be  considered  normal.  One  thing  alone  is  fixed,  namely,  that  plants 
must  at  the  outset  have  a  period  of  vegetative  activity,  but  whether  this 
continues  through  life,  or  whether  the  reproductive  period  begins  early 
or  late  or  not  at  all,  is  a  matter  that  is  determined  by  external  conditions, 
and  one  series  of  events  is  quite  as  normal  as  another.  It  is  quite  as 
normal  for  a  bulrush  to  live  in  deep  water  and  to  vegetate  indefinitely 
as  to  live  in  shallow  water  and  to  flower  annually.  Indeed  it  is  permis- 
sible to  regard  anything  that  a  plant  ever  does  as  normal,  since  in  every 
case  its  particular  reactions  are  due  to  its  life  conditions.  Thus  it  is 
demonstrated  that  plants  as  a  rule  do  not  possess  an  inherent  rhythm, 
since  external  factors  are  the  dominant  determining  causes  of  reproduc- 
tive periods;  if  plants  flower  regularly,  the  requisite  conditions  may 
be  supposed  to  recur  regularly. 

The  exact  analysis  of  the  reproductive  factors  remains  to  be  deter- 
mined, though  the  data  now  available  are  sufficient  to  show  that  suc- 
cessive stages  imply  successive  causes  (i.e.  changed  conditions),  while 
uniform  phenomena  imply  uniform  or  unchanged  conditions.  Broadly 
speaking,  the  conditions  commonly  termed  hydrophytic  and  mesophytic 
seem  especially  to  favor  the  continuance  of  vegetative  activity,  while 
xerophytic  conditions  favor  reproduction.  Hence  it  is  not  unlikely  that 
the  amount  of  available  water  may  be  a  dominant  specific  factor,  es- 
pecially as  its  presence  in  abundance  is  known  to  be  a  primary  requisite 
for  optimum  vegetative  development. 

Another  important  reproductive  factor  is  light.  Careful  experiments  on  Mimulus 
show  that  light  of  high  intensity  favors  flower  production.  In  the  giant  cactus 
there  is  a  ring  of  flowers  about  the  stem,  and  anthesis  begins  on  the  side  toward  the 
sun.  In  the  teasel  ( Dipsacus)  there  is  a  tendency  for  the  basal  flowers  to  blossom 
first,  but  usually  the  lighting  is  better  toward  the  upper  part  of  the  inflorescence; 
as  a  resultant  of  the  two  factors  concerned,  the  first  flowers  to  appear  usually  are 
those  near  the  middle.  High  temperature  also  favors  flowering.  It  has  been  shown 
that  if  radishes  are  grown  in  concentrated  (10  per  cent)  solutions  of  glucose,  they 
bloom  earlier  than  otherwise;  this  may  explain  why  light  favors  flowering,  since 
it  facilitates  the  production  of  carbohydrates;  also  the  girdling  of  trees,  which  hastens 
flowering,  would  tend  to  cause  the  accumulation  of  carbohydrates  in  the  upper  parts 
of  the  plant.  It  has  been  claimed  that  a  minimal  supply  of  food  salts  (especially 
nitrates  and  phosphates)  at  times  favors  flower  production.  Defoliation  occasioned 
by  storms,  by  insects,  or  by  freezing  sometimes  causes  flower  production,  but  the 
reason  is  not  obvious.  The  influence  of  parasites  upon  flower  production  varies; 
the  black  rot  appears  to  stimulate  autumnal  flowering  in  the  apple  ;  in  other  cases 


REPRODUCTION   AND    DISPERSAL  895 

parasites  cause  flower  primordia  to  develop  into  vegetative  organs,  as  in  the  golden- 
rod  (fig.  1097). 

The  influence  of  external  factors  upon  sex  determination.  —  The  great 
majority  of  plants  appear  to  be  strictly  monoclinous  or  diclinous,  the 
latter  being  for  the  most  part  strictly  monoecious  or  strictly  dioecious. 
As  previously  noted,  however,  there  are  a  number  of  species,  which  vary 
between  monocliny  and  dicliny  or  between  monoecism  and  dioecism. 
It  is  particularly  among  such  plants  that  experimentation  has  been  car- 
ried on  regarding  sex  determination.  When  maize  (Zea  Mays)  is  grown 
under  favorable  vegetative  conditions,  the  plants  commonly  are  monoe- 
cious, but  when  it  is  grown  in  dry,  sterile  soil  or  is  exposed  to  weak  light, 
a  small  unbranched  plant  develops,  which  produces  only  a  staminate 
inflorescence.  Even  under  ordinary  growth  conditions,  the  staminate 
flowers  originate  first,  and  it  has  been  suggested  that  the  pistillate  flow- 
ers come  later  when  the  nutritive  conditions  are  more  favorable.1  When 
the  nutritive  conditions  are  very  favorable,  or  when  the  primordia  are 
parasitized  by  smut,  pistillate  flowers  may  be  induced  in  staminate 
inflorescences.  Most  of  the  catkin-bearing  trees  resemble  maize  in  that 
the  primordia  of  the  staminate  flowers  develop  earlier  than  do  the  pri- 
mordia of  the  pistillate  flowers.  In  Picea  and  in  some  other  conifers, 
pistillate  flowers  occur  only  on  the  more  vigorous  and  better  nourished 
shoots,  while  the  staminate  flowers  occur  on  weaker  shoots.  When 
Satureja  hortensis  is  grown  in  rich  soil  and  is  well  illuminated,  79  per 
cent  of  the  flowers  are  monoclinous,  the  remainder  being  pistillate ; 
in  poor  soil  and  under  poor  illumination  only  13  per  cent  are  monoclinous, 
the  remaining  87  per  cent  being  pistillate  only. 

It  is  a  general  belief  that  in  dioecious  plants  xerophytic  conditions  (or  conditions 
of  food  impoverishment)  facilitate  the  development  of  staminate  plants.  The  ex- 
periments cited  above  favor  this  view,  but  there  certainly  are  other  factors  con- 
cerned. For  example,  the  hemp  (Cannabis  saliva),  which  is  a  representative 
dioecious  plant,  may  grow  in  rich,  alluvial  soil,  where  it  displays  great  vigor,  or  in 
dry  and  sterile  waste  soil,  where  the  plants  are  weak  and  impoverished,  but  in  all 
cases  both  staminate  and  pistillate  plants  are  found,  if  there  is  a  large  number  of 
individuals.  It  has  been  claimed  that  in  spinach  certain  salts  (as  those  of  sodium 
and  calcium)  favor  the  development  of  staminate  plants,  while  other  salts  (as  those 
of  potassium  or  phosphates)  favor  the  development  of  pistillate  plants,  but  this  is 
very  doubtful.  It  has  been  suggested  also  that  culture  solutions  which  have  a  high 
osmotic  pressure  tend  to  favor  the  development  of  an  unusually  large  number  of 

1  The  opposite  condition  is  seen  in  Humultis,  in  which  pistillate  plants  sometimes 
develop  staminate  flowers  late  in  summer. 


896  ECOLOGY 

females.  In  dioecious  species  it  has  been  claimed  that  large  seeds  are  likely  to 
develop  into  pistillate  plants,  and  small  seeds  into  staminate  plants. 

There  may  be  noted  some  interesting  cases  of  correlation,  whose  explanation  is 
not  as  yet  forthcoming.  Immediately  after  flowering  it  often  is  possible  to  distin- 
guish at  some  distance  pistillate  from  staminate  mulberry  trees  by  their  much 
smaller  leaves,  as  though  the  constructive  material  in  the  former  were  utilized 
chiefly  in  fruit  development,  and  in  the  latter,  in  leaf  development.  Similarly, 
in  the  box  elder  the  leaves  on  flowering  branches  often  are  much  smaller  than 
on  vegetative  branches.  Later  in  the  season,  both  in  the  mulberry  and  the  box 
elder,  the  leaves  are  equally  large  on  pistillate,  on  staminate,  and  on  vegetative 
shoots. 

Among  dioecious  perennials  (such  as  the  box  elder,  poplars,  and  willows)  the 
same  individual  usually  bears  the  same  kind  of  gametophytes,  regardless  of  external 
conditions  (even  when  transplanted  as  a  whole  or  in  the  form  of  a  cutting  into  a 
very  different  habitat),  so  that  two  individuals  which  appear  alike  when  not  in  flower 
really  are  different,  the  one  transmitting  male  attributes,  and  the  other  female 
attributes.  The  gametophytes  of  Marchantia,  for  example,  have  been  cultivated 
vegetatively  for  thirty  generations  without  undergoing  any  change  of  sex.  There 
are  on  record,  however,  some  noteworthy  cases  of  change  of  sex  on  the  part  of  in- 
dividual plants.  Perhaps  the  best  authenticated  cases  are  those  in  which  the  sex 
has  been  changed  by  wounding  (traumatism).  The  primordia  of  pistillate  inflores- 
cences of  maize  have  been  subjected  to  torsion  and  thereby  changed  to  staminate 
inflorescences ;  also  staminate  inflorescences  have  been  changed  to  pistillate  in- 
florescences. By  injuring  the  terminal  bud  of  a  staminate  plant  of  Carica  Papaya, 
the  plant  has  been  stimulated  to  produce  pistillate  flowers  which  have  matured  into 
fruits.  Pulicaria  dysenterica  commonly  has  monoclinous  flowers,  but  when  the 
subterranean  organs  are  infested  by  Boris  analis,  an  insect  parasite,  the  species 
becomes  dioecious.  The  pistillate  flowers  of  Lychnis  dioica  have  stamen  primordia 
which  rarely  develop  into  mature  stamens ;  if  these  primordia  are  infested  by  a 
smut  ( Ustilago  violacea),  the  stamens  develop  to  a  considerable  size,  though  they 
contain  spores  of  the  smut  instead  of  pollen  grains.  A  staminate  grape  used  as  a 
stock  for  a  monoclinous  scion  has  been  known  to  become  monoclinous  and  to  mature 
fruit.  In  the  strawberry,  ordinary  vegetative  reproduction  has  been  known  to  be 
accompanied  by  sexual  changes;  in  an  imperfectly  dioecious  variety  with  mono- 
clinous and  pistillate  individuals,  the  vegetative  progeny  of  each  kind  of  individual 
has  been  known  to  develop  into  the  other.  A  very  remarkable  change  without 
change  of  conditions  has  been  reported  for  Aucuba  japonica,  in  which  a  plant 
that  had  been  staminate  for  some  years  became  monoecious  and  finally  mono- 
clinous. Similar  changes  have  been  reported  for  the  lower  plants,  particularly 
for  Vaucheria,  in  which  female  branches  have  been  transformed  into  bisexual 
branches. 

Recent  experimentation  has  resulted  in  a  material  change  of  view  regarding  the 
significance  of  the  influence  of  external  conditions  upon  sexual  development  and 
upon  the  change  of  sex,  as  noted  in  the  preceding  paragraphs.  It  is  now  generally 
believed  that  in  most  plants  the  sex  of  an  individual  is  not  due  to  the  external  con- 
ditions to  which  the  individual  itself  may  be  subjected,  but  that  sex  is  determined 
much  earlier  than  had  been  supposed.  In  the  liverwort,  Sphaerocarpus,  sex  is 


REPRODUCTION   AND   DISPERSAL  897 

determined  at  the  time  of  spore  formation,  since  two  spores  in  each  tetrad  give  rise 
to  male  plants  and  two  spores  to  female  plants.  In  heterosporous  pteridophytes 
and  in  seed  plants,  the  sex  of  the  gametophyte  is  determined  long  before  spore  forma- 
tion, since  it  depends  upon  the  kind  of  sporangium  that  is  produced  by  the  preceding 
sporophyte.  On  the  other  hand,  in  homosporous  pteridophytes  and  in  monoecious 
mosses,  sex  determination  appears  to  come  much  later  than  spore  formation,  and 
to  depend  in  part,  at  least,  upon  the  conditions  to  which  the  gametophyte  is  exposed. 
A  remarkable  situation  has  been  disclosed  in  the  dioecious  mosses,  in  which  sex  is 
determined  at  spore  formation,  half  of  the  spores  giving  rise  to  male  plants  and  half 
to  female  plants,  as  in  the  liverwort,  Sphaerocarpus.  Pieces  of  the  sporophyte  may 
give  rise  vegetatively  to  gametophytes,  and  such  gametophytes  are  bisexual,  whereas 
gametophytes  that  develop  from  spores  are  unisexual.  Hence  it  appears  that  the 
supposedly  non-sexual  sporophyte  is  in  reality  bisexual. 

In  the  seed  plants  it  seems  probable  that  the  sex  of  the  gametophyte  is  determined 
far  back  in  the  history  of  the  preceding  sporophyte,  at  least  as  far  back  as  the  seed. 
In  this  event,  ordinary  sporophytes  are  as  characteristically  sexual  as  are  the  gameto- 
phytes to  which  they  give  rise,  so  that  it  is  proper  to  call  a  staminate  plant  male  and 
a  pistillate  plant  female.  There  is  some  evidence  in  favor  of  the  view  that  in  the 
seed  plants  the  sex  of  the  gametophyte  is  determined  farther  back  than  the  seed, 
perhaps  as  far  back  as  the  gametes  of  the  preceding  gametophyte,  or  even  as  far  back 
as  the  spores  from  which  the  latter  gametophytes  arise.  In  Bryonia  dioica  and  in 
Cannabis  saliva,  experiments  seem  to  show  that  the  eggs  have  a  female  potentiality 
and  that  half  of  the  sperms  have  a  male  potentiality  and  half  of  them  a  female 
potentiality.  In  case  a  sperm  with  a  male  potentiality  fuses  with  an  egg,  the  de- 
veloping sporophyte  is  male,  because  the  sperm  dominates  over  the  egg.  If  a  sperm 
with  a  female  potentiality  fuses  with  an  egg,  the  resulting  sporophyte  is  female. 
An  alternative  hypothesis  postulates  that  sperms  have  either  strong  male  potential- 
ities or  weak  male  potentialities,  the  former  dominating  over  the  egg,  and  the  latter 
being  subordinate  to  the  egg.  In  any  event  it  would  seem  that  in  dioecious  plants 
the  sex  of  a  given  sporophyte  or  of  the  subsequent  gametophyte  depends  upon  the 
sexual  potentiality  of  the  preceding  sperm  or  of  the  still  more  antecedent  pollen  grain. 
In  all  of  these  phenomena,  external  factors  seem  to  have  no  part,  unless,  perhaps, 
they  affect  in  some  unknown  way  the  sex  tendency  of  pollen  grains  ;  in  any  case  it 
seems  clear  that  external  factors  operating  upon  a  sporophyte  can  have  no  influence 
upon  the  sex  of  the  subsequent  gametophyte.  Supplementary  evidence  in  favor 
of  the  female  potentiality  of  the  egg  is  afforded  by  the  fact  that  in  Chara  crinlta 
and  in  Antennaria,  eggs  which  develop  parthenogenetically  always  give  rise  to 
female  plants:  Further  data  are  afforded  also  by  Mercurialis  annua,  a  dioecious 
species  whose  pistillate  plants  bear  occasional  staminate  flowers ;  in  the  event  of 
geitonogamy,  these  plants  nearly  always  have  female  progeny;  conversely  the  occa- 
sional female  flowers  of  male  plants  with  geitonogamous  pollination  have  male 
progeny. 

In  animals,  as  in  plants,  sex  determination  appears  to  be  unrelated  to  obvious 
external  factors,  the  sex  potentiality  of  the  gametes  being  predetermined.  The  exact 
factors  involved  in  such  predetermination  are  unknown,  but  it  has  been  suggested 
that  in  those  animals  in  which  the  female  possesses  one  more  chromosome  than  does 
the  male,  the  extra  chromosome  may  be  the  sex  determinant.  Formerly  it  was 


898  ECOLOGY 

believed  that  external  factors  act  as  sex  determinants  in  daphnids,  grape  lice,  aphids, 
and  rotifers,  but  it  is  now  realized  that  such  factors  determine  only  whether  a 
given  generation  is  to  be  sexual  or  parthenogenetic  (p.  881) ;  in  the  case  of  the  sexual 
generation,  the  maleness  or  the  femaleness  of  the  different  individuals  is  predeter- 
mined. In  some  cases  at  least,  the  sex  of  the  progeny  is  determined  before  egg 
formation  and  possibly  as  a  result  of  external  factors ;  in  one  of  the  rotifers  (Hyda- 
tina),  the  better  nourished  females  lay  large  eggs,  which  develop  parthenogeneti- 
cally  into  females,  and  the  more  poorly  nourished  females  lay  small  eggs,  which 
develop  parthenogenetically  into  males.  That  the  sex  potentialities  of  animal 
gametes  may  differ  from  those  of  plant  gametes  is  shown  by  the  fact  that  in  most 
cases,  eggs  which  develop  parthenogenetically  grow  into  male  animals  (as  in  ants, 
bees,  and  wasps) ;  in  those  cases  in  which  certain  eggs  develop  parthenogenetically 
into  females  (as  in  rotifers  and  grape  lice),  there  are  other  and  smaller  eggs,  which 
develop  parthenogenetically  into  males.  In  bees,  in  rotifers,  and  in  grape  lice,  fer- 
tilized eggs  develop  with  equal  certainty  into  females. 

If  it  is  to  be  concluded  from  the  above  data  that  sexuality  but  not  sex  is  deter- 
mined by  external  conditions,  some  further  explanation  is  needed  to  account  for 
the  change  in  sex  noted  above  for  such  plants  as  Zea,  Carica,  Pulicaria,  and 
Lychnis.  These  cases  seem  best  explained  by  assuming  that  all  of  these  forms  are 
potentially  bisexual  and  that  external  factors  either  may  cause  the  suppression  of 
one  of  the  sexes  (as  in  Zea  and  Pulicaria,  and  also  in  most  homosporous  ferns)  or 
may  stimulate  to  development  a  sex  that  commonly  is  suppressed  (as  in  Carica  and 
Lychnis,  and  also  in  Onoclea) ,  in  the  latter  case  acting  as  releasing  stimuli.  This 
view  is  supported  by  the  fact  that  Carica  and  Equisetum  are  known  sometimes  to 
be  monoecious,  and  also  by  the  fact  that  the  staminate  and  pistillate  flowers  of 
Piper  Betel  may  under  proper  conditions  become  monoclinous.  To  what  extent 
other  supposedly  dioecious  species  are  thus  potentially  bisexual  is  unknown ;  it  may 
be  noted  that  even  the  willows,  which  commonly  are  thought  to  be  strictly  dioecious, 
occasionally  have  monoecious  individuals  and,  still  more  rarely,  monoclinous  flowers. 

Variations  in  flower  color.  —  The  most  variable  character  of  flowers 
is  that  of  color.  Many  cases  of  color  variation  in  flowers  of  the  same 
species  clearly  are  due  to  external  factors,  particularly  in  those  flowers 
in  which  colors  are  due  to  anthocyan  *;  such  variations  may  be  quanti- 
tative, involving  differences  in  intensity  only,  or  they  may  be  qualitative, 
involving  differences  in  wave  length.  Light  seems  to  be  the  most  im- 
portant factor  determining  variations  in  color  intensity.  It  was  dis- 
covered long  ago  that  when  bulbs  (as  in  the  tulip  or  hyacinth)  are  grown 
in  the  dark,  they  develop  colored  flowers  much  as  in  the  light,  though 
the  color  intensity  is  less,  sometimes  being  much  less,  as  in  blue  hya- 

1  However,  there  are  some  striking  cases  of  color  variation  in  flowers  whose  color  is 
due  to  chromoplasts,  as  in  Tropaeolum  and  in  Castilleja  coccinea  ;  the  latter  is  more  likely 
to  have  scarlet  flowers  in  rich  soil,  where  the  plants  are  vigorous,  and  lemon-yellow 
flowers  in  peaty  soil,  where  the  plants  are  impoverished. 


REPRODUCTION   AND   DISPERSAL  899 

cinths.  Coloration  takes  place  in  the  dark  in  some  non-bulbous  plants, 
such  as  Lychnis,  Hydrangea,  and  Papaver.  In  striking  contrast  to 
bulbous  plants  are  Antirrhinum  and  Prunella,  where  the  food  necessary 
for  anthesis  does  not  accumulate  during  the  previous  season,  but  is 
manufactured  just  before  the  period  of  floral  development.  In  such 
plants  the  flowers  do  not  become  fully  colored  when  the  entire  plant  is 
grown  in  the  dark,  as  in  the  tulip,  although  they  become  colored  if  the 
vegetative  shoots  are  grown  in  the  light  and  the  floral  shoots  in  the  dark. 
Even  tulip  flowers  do  not  become  colored  in  the  dark  unless  the  previous 
leaf  generation  is  exposed  to  sunlight.  Thus  the  influence  of  light  upon 
color  intensity  appears  to  be  in  part  direct,  as  is  indicated  by  the  deep 
shades  of  hyacinth  flowers  that  are  grown  in  the  sunlight  and  by  the 
high  intensity  of  color  of  alpine  flowers.  However,  to  an  equal  or  greater 
extent  the  light  influence  is  indirect,  as  is  well  shown  by  those  flowers 
that  become  colored  only  when  the  leaves  are  in  the  light.  The  color 
in  this  case  and  in  the  dark  cultures  of  bulbous  plants  seems  to  be  asso- 
ciated with  a  rich  food  supply,  a  fact  which  is  quite  in  harmony  with  the 
sugar  theory  of  anthocyan  formation.  Probably  in  the  majority  of  flow- 
ers, direct  exposure  to  sunlight  is  required  to  produce  full  coloration, 
although  a  certain  amount  of  pigmentation  occurs  in  darkness.  Yellow 
colors  are  much  less  weakened  by  darkness  than  are  the  anthocyan 
colors.  Heat  as  well  as  light  affects  coloration,  the  intensity  often  being 
heightened  at  low  temperatures. 

Variations  in  the  quality  or  kind  of  color  are  much  less  understood 
than  are  variations  in  color  intensity,  though  it  is  known  that  the  cell 
sap  of  red  anthocyan  flowers  is  more  acid  than  is  the  cell  sap  of  blue 
anthocyan  flowers;  hence  it  is  to  be  supposed  that  factors  which  cause 
variations  in  the  acidity  of  the  cell  sap  cause  variations  in  color  also. 
The  flowers  of  Hydrangea  hortensis,  which  usually  are  red,  become  blue 
when  the  plants  are  grown  in  soil  containing  a  considerable  amount  of 
the  sulfates  or  of  other  salts  of  aluminum  and  potassium.  Aluminum 
salts  frequently  change  lilac-colored  flowers  to  blue,  whereas  potassium 
salts  may  change  them  to  green.  Acids,  on  the  other  hand,  frequently 
change  flower  colors  to  red.  White  roses  have  been  changed  to  red  by 
adding  potassium  salts  to  the  soil.  Heat  affects  the  quality  as  well  as 
the  intensity  of  flower  color,  low  temperatures,  for  example,  sometimes 
causing  white  geraniums  to  become  red  or  rose  geraniums  to  become 
carmine.  The  flowers  of  harebells  and  morning-glories  also  vary  with 
the  temperature  in  respect  to  color  quality  (see  also  p.  845). 


9oo 


ECOLOGY 


There  are  some  instances  where  color  variation  may  not  be  due  to  external 
factors.  Hepatica  plants,  in  apparently  similar  conditions,  exhibit  various  colors 
from  pink  to  blue.  Perhaps  the  most  probable  instance  of  "  inherent  "  color  char- 
acters is  in  the  albinos,  which  seem  to  have  white  {i.e.  unpigmented)  flowers  in  any 
habitat;  such  albinos  are  known  in  many  plants  (as  in  Lupinusand  Sisynnchium), 
and  in  some  cases  there  are  comparable  variegated  flowers  (as  in  Viola  cucullata) . 
Albinos  commonly  are  regarded  as  sports  or  mutants,  but  the  possibility  of  external 
determining  factors  even  here  is  suggested  by  the  reported  pigmentation  of  Trillium 
albinos  that  have  been  transplanted  to  a  new  habitat. 

Variations  in  the  size  and  number  of  floral  organs.  —  When  plants 
are  grown  in  very  poor  nutritive  conditions,  the  number  of  flowers  on 
each  individual  is  much  reduced,  and  sometimes  (as  in  the  poppy)  the 
size  of  the  flower  also  is  reduced.  Weakened  illumination  may  cause 
a  decrease  in  flower  size,  particularly  in  the  size  of  the  corolla  (as  in 
Mimulus) ;  by  contrast  it  is  to  be  noted  that  in  xerophytic  alpine  habi- 
tats, in  spite  of  the  marked  reduction  of  other  organs,  there  is  no  marked 
reduction  in  flower  size,  probably  because  of  the  intense  illumination 
(figs.  1051,  1052).  In  poorly  nourished  specimens  of  Agrimonia,  the 
stamen  number  may  be  reduced  from  about  twenty  to  five.  When  the 
poppy  is  grown  in  dense  cultures,  the  number  may  be  reduced  from 
thirty  or  forty  to  six;  this  result  is  most  significant,  since  in  this  group 
the  large  number  of  stamens  is  an  important  taxonomic  character. 
Equally  significant  is  the  carpel  variation  in  the  poppy;  in  well-nourished 
individuals  there  may  be  one  hundred  and  fifty  carpels,  but  in  poorly 
nourished  individuals  the  number  may  be  reduced  to  four.  In  Chrysan- 
themum and  in  some  other  composites,  the  number  of  ray  flowers  varies 
with  the  nutrition,  well-nourished  plants  having  the  largest  number  of 
such  flowers.  A  remarkable  situation  is  presented  in  Sempervivum, 
in  which  there  have  been  produced  all  gradations  between  flowers  and 
vegetative  shoots;  some  flowers  lack  corollas,  others  lack  stamens  and 
pistils,  and  even  the  calyx,  which  usually  is  the  most  certain  of  develop- 
ment of  floral  organs,  sometimes  is  absent,  in  which  event  the  bracts 
alone  represent  the  floral  organs.  In  this  genus  also  it  is  possible 
to  induce  the  transformation  of  stamen  primordia  into  carpels,  or 
of  carpel  primordia  into  stamens.  Similar  results  have  been  obtained 
in  Veronica  (p.  892). 

Variations  in  flower  form.  —  Perhaps  the  most  significant  of  all  the 
variations  in  reproductive  structures  are  those  involving  changes  in  form, 
since  they  have  to  do  with  the  very  fundamentals  of  classification,  and 
therefore  are  likely  to  shed  important  light  upon  the  processes  of  evolu- 


REPRODUCTION   AND   DISPERSAL  901 

tion.  It  was  shown  long  ago  that  various  zygomorphic  flowers  owe  to 
gravity  their  peculiar  shape,  since  they  become  actinomorphic  when  its 
influence  is  equalized.  In  Mimulus,  zygomorphy  is  reduced  in  weak 
illumination.  Floral  symmetry  has  been  modified  also  by  cutting  con- 
ductive strands  that  lead  to  the  flowers  and  by  otherwise  changing  the 
nutritive  conditions.  Of  particular  interest  are  the  experimental  data 
on  cleistogamy,  which  involves  marked  change  of  form  and  structure, 
especially  in  the  corolla.  In  Stellaria,  light  is  required  for  the  opening 
of  the  flowers,  and  in  Linaria,  flowers  that  usually  are  chasmogamous 
in  weak  light  are  cleistogamous.  In  Lamium  the  vernal  and  autumnal 
but  not  the  estival  flowers  are  cleistogamous.  In  Impatiens  noli-tangere 
the  first  flowers  usually  are  cleistogamous  and  spurless,  while  the  later 
ones  are  chasmogamous  and  spurred;  but  if  such  a  plant  is  transferred 
to  sterile  sandy  soil,  only  cleistogamous  flowers  are  produced,  indicating 
that  poor  nutrition  favors  cleistogamy.  Chasmogamous  flowers  may 
be  produced  by  Stellaria  even  in  weak  light,  if  the  plants  are  supplied 
with  glucose.  Parasites  may  induce  cleistogamy;  mildewed  plants  of 
Impatiens  produce  only  closed  flowers,  and  Biscutella  produces  such 
flowers,  when  the  plants  are  attacked  by  cecidomyid  insects.  In  Viola 
mirabilis  the  primordia  of  chasmogamous  flowers  develop  into  closed 
flowers  in  extreme  conditions,  as  in  dry,  sterile  soil  and  in  a  warm, 
humid  greenhouse,  and  in  V.  odorata  the  primordia  of  the  cleistogamous 
flowers  develop  into  showy  open  flowers  in  dryish,  sunny  habitats. 

A  most  interesting  floral  modification  is  that  seen  in  the  so-called 
double  flowers  (figs.  1201,  1202).  Where  the  phenomenon  is  one  of 
the  replacement  of  other  floral  organs  (especially  stamens  and  pistils) 
by  petals,  it  may  be  denominated  petalody  or  petalization.1  There 
are  varying  degrees  of  petalody;  for  example,  the  buttercups,  which 
commonly  have  five  petals,  may  have  the  number  doubled  or  otherwise 
increased  even  to  complete  petalization.  In  the  white  water  lily  (Cas- 
talia),  in  which  there  are  many  petals  disposed  in  several  rows,  the  inner 
members  become  smaller  and  narrower,  and  show  all  transitions  to  sta- 
mens. Passing  outward  from  the  center,  the  stamen  filaments  become 
broader  and  more  petaloid,  while  the  anthers  gradually  become  effaced, 
suggesting  the  possible  origin  of  petals  from  stamens  or  of  stamens  from 
petals;  the  first  theory  is  the  more  reasonable,  but  there  is  no  valid 

1  In  the  composites,  however,  doubling  is  due  to  the  replacement  not  of  stamens  by 
petals,  but  of  disk  flowers  by  ligulate  flowers  (as  in  double  sunflowers  and  chrysanthe- 
mums), so  that  one  should  speak  of  double  heads  rather  than  of  double  flowers. 


902 


ECOLOGY 


evidence  for  either.  The  exact  cause  of  petalization  is  unknown,  but 
in  many  cases  it  appears  to  be  inherent,  double  flowers  usually  being 
regarded  as  sports  or  mutants,  since  they  often  may  be  reproduced  by 
seed  as  well  as  by  cuttings.1  In  other  cases,  petalization  clearly  is  due 
to  external  factors,  notably  in  a  number  of  species  in  which  plants  whose 
roots  are  infested  with  certain  parasitic  fungi  (as  Heterodera  radicicola) 
develop  double  flowers.  Saponaria  sometimes  has  double  flowers 
when  the  roots  are  infested  with  Fusarium.  In  the  tulip,  petalody 

is  facilitated  by 
good  nutrition, 
especially  if  there 
is  an  abundance 
of  nitrogenous  sub- 
stances in  the  soil. 
In  some  cases 
parasites  cause 
not  only  ordinary 
doubling,  but  also 
the  development 
of  green  foliage 
leaves  in  place  of 
floral  organs,  the 
phenomenon  being 
known  as  sepal- 
ody  or  greening. 
In  parasitized  in- 
dividuals of  Heli- 
anthus  strumosus, 
greening  is  a  common  phenomenon,  and  not  infrequently  green  foliage 
leaves  are  intermingled  with  ligulate  flowers  in  place  of  the  usual  disk 
flowers. 

One  of  the  most  remarkable  of  all  reproductive  variations  is  that  in 
which  flowers  are  replaced  by  bulbils,  as  in  the  wild  garlic  (Allium  cana- 
dense,  fig.  1203),  in  whose  umbels  some  primordia  develop  into  flowers  and 
others  into  bulbils.  Sometimes  most  or  even  all  of  the  primordia  develop 

1  Obviously,  completely  petalized  flowers  can  be  reproduced  only  by  cuttings.  In  the 
double  petunia,  which  usually  is  propagated  from  seed,  seeds  are  saved  from  flowers  that 
are  almost  double,  and  only  20  to  30  per  cent  of  the  progeny  have  double  flowers.  In  the 
composites  complete  doubling  does  not  necessarily  prevent  seed  production,  double  asters, 
-daisies,  and  sunflowers  being  raised  regularly  from  seed. 


1201 


FIGS.  1201,  1202.  —  Flowers  of  Narcissus,  illustrating 
petalody:  1201,  an  ordinary  single  flower  with  a  six-parted 
perianth  (p)  and  a  crown  or  corona  (c);  1202,  a  double  flower, 
in  which  there  is  a  considerable  increase  of  the  petaloid  parts 
at  the  expense  of  the  stamens  and  carpels. 


REPRODUCTION   AND    DISPERSAL 


903 


into  flowers,  and  again  nearly  all  of  the  primordia  may  develop  into  bul- 
bils. Usually  the  bulbils  germinate  while  still  attached  to  the  inflores- 
cence, but  they  are  readily  detachable,  and  they  continue  growing  if 
they  fall  into  favorable  situations.  Similar  bulbils  occur  somewhat 
regularly  in  certain  alpine  plants,  such  as  Polygonum  viviparum,  Poa 
alpina,  and  Saxifraga.  The  cause  of  such  bulbil  formation  is  not  clearly 
known,  though  in  the  grasses,  seeds 
rather  than  bulbils  appear  to  be  pro- 
duced, when  the  plants  are  grown  in 
dry  conditions  or  in  media  that  are  poor 
in  nitrogen.  That  bulbil  formation  in 
alpine  plants  probably  is  a  reaction  to 
alpine  conditions  is  indicated  by  the 
fact  that  Gagea  plants  taken  from  low- 
land to  alpine  habitats  produce  bulbils 
rather  than  flowers  in  the  first  season 
of  culture.  In  the  onion  this  habit 
affords  the  plant  the  apparent  advantage 
of  an  additional  method  of  reproduc- 
tion, supplementing  reproduction  by 
seeds  and  by  subterranean  bulbs;  in  FlG  I203._An  umbel  of  the 

alpine   plants    bulbil    formation    may   be     wild  garlic  (Allium  canadense),  in 


more    certain    than    seed     maturation, 
owing  to  the  shortness  of  the  season. 


which  there  are  only  a  few  flowers 
(/),  their  place  being  taken  largely 
by  bulbils  (6),  which  readily  propa- 
gate the  species  vegetatively;  p, 
perianth  segments;  s,  spathe. 


The  factors  determining  the  development 
and  form  of  the  organs  of  vegetative  repro- 
duction have  been  sufficiently  considered  elsewhere.  It  may  be  noted  merely 
that  in  specialized  organs,  such  as  tubers  and  bulbs,  xerophytic  conditions  may 
favor  development,  much  as  in  other  kinds  of  reproductive  organs.  For  ordinary- 
vegetative  reproduction,  however,  particularly  where  it  is  indistinguishable  from 
the  usual  phenomena  of  growth,  mesophytic  or  hydrophytic  conditions  are  more 
favorable. 

In  conclusion,  it  is  now  clear  that  external  factors  play  an  important 
part  in  determining  the  variation  of  reproductive  organs.  How  great 
this  part  may  be,  and  what  the  precise  external  factors  -are,  remain  as 
yet  in  large  part  unknown.  It  cannot  suffice  to  explain  phenomena  by 
such  terms  as  "  bad  nutrition  "  or  "  xerophytic  conditions,"  though  to 
attribute  them  to  such  a  cause-complex  is  vastly  more  satisfactory  than 
to  refer  them  to  inherent  or  internal  causes. 


904  ECOLOGY 

Hybridization.  —  When  related  forms,  not  of  the  same  species  or 
variety,  are  crossed,  the  process  is  known  as  hybridization  and  the  prog- 
eny are  known  as  hybrids.  Plants  differ  widely  as  to  their  hybridizing 
power.  Most  cultivated  races  derived  from  a  common  specific  ancestor 
hybridize  readily;  some  closely  related  species  also  hybridize  readily,  as 
among  the  oaks  and  willows,  while  others  do  not,  as  the  apple  and  the 
pear,  or  the  tomato  and  the  nightshade.  Occasionally  different  genera 
hybridize  with  each  other,  as  Brassica  and  Raphanus.  The  notion  is 
rather  prevalent  that  hybrids  generally  are  sterile,  but  among  plants 
comparatively  few  sterile  hybrids  are  known,  one  of  the  most  probable 
cases  being  that  of  the  horse  radish  (Cochlearia  Armoracia).  Probably 
the  greatest  tendency  to  sterility  is  among  forms  that  are  very  closely 
related  (as  in  the  case  of  close  pollination)  or  in  forms  whose  relation- 
ship is  so  remote  that  crossing  is  difficult  of  accomplishment.  Fre- 
quently hybrids  are  intermediate  between  their  parents,  showing  a 
blending  of  characters,  but  there  may  be  all  degrees  of  likeness  to  one 
parent  or  the  other.  Sometimes,  however,  entirely  new  characters  are 
introduced.  In  recent  years  the  study  of  hybrids  has  assumed  unusual 
proportions  through  the  rediscovery  of  "  Mendel's  law  "  (see  p.  292). 

Bud  variation.  —  Occasionally  plants  have  been  known  to  give  rise 
to  a  branch  that  is  different  from  the  others,  and  whose  progeny  resem- 
bles this  branch  rather  than  the  rest  of  the  plant.  Vegetative  mutations 
of  this  sort  are  known  as  bud  variations.  Among  bud  variations  are: 
witches'  brooms  on  spruce  and  other  trees,  whose  seeds  give  rise  to 
bushy  shrubs  rather  than  to  ordinary  trees;  white  shoots  on  bean 
plants,  whose  progeny  also  is  white;  and  yellow-fruited  branches  on 
red  tomato  plants.  The  nectarine  is  thought  to  have  arisen  as  a  bud 
variation  of  the  peach.  Most  cases  of  bud  variation  seem  to  involve 
the  loss  of  a  character  (such  as  greenness  in  the  bean,  or  hairiness  in 
the  nectarine  fruit),  or  the  change  of  color  (as  in  the  tomato).  The 
factors  involved  in  bud  variation  are  quite  unknown. 

4.     FRUITS  AND   SEEDS 

The  nature  and  role  of  fruits.  —  In  the  seed  plants  the  fusion  of  the 
sperm  and  the  egg  is  followed  usually  by  certain  changes  in  the  floral 
organs,  the  most  conspicuous  of  which  is  the  enlargement  of  the  ovary, 
as  the  ovules  develop  into  seeds.  Of  the  other  floral  organs,  the  corolla 
and  the  stamens  soon  disappear,  but  the  calyx  and  the  receptacle  often 
enlarge  with  the  ovary.  The  structure  thus  arising  from  the  flower, 


REPRODUCTION   AND   DISPERSAL 


90S 


through  the  enlargement  of  the  ovary  with  or  without  the  modification 
of  other  organs,  is  called  the  fruit,  and  obviously  its  chief  role  is  the 
protection  of  the  developing  seeds.  Most  young  fruits  contain  sufficient 
chlorophyll  to  make  them  conspicuously  green,  and  they  doubtless  manu- 
facture much  of  the  food  utilized  in  their  development.  Sometimes,  as 
in  the  elm,  green  fruits  are  prominent  before  the  leaves  appear,  or,  as 
in  Cakile,  after  the  leaves  have  gone;  in  such  cases  the  food-making 
role  would  seem  particularly  significant.  At  maturation  the  fruit  by 
reason  of  its  edibility  and  showiness  often  is  a  means  of  attracting  seed- 
dispersing  animals,  and  in  other  ways  it  is  connected  with  the  process  of 
dissemination. 

General  characteristics  of  seeds.  —  Morphology.  —  The  seed,  which  is 
an  enlarged  and  matured  ovule,  is  one  of  the  most  complex  organs  found 
in  plants.  The  ovule,  which  is  a  sporophytic  organ,  serves  as  a  domi- 
cile for  the  female  gametophyte,  and  afterwards  for  the  new  sporophyte 
that  develops  from  the  fusion  of  the  sperm  and  the  egg.  This  structure, 
which  thus  is  made  up  of  elements  from  three  generations,  grows  to  a 
certain  size,  varying  with  the  species,  whereupon  the  integument  de- 
velops into  the  seed  coat  or  testa,  and  the  growth  of  the  sporophyte  within 
is  checked.  Henceforth  the  structure  may  be  called  a  seed  rather  than 
an  ovule,  so  that  a  seed  may  be  defined  as  a  young  sporophyte  in  a  state 
of  arrested  development,  enclosed  within  a  modified  ovule  integument, 
the  testa ;  in  addition,  the  gametophytic  generation  may  or  may  not  be 
represented  by  the  endosperm  (but  see  p.  270). 

Developmental  features.  —  The  ovules  commonly  arise  on  the  margins 
of  the  carpels,  to  which  they  are  attached  by  a  stalk,  the  funiculus.  The 
central  mass  of  tissue, 

the  nucellus,  usually  is  XffiK  „  /^ffl\  / 1\  I2°5 
enclosed  by  one  or  two 
integuments  arising  from 
the  basal  region  (chalaza) 
just  above  the  funiculus; 
the  integuments  do  not 
close  tightly ,  about  the 
nucellus,  but  leave  a 
slender  canal,  the  mi- 
cropyle,  through  which 
the  pollen  tube  usually  makes  its  way.  The  ovules  may  be  erect  on  the 
funiculus  (orthotropous),  partially  pendent  (campylotropous),  or  more 


1204 


FIGS.  1204,  1205.  —  1204,  cross  and  longitudinal 
sections  of  a  seed  of  Canna,  showing  the  seed  coat  or 
testa  (/),  the  perisperm  (e),  and  the  embryo  (/>);  1205, 
longitudinal  and  cross  sections  of  a  bean  (Phaseolus), 
showing  the  seed  coat  or  testa  (*),.  the  cotyledons  (c). 
and  the  plumule  (/>). 


906  ECOLOGY 

commonly  completely  pendent  (anatropous) ;  in  the  last  case  the  close 
application  of  the  funiculus  to  the  integument  causes  a  suture,  the 
raphe  (figs.  582-584). 1  The  young  sporophyte  or  embryo  at  first  grows 
vigorously,  usually  becoming  differentiated  at  seed  maturity  into  the 
embryo  root  (radicle),  the  embryo  stem  (hypocotyF),  one,  two,  or  more 
seed-leaves  (cotyledons),  and  the  embryo  shoot  (plumule)?1  The  seed 
also  contains  foods  that  are  utilized  by  the  young  sporophyte  during  its 
second  phase  of  activity,  commonly  called  germination?  These  foods 
may  accumulate  within  the  cotyledons  (as  in  peas  and  beans,  fig.  1205), 
which  in  that  event  occupy  most  of  the  space  within  the  testa,  or  they 
may  accumulate  in  a  tissue  surrounding  the  cotyledons  (as  in  most 
monocotyls,  fig.  1204),  this  tissue  being  called  endosperm  if  arising 
within  the  embryo  sac,  and  perisperm  if  arising  from  the  nucellus. 
Most  seeds  mature  in  the  season  of  anthesis.  Some  plants  with  autumnal 
flowers,  such  as  Hamamelis  and  Colchicum,  mature  seeds  the  following 
season,  and  in  some  plants  with  vernal  flowers,  such  as  the  pines 
and  certain  oaks,  maturation  comes  in  the  second  season. 

The  role  of  seeds.  —  Primarily  seeds  are  disseminules,  and  many  of 
their  chief  structural  features  are  associated  with  dispersal.  Of  almost 
equal  importance  in  many  plants,  especially  in  annuals  and  biennials,  is 
their  protective  role,  since  in  no  other  form  is  the  seed  plant  so  immune 
to  danger  as  in  the  seed.  Though  they  are  often  so  regarded,  seeds  are 
in  no  sense  reproductive  organs.  The  reproduction  of  which  the  seed 
is  the  result,  takes  place  previously  within  the  flower,  while  the  seed  rep- 
resents in  a  state  of  arrested  development  the  protected  offspring  of  that 
reproduction.  Thus  the  unique  feature  of  the  seed  plants  is  the  sepa- 
ration of  reproduction  from  protection  and  dispersal;  post-reproductive 
disseminules,  the  seeds,  take  the  place  of  reproductive  disseminules,  the 
asexual  spores. 

The  protective  structures  and  relations  of  seeds.  —  The  protection  of 
developing  seeds.  —  Developing  seeds  are  protected  from  transpiration 
and  from  other  dangers  by  the  ovary  wall,  which  thickens  and  hardens 
into  the  fruit  wall  or  pericarp.  It  has  been  thought  that  grazing  ani- 
mals might  eat  the  young  fruits,  so  that  the  sourness,  bitterness,  or  hard- 
ness of  fruits  that  later  become  edible  have  been  regarded  as  advantageous 
in  protecting  them  from  such  dangers.  In  some  cases,  as  in  the  jimson 

1  Most  of  these  terms  apply  also  to  seeds. 

2  In  the  orchids  and  in  some  parasites  the  embryo  remains  undifferentiated. 
8  Little  or  no  food  is  found  in  minute  seeds,  as  in  those  of  the  orchids. 


REPRODUCTION   AND   DISPERSAL  907 

weed  (Datura),  the  chestnut,  and  certain  gooseberries  (as  Ribes  Cynos- 
bati),  the  fruits  are  spinescent. 

The  prickly  pear  (Opuntia)  is  especially  interesting  from  this  viewpoint,  since 
the  unpleasant  bristles  of  the  young  fruits  fall  off  as  the  fruit  ripens,  from  which  it 
has  been  inferred  that  the  young  fruit  is  protected  from  the  fruit-eating  animals 
which  later  scatter  the  ripe  seeds.  Such  views  are  misleading  in  their  implications, 
since  most  young  fruits  are  not  especially  attractive  to  animals.  Their  unpalata- 
bility  is  a  sign  of  immaturity  rather  than  of  protection. 

The  protective  structures  of  mature  seeds.  —  Seeds  as  a  class  are  the 
most  xerophytic  of  plant  structures,  since  not  alone  in  xerophytes,  but 
also  in  mesophytes  and  even  in  hydrophytes,  they  generally  are  covered 
with  hard  and  impermeable  coats.  So  universal  is  the  xerophytism  of 
the  seed  that  usually  it  is  impossible  to  determine  from  its  structure  the 
habitat  in  which  it  grew.  This  xerophytism  consists  in  three  features: 
the  thick  and  impermeable  coat,  the  compactness  of  the  tissues  within 
the  testa,  and  the  small  amount  of  water.  The  testa,  or  seed  coat,  com- 
monly is  single,  being  derived  from  the  ovule  integument  (from  the  outer 
integument,  in  case  there  are  two)  through  thickening,  hardening,  and 
other  modification.  In  some  seeds  there  is  a  second  coat  within  the 
testa,  and  in  others  there  is  a  structure  outside  the  testa,  which  is  known 
as  an  aril  (e.g.  in  the  water  lily) .  The  testa  at  maturity  usually  is  hard 
and  bony,  being  composed  of  several  or  more  layers  of  cells  with  greatly 
thickened  walls;  :in  the  hickory  nut  it  is  made  up  of  a  number  of  layers 
of  sclereids.  Sometimes  the  testa  is  so  hard  that  it  is  difficult  to  cut  it 
with  a  knife,  as  in  Gymnocladus.  In  most  one-seeded  fruits,  such  as  the 
grains  of  cereals  (fig.  1211)  and  the  achenes  of  the  composites,  the  fruit 
wall  or  pericarp  remains  closed  about  the  seed  at  detachment,  and  often 
is  the  chief  protective  layer,  especially  where  it  is  hard  and  bony  (as  in 
Lithospermum).  In  some  instances  seeds  are  essentially  without  a  pro- 
tective outer  layer;  this  is  the  case  particularly  in  the  Amaryllidaceae, 
where  the  outer  integument  or  the  endosperm  may  become  fleshy  and 
green  (as  in  Hymenocallis  and  Crinum). 

The  advantages  of  seed  protection.  —  The  chief  dangers  which  beset 
seeds  are  premature  germination,  loss  of  viability,  and  destruction  by 
herbivorous  animals.  Adequate  protection  is  especially  important  in 
monocarpic  species,  above  all  in  annuals,  since  the  maintenance  of  the 
species  depends  absolutely  upon  the  viability  of  its  seeds.  For  months 
at  a  time  annuals  may  be  non-existent  over  vast  tracts  of  country 
except  in  the  form  of  seeds.  While  most  trees,  as  the  pines,  spread 


908  ECOLOGY 

only  through  the  agency  of  seeds,  the  situation  is  different,  since  the 
same  individual  produces  seeds  a  number  of  times.  The  adequacy 
of  seed  protection  is  well  illustrated  by  the  abundant  annual  recur- 
rence of  such' weeds  as  the  ragweeds,  pigweeds,  purslane,  and  Russian 
thistle. 

Seed  protection  in  relation  to  animals.  —  Many  seeds  are  used  as  food 
by  herbivorous  animals.  Often,  as  in  the  nuts  that  are  eaten  by  squirrels 
and  in  the  many  small  seeds  that  are  eaten  by  birds,  the  protective  coats 
are  insufficient  to  give  adequate  protection,  the  survival  of  the  species 
depending  upon  those  seeds  that  chance  not  to  be  eaten.  The  likeli- 
hood of  such  survival  is  not  so  slight  as  it  might  seem,  since  most  species 
produce  many  more  seeds  than  would  commonly  be  eaten,  and  many 
seeds  fall  to  the  ground  and  become  hidden  by  leaves.  The  seeds  of 
edible  fruits  might  be  thought  to  be  in  especial  danger,  but  in  most  cases 
they  pass  through  the  digestive  tracts  unharmed.  The  smooth  and 
slippery  surfaces  and  the  pointed  ends  of  most  such  seeds  make  it  prob- 
able that, they  will  be  swallowed  whole  rather  than  masticated,  and  the 
thick  and  hard  testa  prevents  the  destructive  action  of  digestive  juices 
upon  the  living  contents.  Sometimes  the  seeds,  as  in  the  grape,  are  en- 
closed by  a  mucilaginous  pulp  that  is  likely  to  be  swallowed  whole,  and 
sometimes  they  are  protected  by  special  structures,  such  as  the  car- 
tilaginous layers  within  the  apple. 

The  vitality  of  seeds.  —  The  amount  of  protection  exhibited  by  seeds 
is  shown  in  no  other  respect  so  well  as  by  their  remarkable  longevity. 
While  some  seeds  (as  in  the  willow  and  the  cacao)  die  unless  they 
germinate  almost  immediately,  most  seeds  retain  their  viability  for 
several  months  or  even  years,  and  a  few  may  remain  alive  for  many 
years. 

There  is  a  popular  belief  in  the  possession  of  extreme  longevity  by 
certain  seeds.  For  example,  it  often  is  asserted  that  the  reason  for  the 
development  of  a  totally  new  kind  of  vegetation  when  a  forest  is  cleared 
is  that  seeds  which  have  lain  dormant  for  years  or  even  for  centuries 
then  for  the  first  time  have  a  chance  to  germinate;  a  much  simpler  ex- 
planation, however,  is  found  in  the  ease  of  seed  dissemination.  Many 
people  have  believed  that  wheat  buried  many  centuries  ago  with  the 
Egyptian  mummies  has  germinated  in  recent  times  when  properly 
planted.  While  stories  of  this  character  are  without  foundation,  never- 
theless it  is  true  that  under  proper  conditions  certain  seeds  may  remain 
alive  for  many  years.  Probably  the  longest-lived  seeds  are  those  of  the 


REPRODUCTION   AND   DISPERSAL  909 

legume  family.  In  experiments  made  with  dried  'seeds  of  considerable 
age  (none  under  25  years  old),  involving  over  500  species  belonging  to 
30  families,  those  of  23  species  distributed  among  4  families  proved 
viable,  18  of  these  being  among  the  legumes;  in  these  experiments  the 
oldest  viable  seeds  had  an  age  of  87  years.  Other  experiments  indicate 
the  maximum  retention  of  viability  by  legume  seeds  to  be  150  to  250 
years.  Other  long-lived  seeds  are  those  of  the  water  lilies,  the  mallows, 
and  some  of  the  mints. 

In  the  phenomena  of  longevity  there  are  three  features  of  special  in- 
terest: the  status  of  the  Irving  cells  during  this  long  period,  the  features 
of  the  seed  that  cause  the  retardation  of  death,  and  the  nature  of  the 
factors  that  ultimately  cause  death.  There  have  been  two  theories  con- 
cerning the  status  of  the  living  cells,  namely,  that  they  manifest  very 
slight  respiratory  activity,  and  that  they  are  in  a  state  of  suspended  ani- 
mation. 

It  is  not  possible  at  present  to  determine  which  theory  of  cell  life  is  the  more  valid, 
but  recent  experiments  seem  to  give  strong  support  to  the  theory  of  suspended  ani- 
mation. There  is  no  adequate  evidence  of  respiratory  gas  exchanges  nor  of  any 
other  metabolic  activity  in  dry  seeds;  the  very  small  gas  exchanges  that  have  been 
noticed  are  quite  as  characteristic  of  dead  seeds  as  of  living  seeds,  and  in  the  latter 
they  are  fully  as  prominent  in  the  dead  testa  as  in  the  embryo.  Furthermore,  the 
theory  of  suspended  animation  best  accounts  for  the  wonderful  resistance  of  seeds 
to  extreme  temperatures;  indeed,  seeds  can  endure  a  temperature  so  low  that 
activity  of  any  kind  under  such  conditions  seems  quite  impossible.  The  likeli- 
hood that  activities  may  take  place  in  seeds  has  been  suggested  from  the  fact  that 
recently  matured  seeds  of  certain  species  germinate  poorly,  if  at  all,  while,  without 
any  obvious  structural  change  or  physiological  activity,  they  germinate  readily 
after  a  lapse  of  some  months.  However,  this  theory  has  become  less  tenable  in 
view  of  the  discovery  that  differences  in  the  germinability  of  seeds  are  due  chiefly 
to  changes  in  the  permeability  of  the  dead  seed  coat.  In  any  case  the  life  processes 
of  seeds,  if  present,  are  intracellular  and  anaerobic  and  are  exceedingly  minute  in 
amount. 

It  is  practically  certain  that  the  chief  feature  of  seeds  which  retards 
premature  germination  and  facilitates  longevity  is  the  impermeability 
of  the  enveloping  coat,  especially  of  the  testa.  In  many  instances  the 
seed  coats  of  desiccated  seeds  have  been  found  to  be  nearly  impermeable 
to  water  and  to  gases;  the  most  impermeable  of  such  envelopes  are 
those  of  legume  seeds,  which,  as  has  been  noted,  are  the  longest-lived  of 
all.  The  compact  structure  and  the  low  water  content  of  seeds  are  un- 
favorable to  activity,  and  hence  facilitate  longevity.  Certain  short-lived 


ECOLOGY 

seeds  live  longer  in  the  soil  than  when  dried,  possibly  because,  unlike 
most  seeds,  they  are  unable  to  withstand  prolonged  desiccation. 

It  is  not  unMkely  that  in  some  cases  longevity  is  due  to  much  less  obvious  features 
than  to  seed  coats.  Seeds  similar  in  structure  and  with  envelopes  equally  imper- 
meable vary  widely  in  longevity.  Still  more  striking  in  this  respect  are  the  minute 
asexual  spores  of  the  seedless  plants.  While  the  spores  of  Equisetum  die  if  they 
fail  to  germinate  almost  immediately,  moss  spores  that  have  lain  dry  in  a  her- 
barium for  fifty  years  have  been  known  to  retain  their  viability;  it  seems  improb- 
able that  such  differences  can  be  accounted  for  by  differences  in  the  spore  coat, 
which  is  not  noticeably  dissimilar  in  the  two  cases.  In  the  liverworts,  however,  it 
has  been  observed  that  the  spores  of  xerophytic  species  may  withstand  desiccation 
for  two  years,  whereas  the  thin-walled,  green  spores  of  semi-hydrophytic  species 
lose  their  vitality  very  quickly. 

The  causes  of  the  death  of  seeds  are  in  part  known  and  in  part  open 
to  question.  While  water  is  necessary  for  the  initiation  of  germination, 
it  often  is  absorbed  by  seeds  under  conditions  that  are  unfavorable  for 
the  continuance  of  the  germinative  processes.  This  is  the  case  with 
many  seeds  which  fall  into  the  water,  or  which  are  subjected  to  low 
temperatures  or  to  desiccation,  immediately  after  the  absorption  of 
water  has  begun.  Such  seeds  soon  decay,  or  at  any  rate  lose  their 
vitality.  Submergence  in  water  for  a  month  results  in  the  death  of  the 
seeds  of  many  land  plants,  such  as  rye,  oats,  and  maize.  However,  the 
seeds  of  many  water  plants  (such  as  Alisma  and  Sagittaria)  can  with- 
stand submergence  for  some  years,  probably  because  of  the  extreme 
resistance  offered  by  the  seed  coats  to  the  penetration  of  water.  Even 
when  seeds  are  kept  in  ordinary  rooms,  the  changes  in  atmospheric 
humidity  probably  are  sufficient  to  reduce  longevity  seriously,  because 
of  the  hygroscopic  properties  of  the  integuments.  Most  seeds  die 
within  three  months  if  they  are  continuously  exposed  to  saturated  air, 
the  longevity  increasing  somewhat  regularly  as  the  percentage  of  hu- 
midity is  reduced.  Parsnip  seeds  die  within  two  months  at  a  humidity 
of  70  per  cent,  although  they  may  be  kept  alive  for  three  years  when 
desiccated  and  placed  in  a  vacuum.  Apparently,  then,  the  exemption 
of  seeds  from  conditions  that  tend  to  incite  water  absorption,  respira- 
tion, or  activity  of  any  kind  is  a  necessity  for  longevity.  It  is  probably 
for  this  reason  that  most  seeds  retain  their  vitality  best  when  they  are 
stored  where  conditions  are  uniformly  cool  and  dry.  Experiments  show 
that  certain  seeds  retain  their  longevity  for  a  very  long  time  when  they 
are  buried  in  the  soil,  though  not  so  long  as  in  dry  storage.  For  ex- 
ample, seeds  of  mustard,  dock,  and  purslane  have  been  known  to  retain 


REPRODUCTION    AND    DISPERSAL  gn 

their  vitality  for  twenty-five  years,  when  buried  at  a  depth  of  fifty  centi- 
meters. It  has  been  shown  also  that  deep  burial  insures  greater  longev- 
ity than  does  shallow  burial.  Under  such  conditions,  longevity  would 
seem  to  depend  largely  upon  the  resistance  of  the  seed  coats  to  water. 
If  the  seeds  are  deeply  buried,  the  conditions  are  relatively  favorable  for 
longevity,  because  of  the  uniformly  low  temperature  and  because  of 
comparative  freedom  from  exposure  to  air  and  to  alternations  of  wetness 
and  dryness  in  the  soil. 

While  the  amount  of  water  in  seeds  is  small,  a  portion  of  this  amount 
is  essential  to  life.1  Hence,  it  is  probable  that  any  seed  would  die,  if 
it  is  exposed  to  evaporation  for  a  sufficient  length  of  time,  but  the  time 
may  vary  with  the  species  from  a  few  hours  or  days  to  hundreds  of  years. 
If  continued  respiration  takes  place  in  dry  seeds,  however  slowly,  it  is 
obvious  that  death  must  sooner  or  later  ensue.  Conditions  which  are 
fatal  to  most  other  plant  organs  often  have  no  deleterious  influence 
upon  seeds.  For  instance,  dry  seeds  can  be  kept  for  some  time  with- 
out injury  at  a  temperature  of  —  210°  C.,  even  if  the  testa  is  perforated, 
and  a  long  sojourn  in  a  vacuum  or  in  an  atmosphere  of  carbon  dioxid  or 
nitrogen  is  not  injurious.  Extremely  high  temperatures  also  may  be 
withstood  without  harm,  but  with  them  there  is  a  recognizable  limit, 
as  is  not  the  case  with  low  temperatures.  Most  desiccated  seeds  can 
withstand  for  one  or  two  hours  a  temperature  of  100°  C.,  and  alfalfa 
seeds  can  withstand  a  short  exposure  to  a  temperature  of  120°  C.,  even 
when  placed  in  water.  Perhaps  the  severest  test  yet  made  has  been 
with  the  seeds  of  alfalfa,  mustard,  and  wheat,  whose  coats  had  been 
perforated  and  thus  made  permeable;  these  seeds  germinated  after 
having  been  subjected  to  desiccation  for  six  months,  and  then  placed  in 
a  vacuum  for  a  year,  and  finally  subjected  for  three  weeks  to  a  tem- 
perature of  —  190°  C.,  and  for  three  days  to  a  temperature  of  —  250°  C.  It 
may  well  be  wondered  why  seeds  should  ever  die  if  they  can  withstand 
such  severe  conditions.  The  seeds  which  have  been  reported  to  have 
retained  their  vitality  for  more  than  two  centuries  were  subjected  during 
this  time  to  constant  changes  of  humidity  and  temperature.  It  is  im- 
possible to  conjecture  how  long  they  might  have  lived,  had  they  been 
stored  under  conditions  of  uniform  desiccation  and  refrigeration. 

Seeds  as  organs  of  food  accumulation.  —  Introductory  statement.  —  It 
has  been  seen  elsewhere  that  foods  accumulate  in  various  organs,  par- 

1  When  seeds  are  placed  in  a  desiccator,  they  retain  six  per  cent  or  more  of  their  water 
for  weeks ;  when  at  last  this  hygroscopic  water  evaporates,  death  ensues. 


ECOLOGY 


ticularly  in  stems  and  in  roots,  but  it  is  in  seeds  that  food  accumulation 
is  most  conspicuous,  so  that  the  chief  discussion  of  plant  foods  has  been 
reserved  for  this  place.  Seeds  are  filled  with  food  more  generally  than 
is  any  other  plant  organ,  and  the  kinds  of  foods  reach  here  their  greatest 
diversity  in  composition,  form,  and  distribution.  The  foods  in  seeds 
and  in  other  organs  may  conveniently  be  divided  into  those  without 
nitrogen  (such  as  the  carbohydrates)  and  those  containing  nitrogen 
(notably  the  proteins). 

Starch.  —  Starch  probably  is  more  generally  accumulated  in  seeds 
and  in  other  plant  organs  than  is  any  other  kind  of  food,  being  particu- 
larly well  known  in  the  grains,  in  peas  and  beans,  and  in  potato  tubers. 
Starch  grains  differ  widely  in  size,  in  shape,  and  in  structure,  these  dif- 
ferences serving  often  to  characterize  particular  species,  genera,  or 
families  (figs.  1206,  1211).  As  previously  noted,  starch  grains  are  pro- 
duced through  the  activity  of  plastids;  in  seeds  the  plastids  concerned 
are  the  colorless  leucoplasts,  the  sugar  that  enters  the  developing  seeds 
being  transformed  by  them  into  starch.  As  the  starch  accumulates  in  the 
plastid,  the  peripheral  portion  of  the  latter  expands  until  finally  the  pro- 
toplasm consists  merely  of  a  thin  film 
enveloping  the  starch  grain  (see  figs. 
660-662).1 

The  most  obvious 
structural  features  of 
starch  grains  are  their 
lines  of  stratification, 
which  are  due  to  alter- 
nating layers  of  different 
density  (fig.  1207).  In 
the  large  grains  of  Pel- 
lionia  and  probably  else- 
where the  dense  layers 
have  been  thought  to 
represent  accumulation 
by  day  when  the  sugar 
layers  representing  accumulation  by  night 
Thus  the  layers  of  starch 


FlG.  1206.  —  Cortical  cells  of  a 
potato  tuber  (Solanum  tuberosum), 
showing  starch  grains  (s)  of  differ- 
ent sizes,  and  also  a  protein  crystal 
(c);  highly  magnified. 


FIG.  1207.  —  A 
starch  grain  from 
the  cortex  of  a 
potato  tuber  (So- 
lanum tuberosum), 
showing  the  eccen- 
tric development  of 
the  ringsof  growth; 
very  highly  mag- 
nified. 


is   abundant,    the   other 

when  the  sugar  supply  is  less   (fig.  660). 


1  All  gradations  between  ordinary  chloroplasts  and  chloroplasts  which  are  reduced  to  a 
thin  enveloping  film  often  may  be  seen  in  the  stems  of  various  water  plants,  as  Myrio- 
phyllum.  * 


REPRODUCTION   AND   DISPERSAL 


9*3 


grains  appear  comparable  to  the  growth  rings  of  trees,  like  them  being 
caused  by  alternations  in  growth  conditions.1  Differences  in  size  and 
shape  are  due  partly  to  growth  conditions  in*  the  plastid.  Commonly 
growth  starts  in  the  center  (as  in  peas  and  beans),  and  the  rings  are  paral- 
lel to  the  plastid  periphery,  the  resulting  grain  being  a  symmetrical 
spheroid  or  ellipsoid.  Sometimes  growth  begins  at  one  end,  resulting 
in  eccentric  rings  (as  in  the  potato  tuber,  fig.  1207).  Sometimes  more 
than  one  grain  forms  in  a  plastid,  resulting  in  a  compound  grain  through 
mutual  crowding  in  growth  (as  in  oats  and  rice) ;  crowded  grains  often 
are  polyhedral  in  shape. 

The  minute  structure  of  starch  grains  is  in  doubt.  One  view  is  that  they  are 
sphcrocrystals,  that  is,  structures  composed  of  a  vast  number  of  needle-like  crystals 
or  trichites,  radiating  in  all  directions  from  the  growth  center.  This  conception  is 
based  upon  their  behavior  in  polarized  light,  which  is  comparable  to  that  of  inulin 
when  precipitated  by  alcohol  (see  below).  Another  view  is  that  starch  is  an  amor- 
phous colloid;  formerly  this  view  was  supposed  to  be  supported  by  the  fact  that 
starch  grains  readily  absorb  stains  and  swell  as  they  absorb  water,  but  certain  un- 
doubted crystals  exhibit  similar  phe- 
nomena. Starch  grains,  because  of 
strains  arising  from  desiccation  or 
otherwise,  often  exhibit  cracks  radi- 
ating from  the  center  (fig.  1017). 
The  exact  chemical  formula  of  starch 
is  not  known,  but  it  is  generally 
written  n  (CeHioOs)  (see  p.  358). 


Various  non-nitrogenous foods. 
—  Scarcely  second  in  impor- 
tance to  starch  among  the  non- 
nitrogenous  foods  in  seeds  are 
the  fats  or  glycerids,  which  are 
compounds  of  fatty  acids  and 
glycerin,  and  are  well  illustrated 
in  the  seeds  of  the  castor  bean, 
cotton,  and  sunflower,  and  in 
many  nuts.  The  fats  usually 
exist  as  drops  of  oil  in  the  cell 
lumina.  A  third  form  in  which  non-nitrogenous  food  accumulates  is 
the  so-called  reserve  cellulose  or  hemicellulose,  which  makes  up  the 


FIG.  1208.  —  A  section  through  part  of  the 
endosperm  of  a  persimmon  seed  (Diospyros 
virginiana),  showing  greatly  thickened  walls 
of  "reserve  cellulose";  the  lines  traversing 
the  cell  walls  indicate  the  paths  of  communi- 
cation between  adjacent  cells;  highly  magnified. 


1  However,  starch  grains  exhibit  some  stratification  when  exposed  during  development 
to  continuous  illumination. 


914 


ECOLOGY 


greater  part  of  the  thickened  endosperm  walls  of  vegetable  ivory  and 
of  the  seeds  of  the  persimmon  (fig.  1208)  and  the  date;  it  is  "  reserve 

cellulose"  that  gives  the  characteristic  horny  hardness 

to  these  and  to  similar  seeds. 

Although  they  rarely  if  ever  accumulate  in  quantity  in 
seeds,  a  word  may  be  said  as  to  sugars  and  similar  substances. 
Sugar  (particularly  saccharose x)  frequently  accumulates  in 
quantity  in  stems  (as  in  sugar 
cane)  and  in  roots  (as  in  beets), 
being  in  solution  in  the  cell  sap. 
Related  to  sugar  is  inulin,  a  car- 
bohydrate occurring  in  solution  in 
the  roots  of  composites  and  of 
various  other  plants.  When  these 
roots  are  immersed  in  alcohol, 
the  inulin  is  precipitated  in  solid 
bodies  with  concentric  stratifica- 
tion layers,  as  in  starch,  and  also 
with  lines  radiating  in  all  directions 
from  the  center,  suggesting  the 

trichites  that  characterize  sphero-  sperm  cell  from  a  seed  of 
crystals  (fig.  1209).  As  with  the  castor  bean  (Rtcinus 
starch,  the  behavior  of  these  bodies  communis),  showing  protein 
in  polarized  light  is  that  of  sphero-  8rains  (/>)  made  UP  of  amor- 
crystals,  yet  some  investigators  Phous  Proteins'  crvstalline 

root  cell  of  the  ele-     sti11  regard   them  as   amorphous 

campane(7»M/oife-     colloids. 

leniuni)  taken  from 


FIG.    1209.  —  A 


FIG.    1210.  —  An   endo- 


proteins  (c\  and  globular 
compounds  of  protein  with 
calcium  and  magnesium. 


TVT«J  x     j  -NT-       the  globoids  (?);   highly 

a      specimen      pre-          Nitrogenous    foods.-Kl-     magmfied.  _  From  BARN/S 

served    in    alcohol;     trogenOUS  foods,  such   as  the     (Part  II). 
note    the    spherites     protei        are  much  less  abun_ 
of  mulm  with  their     c 

growth  rings  and  dant  in  seeds  than  are  starches  and  fats,  but  they  are 
with  cracks  radiat-  universally  distributed  and  of  much  significance.  The 
ing  from  the  center;  ordi  protoplasm  of  the  living  cells  is,  of  course, 

note  also  that  where  *    • 

growth  begins  at  the    nitrogenous;  during  seed  development  it  is  active,  but 

wall,  only  half  of  a    jt  enters  a  period  of  comparative  quiescence  at  ma- 

°rn*e  '    turity.  again  becoming  active  at  germination.     Nitrog- 

highly  magnified.  ,  .   , 

enous  substances  also  develop  from  vacuoles  rich 
in  nitrogenous  materials  and  later  hardening  into  aleurone  grains  (fig. 
1210).  In  the  wheat  grain,  as  in  grasses  generally,  most  of  the  endo- 
sperm cells  are  packed  with  starch,  but  the  peripheral  layer,  often  called 


The  sugar  of  onion  bulbs  is  dextrose. 


REPRODUCTION   AND   DISPERSAL 


915 


the  gluten  layer,  is  filled  with  small  aleurone  grains  (fig.  1211);  in  most 
other  seeds  the  aleurone  grains  are  scattered  among  the  starch  grains 
or  the  drops  of  fat.  Some- 
times, as  in  Ricinus  (fig.  1210), 
the  protein  grains  are  large  and 
contain  inclusions,  such  as  pro- 
tein crystals  and  globoids,  the 
latter  composed  in  part  of 
calcium-magnesium  phosphate. 
Protein  crystals  also  may  lie 
free  in  the  cell  sap,  as  in  the 
cortex  of  the  potato  tuber  (fig. 
1 206) ;  such  crystals  differ  from 
inorganic  crystals  in  being  able 
to  take  stains  and  to  swell  in 
certain  media.  In  the  algae, 
nitrogenous  foods  occur  in  the 
pyrenoids  (fig.  106). 

The  distribution  of  foods  in 
seeds  and  the  associated  advan- 
tages. —  In  nearly  all  seeds 
there  occur  nitrogenous  and 
non-nitrogenous  foods,  the  lat- 
ter always  dominating  in  amount.  Commonly  one  form  of  non-nitrog- 
enous food  dominates  in  any  given  case,  so  that  one  may  speak  of 
starchy,  oily,  or  horny  seeds.  The  percentage  of  carbon  in  fat  is  about 
77  per  cent,  as  compared  with  44  per  cent  in  starch,  yet  because  of  its 
greater  density,  a  given  volume  of  starch  contains  about  as  much  carbon 
as  does  the  same  volume  of  fat.  The  chief  advantage  of  fatty  seeds 
would  seem  to  be  that  their  relative  lightness  facilitates  dispersal,  while 
on  the  other  hand  starchy  seeds  are  better  fitted  for  quick  germination, 
since  the  amount  of  oxygen  required  to  make  starch  available  for  growth 
is  much  less  than  that  for  fat.  Thus  it  is  distinctly  advantageous  that 
large  seeds  generally  are  starchy ;  where  they  are  not  starchy,  germination 
is  very  slow  (as  in  the  coconut).  Among  the  seeds  that  are  slow  to 
germinate  are  those  with  "  reserve  cellulose,"  as  in  the  date. 

The  influence  of  external  factors  upon  the  formation  of  accumulating  foods.  — 
Moderately  high  temperatures  are  favorable  for  seed  maturation  and  also  for  maxi- 
mum starch  production,  the  optimum  temperature  for  the  latter  being,  in  general, 


FIG.  121 1.  —  A  cross  section  through  the  outer 
part  of  a  wheat  grain  (Triticum  sativum),  show- 
ing the  husk  (h)  whose  outer  part  is  the  peri- 
carp and  whose  inner  part  is  the  testa,  the  aleu- 
rone or  gluten  layer  (a)  whose  cells  are  filled 
with  protein  grains,  and  a  part  of  the  starch 
region  (&)  which  makes  up  the  body  of  the  grain ; 
highly  magnified.  —  From  COBB. 


916  ECOLOGY 

in  the  neighborhood  of  25°  C.  In  cold  weather,  sugar  is  not  readily  transformed 
into  starch;  indeed,  the  reverse  process  often  takes  place  (as  in  the  sweetening  of 
potatoes).  An  important  variable  in  starch  production  is  the  supply  of  available 
sugar;  if  the  sugar  concentration  is  high,  starch  forms  more  rapidly  and  at  lower 
temperatures  than  usual,  even  at  o°  C.  Indirectly,  light  favors  starch  formation  in 
that  it  induces  a  considerable  production  of  sugar,  from  which  starch  can  be  made, 
but  the  latter  can  form  in  the  darkness  as  well  as  in  the  light.  The  influence  of 
external  factors  upon  the  accumulation  of  fats,  proteins,  and  "  reserve  cellulose  " 
is  not  known.  For  a  consideration  of  food  accumulation  in  tubers  and  galls,  see 
pp.  719,  782.  The  role  of  food  in  seeds  will  be  considered  in  connection  with 
germination  (p.  934). 

The  structure  of  the  food-containing  cells.  —  Practically  all  cells  in  which  food 
accumulates,  whether  in  the  endosperm,  perisperm,  or  cotyledons  (and  also  in  galls 
and  tubers),  are  parenchymatic  and  also  are  thin-walled  except  in  those  cases  where 
the  food  accumulates  in  the  walls  rather  than  in  the  lumina  as  in  "  reserve  cellu- 
lose ").  There  are  protoplasmic  connections  between  adjoining  endosperm  cells, 
and  where  the  walls  are  thick,  as  in  "  reserve  cellulose,"  the  canals  containing  the 
connecting  protoplasmic  threads  are  quite  conspicuous  (fig.  1208). 

Variations  in  seeds  and  fruits  in  relation  to  external  factors.  —  The 

fusion  of  gametes  in  relation  to  fruit  development.  —  Were  the  phe- 
nomenon not  so  universal,  it  would  seem  amazing  that  large  fruits  are 
able  to  develop  as  a  result  of  so  slight  an  external  stimulus  as  that  in- 
troduced by  a  male  gamete  upon  fusion  with  an  egg.  In  general  those 
pistils  in  which  this  fusion  takes  place  develop  into  fruits,  while  other 
pistils  show  no  such  changes,  soon  dropping  off,  as  do  the  stamens. 
In  the  simpler  cases  fruit  development  involves  only  the  enlargement 
or  elongation  of  the  ovary,  but  in  other  cases  various  organs  may  be  in- 
volved, for  example,  the  calyx  and  the  receptacle  (as  in  the  apple). 
Sometimes  the  stimulation  appears  greater  in  the  case  of  xenogamy  than 
of  autogamy;  for  example,  in  Cheiranthus  the  fruits  are  twice  as  large, 
and  the  seeds  are  heavier  and  more  numerous  on  the  cross-pollinated 
individuals. 

There  is  much  in  common  in  the  formation  of  fruits  and  galls>  and  in  each  case 
it  has  been  held  by  some  investigators  that  the  growth  arises  solely  through  the 
influence  of  a  momentary  stimulus  at  the  inception  of  the  process,  and  by  others 
that  the  activities  within  the  growing  structures  afford  constant  stimuli  for  further 
development.  Of  interest  in  this  connection  is  the  fact  that  staminate  flowers 
may  be  transformed  into  galls  if  stimulated  by  the  proper  insects  (as  in  the  ash). 
In  this  event,  instead  of  dropping  off,  they  enlarge  and  remain  for  a  year  or  more. 
Here  a  foreign  stimulus  given  by  the  gall  insect  causes  the  retention  and  the  further 
development  of  the  staminate  flowers  much  as  another  foreign  stimulus  given  by 
the  male  gamete  more  commonly  causes  the  retention  and  the  further  development 
of  the  pistillate  flowers.  The  precise  nature  of  the  fruit-forming  stimulus  varies 


REPRODUCTION   AND   DISPERSAL  917 

considerably  with  the  species.  In  many  cases  the  act  of  pollination  forms  a  stimu- 
lus of  sufficient  intensity  to  inaugurate  continued  development;  this  condition  is 
well  illustrated  in  certain  orchids  in  which  fruit  development  has  been  started  by 
dead  pollen  or  pollen  extract  placed  upon  the  stigma.  In  other  cases  it  is  the  grow- 
ing pollen  tube  which  initiates  fruit  development,  as  in  Geranium  and  in  various 
orchids.  In  the  Cucurbitaceae  the  fusion  of  gametes  is  necessary  for  complete  fruit 
development,  although  pollination  alone  stimulates  considerable  growth.  In  still 
other  cases  the  growing  ovules  are  an  important  stimulus,  as  in  the  grape,  where 
the  size  of  the  fruit  increases  with  the  number  of  seeds. 

Parthenocarpy.  —  In  striking  contrast  to  ordinary  fruit  production 
is  parthenocarpy  j  or  the  development  of  fruit  without  the  fusion  of 
gametes.  Familiar  illustrations  of  parthenocarpy  are  afforded  by  a 
number  of  seedless  varieties  of  cultivated  fruits,  as  in  oranges,  grapes, 
and  bananas;  while  only  certain  varieties  of  grapes  and  oranges  are 
seedless,  the  cultivated  banana  never  produces  seeds.1  In  some  cases 
of  parthenocarpy,  pollination  seems  to  be  necessary  for  fruit  development, 
but  it  is  quite  unnecessary  in  certain  figs,  where  fruit  development  occurs 
without  the  aid  of  any  known  external  stimulus.  The  most  striking 
case  of  all  is  in  Balanophora,  a  plant  which  is  quite  without  functional 
pistillate  flowers,  but  which  produces  fruits  containing  viable  seeds.  In 
this  genus  the  pistillate  flower  is  reduced  to  a  protuberance  with  rudi- 
ments of  a  style  and  an  embryo  sac.  One  species  (B.  globosa)  lacks 
staminate  flowers,  and  even  in  those  species  which  produce  pollen,  it  is, 
of  course,  entirely  useless,  affording  one  of  the  best  illustrations  of 
the  retention  by  a  plant  of  a  useless  organ.  Balanophora  is  a  holopara- 
site,  and  it  may  be  that  there  is  some  connection  between  its  parasitism 
and  its  loss  of  sexuality. 

In  recent  years  the  number  of  plants  which  are  known  to  be  able  to  develop 
parthenocarpic  fruits  has  been  considerably  increased;  among  such  plants  are  the 
persimmon,  gooseberry,  hop,  and  certain  varieties  of  the  apple  and  the  pear.  It  is 
also  becoming  clear  that  in  most  cases  neither  pollination  nor  any  other  known 
external  stimulus  is  necessary  to  secure  fruit  development.  In  several  cases,  as  in 
the  gooseberry  and  the  persimmon,  the  seedless  fruits  mature  earlier  than  do  the 
seed-bearing  fruits.  Obviously  fruit  production  without  seeds  is  wholly  useless  so 
far  as  the  perpetuation  of  the  plant  is  concerned. 

Variations  in  the  size  and  the  structure  of  fruits  and  seeds.  —  Probably 
no  other  plant  organs  are  as  invariable  as  are  fruits  and  seeds,  and  for 
this  reason  the  few  variations  which  are  known  have  an  unusual  interest. 
Seeds  which  develop  singly  or  which  are  not  crowded  during  develop- 

1  Plants  with  parthenocarpic  fruits  are,  of  course,  propagated  vegetatively ;  it  is  sup- 
posed commonly  that  they  originated  suddenly  as  mutants. 


918  ECOLOGY 

ment  are  likely  to  be  spherical,  while  crowded  seeds  commonly  are 
angular.  In  the  two-seeded  fruits  of  Xanthium  and  Cakile,  each  seed 
differs  considerably  in  shape  and  in  size  from  the  other.  In  certain 
composites  the  achenes  of  the  ray  flowers  and  of  the  dislc  flowers  differ 
strikingly  in  shape.  In  the  parasitic  Scrophulariaceae  it  has  been  dis- 
covered that  vigorous  plants  give  rise  to  larger  seeds  than  do  weak  plants, 
and  that  the  large  seeds  give  rise  in  turn  to  more  vigorous  plants  than  do 
the  smaller  seeds;  furthermore,  the  larger  seeds  are  more  likely  than  are 
the  others  to  grow  into  autophytic  individuals,  while  the  plants  coming 
from  small  seeds  in  order  to  thrive,  apparently  must  be  parasitic.1 
The  achenes  of  hemp  vary  considerably  in  size  and  in  weight,  those  pro- 
duced in  moist  habitats  being  larger  and  heavier  than  those  produced 
in  dry  habitats.  The  larger  achenes  germinate  more  quickly  than  do 
the  others,  forming  stronger  plants.  Similar  differences  have  been  ob- 
served in  the  seeds  and  seedlings  of  tobacco.  In  a  crowded  group  of 
natural  seedlings  such  a  difference  in  size  might  be  of  great  significance, 
since  the  stronger  seedlings  would  tend  to  crowd  out  the  others.  The 
influence  of  grafting  upon  the  character  of  fruits  has  been  noted  elsewhere, 
but  it  may  be  recalled  that  changes  in  the  size  and  in  the  flavor  of  culti- 
vated fruits  often  result  from  the  reciprocal  influence  of  the  stock  and 
the  scion.  Pollination  may  affect  the  character  of  the  fruit ;  for  example, 
when  the  flowers  of  watermelons  are  pollinated  by  cucumber  pollen, 
the  resulting  fruit  is  very  poor  in  sugar. 

Seed  variations  manifested  in  behavior.  —  Seeds  of  the  same  species, 
though  apparently  alike  in  structure,  in  reality  may  be  very  different. 
Perhaps  the  best  instance  of  this  is  seen  in  a  comparison  of  seeds  raised 
in  different  climates.  Farmers  in  the  United  States  have  long  known 
that  northern-grown  seeds  produce  crops  that  ripen  earlier  than  do  crops 
raised  from  seeds  grown  farther  south.  It  appears  as  if  the  progeny  of 
the  northern  plants  have  inherited  from  them  their  short  maturation 
period,  thus  furnishing  evidence  in  favor  of  the  theory  of  the  inheritance 
of  acquired  characters  (p.  947).  After  a  few  years,  however,  it  is  neces- 
sary once  more  to  use  northern  seeds,  since  the  progeny  of  northern- 
grown  plants  come  to  have  the  same  period  that  is  characteristic  of  the 
climate  to  which  they  are  transferred. 

1  In  this  connection  it  is  of  interest  to  note  that  various  parasites  (as  Hydnora,  Rafflesia, 
and  Balanophora)  and  mycophytes  (as  Monotropa  and  the  orchids)  have  minute  seeds 
with  rudimentary  undifferentiated  embryos  and  almost  no  food,  nutritive  dependence 
upon  other  plants  being  necessary  very  early,  in  most  cases  even  in  the  earliest  stages  of 
germination. 


REPRODUCTION   AND   DISPERSAL  919 

The  dehiscence  of  fruits.  —  Fruits  that  open  on  maturity,  thus  per- 
mitting the  ready  scattering  of  seeds,  are  known  as  dehiscent,  while  those 
that  do  not  open  are  called  indehiscent.  Dehiscent  fruits  are  illustrated 
by  capsules  (figs.  1213,  1214)  and  pods  (fig.  1212),  while  berries  (fig.  1222), 
stone  fruits  (drupes},  and  acorns  (fig.  1223)  represent  indehiscent  fruits. 
Many  indehiscent  fruits  are  one-seeded,  and  may  easily  be  mistaken  for 
seeds;  among  such  are  the  small,  dry  fruits,  known  as  achenes,  especially 
characteristic  of  the  composites  (figs.  1217,  1220),  and  also  grains,  nuts, 
and  acorns.  In  the  umbellifers  the  fruits  are  known  as  schizocarps,  the 
one-seeded  carpels  splitting  at  maturity  but  not  dehiscing  (fig.  1221). 
Although  the  habit  seems  relatively  useless,  dehiscence  occurs  in  some 
one-seeded  fruits,  as  in  the  nutmeg. 

Usually  the  opening  of  dehiscent  fruits  is  due  to  an  unequal  contrac- 
tion of  the  pericarp  tissues,  resulting  from  desiccation,  some  cells  or  tis- 
sues losing  more  water  than  do  others;  often  the  cells  are  arranged  trans- 
versely on  the  concave  side,  and  longitudinally  on  the  convex  side. 
Capsules  usually  open  in  such  a  way  as  to  expose  as  many  valves  as  there 
are  carpels,  the  splitting  taking  place  along  the  separating  walls  or  along 
the  middle  line  of  the  individual  carpels.  Characteristic  pods  are  illus- 
trated by  the  crucifers  and  legumes;  among  the  latter  the  valves  not  only 
are  separated  by  longitudinal  splitting,  but  there  may  be  torsion  within 
the  individual  valves  (fig.  1212).  Some  fruits  dehisce  through  pores, 
as  in  the  poppy;  and  in  others  the  lines  of  dehiscence  are  transverse 
rather  than  longitudinal,  resulting  in  the  detachment  of  the  top  like  a 
lid,  as  in  Portulaca  and  Plantago.  At  the  time  of  dehiscence  the  seeds 
readily  become  detached  from  the  carpel  wall  and  are  exposed  to  dispers- 
ing agencies;  the  scar  left. on  the  seed  at  the  point  of  attachment  is  known 
as  the  hilum.  In  the  pines  and  in  other  conifers  the  cone  scales  commonly 
separate  from  one  another  at  maturity,  exposing  the  winged  seeds  to  the 
wind;  in  some  species  the  persistent  cones  may  remain  closed  for  many 
years  (as  in  Pinus  Banksiana),  the  seeds  thus  retaining  their  viability 
much  longer  than  otherwise.  In  some  of  these  trees,  extreme  desicca- 
tion, such  as  is  caused  by  forest  fires,  seems  necessary  to  effect  the  open- 
ing of  the  cones.  In  some  indehiscent  fruits  there  is  an  outer  dehiscent 
envelope,  as  in  the  involucre  of  the  chestnut  and  the  hickory  nut  and 
in  the  aril  of  the  bittersweet. 

The  dispersal  of  fruits  and  seeds.  —  Dispersal  by  propulsion.  —  In 
dehiscent  fruits  it  is  generally  the  seed,  and  in  indehiscent  fruits,  the 
fruit  as  a  whole,  that  is  scattered.  In  some  cases  the  act  of  dehiscence 


920  ECOLOGY 

is  so  sudden  and  violent  that  the  seeds  are  expelled  at  the  same  time. 
At  dehiscence  the  seeds  of  the  violet  and  lupine  are  shot  out  several 
centimeters  (sometimes  nearly  a  meter),  while  those  of  the  witch-hazel 
are  expelled  much  more  violently,  and  may  be  scattered  for  several 
meters.  In  the  lupine  the  seeds  are  expelled  spirally  by  reason  of  the 
torsion  of  the  valves  (fig.  1212). 


In  Geranium  the  carpels  separate  from  the  central  axis,  coiling  upwards  and 
discharging  the  seeds.     In  Hura  crepitans  the  dehiscence  is  so  violent  that  the 
seeds  are  discharged  with  an  explosive  report.     In  Eeballium 
the  seeds  are  squirted  out,  together  with  some  of  the  fruit 
tissues,  whence  the  name,  squirting  cucumber.     In  Impatiens 
the  fruit  tissues  are  in  a  state  of  such  delicate  balance  that 
a   mere   touch  causes  violent  dehis- 
cence and  dispersal,  whence  the  sig- 
nificance of  the   scientific   name  as 
well  as  of  the  common  name,  touch- 
me-not.       In    a    western    mistletoe, 
Arceuthobium    occidentale,    the    ripe 
fruits  explode,  ejecting  the  seeds  for 
several  meters;    as  in  other  mistle- 
toes,   the    seeds    adhere    readily    to 
leaves  or  bark. 


V  —  • 


1213    1214] 


In  many  cases  there  is  no  vio- 


FIGS.  1212-1214.  —  1212,  an  opening  pod  or 
legume  of  the  lupine  (Lupinus  perennis),  illus- 
trating violent  dehiscence  through  the  torsion 
of  the  valves  (v)  when  desiccated;    the  seeds      .          .   .  .  ... 

(*)  are  mechanically  expelled  for  some  distance;      lent  dehiscence,  but  the  Seeds  he 

1213,  1214,  dehiscence  of  the  capsules  of  the    in  such  a  position  that  a  mechan- 

evening  primrose  (OenotheraUennis}:  1213,  a  jcal     impact    causes     Scattering, 

mature  capsule  in  which  the  four  valves  (v)  are  _  _  .        .       .       _ 

beginning  to  split  at  the  apex;  12 14,  a  later  stage  Most  capsules  (as  in  Oenothera, 

in  which  desiccation  has  caused  the  valves  to  figs.  1213, 121 4,  and  Pedicularis) 

spread  apart,  exposing  the  seeds  (5)  in  such  a  and   man  dg    lie    with    their 

way  that  they  may  readily  be  shaken  out. 

valves  open,  and  the  wind  may 

shake  out  the  seeds,  or  animals  may  brush  them  out.  In  many  mints, 
if  one  presses  down  a  calyx  having  mature  nutlets,  the  latter  shoot 
out  upon  release.  Of  especial  interest  is  Polygonum  virginianum, 
whose  achene  is  fastened  to  an  elastic  cushion  of  tissue  in  such  a  way 
that,  when  pressed  back,  it  bounds  off  upon  release  for  a  distance  of 
three  or  four  meters.  Obviously  the  dispersal  of  seeds  by  propulsion 
is  relatively  ineffective,  since  at  best  the  seeds  are  scattered  but  a  few 
meters  from  the  parent  plant,  and  commonly  much  less. 

Dispersal  by  wind.  —  With  seeds,  as  with  spores,  the  most  effective 
of  dispersal  agents  is  the  wind,  especially  from  the  standpoint  of  the 


REPRODUCTION   AND   DISPERSAL 


921 


FIGS.  1215,  1216.  —  Winged  fruits: 
1215,  a  samara  or  key  fruit  of  the  moun- 
tain maple  (Acer  spicatuni);  1216,  a 


number  of  disseminules  carried.  As 
seeds  are  much  larger  than  spores,  the 
distances  covered  are  much  less,  though 
in  the  case  of  small  seeds,  as  in  the 
orchids,  it  is  possible  that  the  dis- 
tances may  be  very  great.  Among 
the  commonest  of  wind-scattered  dis- 
seminules are  those  with  wings,  as  in 
the  seeds  of  the  catalpa,  and  in  the 
fruits  (known  as  samaras)  of  the 

i     tr  \    i_  /c.  z\ 

maple  (fig.  1215),  hop  tree  (fig.  1216), 

and     elm.      Such     disseminules     com- 

monly  are  much  flattened,  and  hence  samara  of  the  hop  tree  (Ptdea  trifoliata)  • 
are  unlikely  to  fall  rapidly  to  the  w' w 

ground;  furthermore,  the  wings  are  light,  often  containing  air  spaces  of 
considerable  size.  The  wings  may  be  terminal,  as  in  the  ash  and  the 
maple,  or  they  may  form  a  margin  about  the  seed-bearing  portion,  as  in 
the  hop  tree,  elm,  and  bugseed.  Similar  winged  disseminules  are  found 

in  the  pines  and  birches.  In  the  linden 
there  are  relatively  heavy,  globular,  in- 
dehiscent  fruits,  but  they  are  borne  on 
a  large,  membranous  bract  attached  to 
the  peduncle. 

Many  wind-scattered  disseminules  are 
crowned  with  hairs.  Perhaps  the  most 
representative  of  these  are  found  in  the 
composites,  especially  in  those  with  milky 
juice;  in  the  latter,  at  maturation,  the 
involucre  falls  back  once  more  as  at  an- 
thesis,  exposing  the  achenes,  with  their 
crowns  of  hairs  (known  as  the  pappus) 

FIG.  1217.  -A  fruiting  head  of      sPread  out  in  such  a  Wa7  that  the  entire 

the  prickly  lettuce  (Lactuca  scan-  structure  resembles  a  parachute  (fig.  1217) ; 
ola},  from  which  all  of  the  mature  as  in  parachlltes,  also,  the  resistance  to 

fruits  but  one   have  been   blown  ...,,. 

the  air  in  falling  is  considerable,  so  that 
wind  currents  are  apt  to  scatter  the 

which  is  prolonged  into  a  beak  (6)  achenes  for  some  distance.  Dispersal  IS 
and  is  crowned  with  pappus  com-  r  ... 

posed  of  capillary  bristles  (/>);  r,  facilitated  still  further,  if  there  is  a  long, 
receptacle;  p't  peduncle.  slender  process  (known  as  the  beak)  sepa- 


away;   note  the  reflexed  involucral 
scales    (s)    and    the    achene    (a), 


922 


ECOLOGY 


rating  the  achene  from  the  pappus,  as  in  the  lettuce  and  the  dandelion, 
or  if  the  pappus  hairs  are  branched,  as  in  the  thistle. 

In  the  milkweed  (Asclepias)  the  seeds  bear  a  crown  of  long,  silky  hairs  at  the 
hilum  end,  which  enables  them  to  float  in  the  air  much  as  do  the  achenes  of  the 
composite.  Similar  hairs  facilitate  dispersal  in  the  willows  and  poplars,  the  cotton- 
wood  deriving  thus  its  common  name.  Commercial  cotton  is  derived  from  the 
copious  hairs  that  are  attached  to  the  seeds  of  the  cotton  plant  (Gossypium);  similar 
cottony  hairs  are  attached  to  the  fruits  of  some  anemones  and  of  the  cotton  grass 
(Eriophorum). 

A  remarkable  instance  of  wind  dispersal  is  afforded  by  the  tumbleweeds, 
a  class  of  plants  that  at  maturity  break  off  from  the  roots  as  a  whole  or 


FIG.  1218.  —  A  general  view  of  mature  plants  of  the  winged  pigweed  (Cycloloma  atripli- 
cifolium),  a  representative  tumbleweed;   Gary,  Ind.  —  Photograph  supplied  by  MEYERS. 

in  part,  whereupon  they  are  tumbled  along  over  the  ground  by  the  wind, 
scattering  seeds  as  they  go  (fig.  1218).  Among  such  plants,  which  are 
especially  common  on  the  prairies  and  plains,  are  the  winged  pigweed 
(Cycloloma),  the  Russian  thistle  (Salsola  Kali  tenuifolia),  and  Amaran- 
thus  graecizans;  these  generally  break  off  entire,  but  in  the  old  witch 
grass  (Panicum  capillare)  and  in  some  other  plants,  portions  break  off 
and  blow  about  alone  or  attached  to  other  tumbleweeds. 

Dispersal  by  water.  —  Water,  though  less  effective  than  wind  in  the 
number  of  seeds  carried  to  places  where  they  can  germinate  and  grow, 
is  none  the  less  a  dispersal  agent  of  great  importance,  particularly  be- 
cause it  may  carry  disseminules  for  long  distances.  Sometimes  the  dis- 


REPRODUCTION    AND   DISPERSAL  923 

seminules  move  in  definite  directions,  as  in  rivers  and  in  the  better- 
defined  ocean  currents,  but  in  ponds  and  lakes  the  direction  of  move- 
ment commonly  varies  with  the  winds.  All  seeds  are  heavier  than  air, 
and  hence  are  incapable  of  indefinite  propulsion  in  that  medium,  but 
many  seeds  and  fruits  are  lighter  than  water,  and  hence  may  be  carried 
for  great  distances;  among  the  latter  are  the  fruits  of  many  water  plants 
and  swamp  plants,  such  as  Sagittaria  and  Sparganium,  whose  lightness 
is  due  largely  to  the  presence  of  prominent  air  chambers  in  the  pericarp 
or  testa.  Many  seeds,  however,  sink  in  water,  some  rapidly  and  others 
more  slowly,  so  that  the  distance  they  may  traverse  is  more  or  less  limited, 
as  with  wind-scattered  seeds;  among  the  seeds  which  sink  at  once  are 
included  those  of  such  pronounced  hydrophytes  as  Ceratophyllum  and 
Subularia. 

Of  great  significance  in  connection  with  water  dispersal  is  the  degree 
of  resistance  to  the  entrance  of  water  offered  by  floating  seeds  and  fruits. 
Many  seeds  capable  of  floating  soon  lose  their  vitality  through  the  en- 
trance of  water,  which  thus  institutes  decay.  Particularly  is  this  the 
case  if  the  water  is  rough,  and  more  particularly  if  it  is  salt  as  well  as 
rough.  For  example,  the  coconut,  whose  fruit  often  is  seen  floating  on 
tropical  seas,  loses  its  vitality  within  a  few  days  through  infiltration,  so 
that  it  is  doubtful  if  it  could  populate  a  new  land  at  a  great  distance, 
though  no  illustration  of  water  dispersal  is  quoted  more  frequently. 
In  contrast  with  the  coconut  are  such  fruits  as  that  of  Suriana  maritima, 
a  common  plant  of  tropical  strands;  these  have  been  shown  experimen- 
tally to  be  uninjured  after  floating  for  143  days  in  rough  salt  water,  and 
the  seeds  of  Hibiscus  tiliaceus  similarly  have  been  shown  to  be  capable 
of  floating  for  121  days  without  injury.  The  presence  of  air  chambers, 
especially  in  the  pericarp,  greatly  retards  water  infiltration.  In  Barring- 
tonia  the  resistance  to  infiltration  is  so  great  that  broken  pieces  of  the 
fruit  float  for  more  than  twenty  weeks  in  a  3  per  cent  salt  solution.  The 
seeds  of  Asparagus  may  retain  their  vitality  when  soaked  in  water  for 
a  year,  and  in  many  water  plants  (as  Sagittaria  and  Proserpinaca} 
the  seeds  may  retain  their  vitality  at  the  bottom  of  ponds  for  several 
years.  It  can  hardly  be  doubted  that  in  all  cases  the  retention  of  vitality 
in  immersed  seeds  is  due  to  the  resistance  of  the  various  coats  to  infiltra- 
tion. 

Dispersal  by  animals.. —  Many  fruits,  mainly  indehiscent,  are  scat- 
tered involuntarily  by  animals,  particularly  the  bur  fruits  and  others  with 
hooked  appendages.  Unpleasantly  familiar  fruits  of  this  character  are 


924 


ECOLOGY 


those  of  the  cocklebur  (Xanthium,  fig.  1219),  burdock  (Arctiuni),  beggar- 
ticks  (Bidens,  fig.  1220),  hound's-tongue  (Cynoglossum),  sweet  cicely 
(Osmorhiza,  fig.  1221),  and  bur  grass  (Cenchrus).  These  and  similar 
fruits  are  scattered  abundantly  by  man  and  by  domestic  animals,  and 
some  plants  (as  Xanthium)  have  thus  made  a  rapid  in- 
vasion of  all  continents. 

An  interesting  class  of  fruits  from  the  standpoint  of 
dispersal  consists  of  those  which  are  fleshy  and  possess  a 
more  or  less  juicy  and  edible  pulp 
(fig.  1222).  Birds  and  other  ani- 
mals commonly  eat  such  fruits 
abundantly,  often  aiding  in  the 
scattering  of  the  seeds.  Some 
birds  eject  the  seeds  immediately 
after  divesting  them  of  the  edible 
portion  of  the  fruit,  but  the  ma- 
]2ig  jority  of  fruit-eating  animals  prob- 

FIGS.  I2IO-X22I. -Fruits  with  append-      ably   SWall°W    the   SCeds>    especially 

ages  which  become  fastened  to  animals     those  that  are  small;  even  stones 


and  thus  dispersed:    1219,  a  fruit  of   the 
cocklebur  (Xanthium},  whose  body  is  cov- 


as  large  as  those 


ered  with  stiff  recurved  prickles;  1220,  an  °f  the  cnerrv  are 
achene  of  the  bur  marigold  (Bidens),  Swallowed  by  ani- 
crowned  with  two  sharp  and  stiff  teeth  or 
awns  (a)  which  are  covered  with  reflexed 
barbs  (b);  1221,  a  mature  fruit  (schizo- 
carp)  of  the  sweet  cicely  (Osmorhiza  lo;i- 
gistylis\  consisting  of  two  one-seeded  car- 
pels (c)  which  separate  along  the  inner 
face,  remaining  delicately  suspended  on 

slender  prolongations  of  the  axis,  the  car-  Seeds  commonly 
pophore  (c') ;  the  carpels  readily  adhere  to  are  destroyed  in 
passing  animals  by  means  of  the  barbs  ( *  * 


mals  as  small  as 
the  raven.  In 
some  cases,  as  in 
the  dove  and  the 
domestic  fowl,  the 


FIG.  1222.  —  An 
aggregate  fleshy 
fruit  of  the  mul- 

passmg    through     berry      (Morus}; 

fruits    are 
r   animals, 


the  alimentary  tract.  The  most  useful  animals  from  such 
the  standpoint  of  dispersal  are  such  birds  as  the  eaten 
robins,  thrushes,  and  blackbirds,  which  eat  fleshy  Undigelttd  through 
fruits  in  abundance,  swallowing  the  seeds,  and  void-  the  alimentary 
ing  them  without  harming  them  in  the  alimentary  tract- 
tract.  Obviously  such  birds  are  likely  to  carry  the  seeds  to  some  dis- 
tance from  the  parent  plant,  as  would  not  be  the  case  with  those  that 
reject  the  seeds  while  eating. 

Fleshy,  edible  fruits  when  ripe  usually  are  conspicuous  by  reason  of 


REPRODUCTION   AND   DISPERSAL  925 

their  color,  though  green  and  relatively  inconspicuous  when  immature. 
Of  the  showy  fruits  of  this  sort  some  are  white  (as  in  the  snowberry  and 
mistletoe),  others  red  (as  in  the  holly,  bittersweet,  and  cherry),  others 
blue  (as  in  the  red  cedar  and  blueberry),  and  still  others  black  (as  in  the 
blackberry  and  black  haw),  or  yellow  (as  in  various  Solanaceae). 
Though  showy  fruits  doubtless  attract  fruit-eating  animals  and  thus 
facilitate  seed  dispersal,  it  is  likely  that  the  advantage  of  such  showiness 
has  been  overestimated.  Probably  the  animals  would  find  the  fruits  if 
they  were  not  highly  colored;  indeed,  some  edible  fruits,  as  in  Asimina 
and  Ribes  Cynosbati,  are  green  at  maturity.  Furthermpre,  species  with 
fleshy  fruits  doubtless  would  become  dispersed,  even  if  all  fruit-eating 
animals  should  disappear  (see  below  concerning  nut  dispersal).  Showi- 
ness, therefore,  probably  is  merely  an  accompaniment  of  ripening,'  in- 
dicating the  occurrence  of  certain  chemical  changes;  incidentally  they 
also  are  of  some  advantage  in  that  animals  thereby  are  attracted.  Some 
fruits,  as  the  blackberry,  are  showiest  when  red  and  immature,  and  some 
showy  fruits  (as  in  Physocarpus)  are  quite  dry  and  inedible. 

Doubtless  various  large  wading  birds,  such  as  the  herons,  carry  seeds  in  the  mud 
that  adheres  to  their  feet,  thus  accounting,  perhaps,  for  the  wide  distribution  of 
some   swamp   plants.     Fruit-eating    animals    do    not    always 
facilitate  dispersal.     For  example,  in  autumn,  birds  feed  abun- 
dantly on  the  fruits  of  various  plants  (such  as  the  ragweeds, 
sunflowers,  and  certain  grasses,  as  wild  rice),  eating  the  seeds, 
and  thus  preventing  rather  than  advancing  dispersal.     Recently 
it  has  been  shown  that  ants  play  an  important  part    in   the 
dispersal  of   many  small    seeds,  particularly  where  the  seeds 

have  oily  appendages  which    the    ants  utilize  as  food.     Cer- 

,      ,      ,                                                    ,.  FIG.   1223. —A 

tain    heavy   seeds    (such   as   the   nuts   and   acorns,  fig.  1223)  ,    , 

,    J'  nut  (acorn)  of  the 

usually  are  not  scattered  in  any  of  the  above  ways;    further-  yac^ oak / Quercus 

more,  they  are  gathered  and  eaten  in  large  numbers  by  squirrels,  velutina),  partially 
Occasionally  nuts  that  are  carried  off  by  animals  are  not  eaten,  enclosed  by  its  cup 
and  thus  may  germinate,  but  at  best  such  a  means  of  dispersal  (c)t  which  has  de- 
is  rather  precarious.  veloped  from  the 

involucre ;  note  the 

The  relative  efficiency  of  the  various  means  of  dis-    >'™bricated    *cales 

of  the  cup  (5). 

persal.  —  Three  considerations  seem  to  be  involved  in 
successful  dispersal:  the  number  of  disseminules  transported,  the 
distance  they  are  taken,  and  the  degree  of  precision  with  which 
they  lodge  in  places  favorable  for  germination  and  for  subsequent 
development.  Some  seeds  and  fruits  are  not  transported  at  all,  the 
most  notable  examples  being  those  that  ripen  under  ground,  as  in 


926  ECOLOGY 

the  peanut  and  in  the  fruits  of  the  cleistogamous  flowers  of  Polygala 
(fig.  1191)  and  Viola;  though  this  habit  might  seem  disadvan- 
tageous, no  seeds  are  better  placed  for  germination.  A  vast  number 
of  seeds  and  fruits  have  no  regular  means  of  dispersal  apart  from  drop- 
ping to  the  ground  beneath  the  plant  that  bore  them;  among  such  are 
the  nuts,  the  acorns,  and  many  other  heavy  fruits  or  seeds.  Scarcely 
more  effective  are  the  numerous  cases  of  mechanical  propulsion  from  de- 
hiscent fruits.  However,  in  all  these  cases  the  seeds  are  likely  to  lodge 
in  places  that  are  relatively  fit  for  germination. 

The  effective  agents  of  distant  dispersal  are  water,  wind,  and  animals. 
Water  probably  is  the  most  likely  to  carry  disseminules  for  great  dis- 
tances, but  the  number  of  seeds  which  fall  into  the  water  is  limited;  a 
great  many  of  these  seeds  also  are  injured  in  transit,  and  still  more  fail 
to  lodge  in  a  suitable  habitat.  Water,  however,  is  of  the  utmost  impor- 
tance as  a  transporter  of  the  fruits  and  seeds  of  plants  which  grow  in 
the  water,  or  in  swamps,  and  along  shores,  since  deposition  is  likely  to 
be  in  a  place  that  is  fit  for  subsequent  growth.1  Temporary  streams, 
such  as  torrents  following  heavy  rains,  and  permanent  streams  in  times 
of  flood,  are  highly  important  agents  in  the  dispersal  of  the  seeds  of  land 
plants.  Wind  is  the  most  likely  of  all  agents  to  pick  up  and  transport 
seeds  and  fruits  in  great  numbers  from  all  habitats,  but  it  is  also  the  most 
indiscriminate  of  scatterers,  depositing  all  kinds  of  seeds  in  all  kinds  of 
places,  so  that  the  waste  of  disseminules  is  enormous.  Seeds  and  fruits 
scattered  by  animals  may  or  may  not  be  carried  far,  but  they  are  likely 
to  lodge  in  a  favorable  situation,  since  animals  of  a  given  species  tend  to 
frequent  similar  habitats;  wading  birds,  for  example,  fly  from  swamp 
to  swamp,  and  grazing  animals  scatter  hooked  fruits  in  places  similar 
to  those  in  which  they  were  gathered. 

Probably,  in  spite  of  its  wastefulness,  wind  is  the  most  efficient  of  dis- 
persing agents.  On  newly  formed  islands  the  pioneer  plants  of  the  in- 
terior portions  are  mainly  those  whose  disseminules  are  scattered  by 
wind,  a  smaller  number  being  scattered  by  birds,  while  the  shore  plants 
are  brought  largely  by  water  currents.  For  example,  on  the  island  of 
Krakatoa,  whose  vegetation  was  entirely  destroyed  by  a  volcanic  eruption 
in  1883,  the  first  plants  were  thallophytes  and  bryophytes  with  wind- 
borne  spores,  and  the  first  higher  plants  to  reappear  in  abundance  were 

1  However,  the  water  may  carry  seeds  so  far  that  the  new  climate  is  unsuited  for  de- 
velopment, as  in  the  West  Indian  seeds  carried  to  the  shores  of  Norway  by  the  ocean 
currents. 


REPRODUCTION   AND   DISPERSAL  927 

ferns,  whose  spores  are  readily  scattered  by  wind.  Fifteen  years  after 
the  eruption,  fifty-three  species  of  seed  plants  had  reached  the  island, 
and  of  these  it  was  estimated  that  60  per  cent,  chiefly  shore  species,  were 
brought  by  ocean  currents,  32  per  cent  by  wind,  and  8  per  cent  by  animals. 

The  dispersal  of  epiphytes  is  of  interest  because  of  the  difficulties  attending  the 
lodgment  of  disseminules  in  places  fit  for  germination.  Most  epiphytes  have  wind- 
scattered  disseminules,  as  in  the  spores  of  the  lichens,  mosses,  and  ferns,  or  the 
seeds  of  the  orchids  and  bromelias.  Most  such  disseminules  are  minute,  and, 
while  many  are  wasted,  a  few  find  lodgment  in  bark  crevices.  The  seeds  of  some 
epiphytes  are  scattered  by  birds,  as  is  the  case  also  with  many  of  the  pseudo-epi- 
phytes of  temperate  climates,  which  occur  in  soil  in  the  crotches  of  trees  (as  the 
raspberry,  gooseberry,  and  nightshade).  Mistletoe,  which  is  parasitic  on  trees, 
is  also  scattered  by  birds;  after  eating  the  enveloping  fleshy  rind,  the  slimy  seeds 
which  often  stick  to  their  bills  may  be  wiped  off  upon  the  limbs  where  they  are 
perched,  and  hence  in  places  suitable  for  germination. 

A  study  of  the  geographic  distribution  of  plants  shows  that  some 
species,  which  are  known  as  endemic,  are  confined  to  restricted  areas, 
and  that  other  species,  which  are  known  as  cosmopolitan,  are  almost 
wo  rid- wide  in  distribution;  the  members  of  a  third  class,  embracing  a 
much  greater  number  of  species,  occupy  relatively  large  but  not  world- 
wide areas.  It  might  be  supposed  that  the  size  of  the  area  occupied  by 
a  species  is  determined  by  its  means  of  dispersal,  but  this  is  not  obviously 
the  case.  While  many  mobile  species  (i.e.  those  with  easily  scattered 
disseminules)  are  widely  distributed  (as  in  the  willows  and  cat-tails), 
and  while  some  immobile  species  are  endemic  (as  in  Torreya),  there  are 
many  cases  in  which  the  reverse  is  true;  for  example,  the  immobile  oaks 
and  beeches  are  among  the  most  widely  distributed  trees,  while  the 
wonderfully  mobile  orchids  furnish  many  cases  of  endemism. 

In  explaining  the  distribution  of  species,  many  factors  other  than  the 
mobility  of  disseminules  are  to  be  considered.  An  important  element 
in  the  problem  is  time.  For  example,  even  though  the  oak  or  beech 
in  a  century  might  be  able  to  migrate  only  a  few  meters,  in  contrast  with 
as  many  kilometers  in  the  case  of  the  willows,  such  a  difference  is  of 
little  consequence  in  the  eons  of  geological  time.  Hence  it  may  be 
stated  as  a  somewhat  general  truth  that  the  rapid  occupation  of  a  new 
area  depends  largely  upon  the  mobility  of  plant  disseminules,1  but  that 

1  There  are  some  cases  of  rapid  migration,  where  the  disseminules  are  not  conspicu- 
ously mobile,  as  in  Galinsoga  parviflora  and  Artemisia  Stelleriana,  two  composites  without 
the  usual  hairlike  pappus,  which  have  spread  over  the  world  in  a  comparatively  few  years. 
Apparently  such  cases  are  associated  in  some  way  with  man,  whose  various  means  of 


928  ECOLOGY 

this  is  usually  a  matter  of  small  moment  in  determining  the  ultimate 
population.1  Geological  history  shows  that  the  endemism  of  Torreya, 
noted  above,  is  in  no  wise  due  to  disseminule  immobility,  for  it  was  once 
widely  distributed,  a  fact  that  suggests  that  the  most  important  of  all 
factors  in  distribution  may  be  the  fitness  of  a  species  to  exist  under  the 
given  conditions. 

The  origin  of  seed  structures.  —  Nothing  is  known  concerning  the 
factors  involved  in  the  origin  of  the  manifold  features  of  seeds  and  fruits 
which  fit  them  for  the  role  of  disseminules.  It  has  been  suggested  that 
these  features  have  arisen  through  natural  selection,  but  such  a  hypothe- 
sis seems  incredible  in  view  of  the  obvious  difficulty  in  grouping  seed 
plants  in  the  order  of  successfulness  in  such  a  way  as  to  show  a  definite 
relation  to  their  kinds  of  disseminules.  Even  in  the  annuals,  which 
depend  most  upon  seeds,  there  is  no  obvious  relation  in  most  cases  be- 
tween mobility  and  success.  Nor  is  anything  definitely  known  as  to  the 
factors  involved  in  seed  formation,  except  that  it  is  the  final  process  of 
the  series  initiated  by  flower  formation,  which  has  been  seen  to  be  facili- 
tated by  xerophytic  conditions  and  by  poor  nutrition.  The  seed  is  by 
far  the  most  xerophytic  structure  of  the  entire  series,  and  thus  may  bear 
a  definite  relation  to  the  causative  factors  of  the  reproductive  processes. 

The  planting  of  seeds.  —  While  gardeners  are  particular  as  to  the 
depth  at  which  seeds  of  various  sizes  are  planted,  there  is  no  such  sorting 
in  nature.  Large  and  small  seeds  alike  fall  to  the  ground  and  gradually 
become  covered  by  falling  leaves,  by  decaying  herbage,  or  by  soil  that 
is  deposited  by  winds  or  waters.  Doubtless  many  small  seeds  become 
buried  too  deeply  to  permit  of  successful  germination;  such  a  fate  is 
rarer  with  large  seeds,  except,  perhaps,  where  they  are  covered  by  the 
deep  alluvium  of  streams.  While  superficial  planting  doubtless  is  more 
favorable  for  small  seeds  than  for  large  seeds,  the  latter  may  none  the  less 
germinate  successfully  at  the  surface;  perhaps  the  chief  danger  in  the 
shallow  planting  of  large  seeds  is  that  there  may  not  be  sufficient  water 
for  germination. 

transportation  seem  to  have  made  up,  in  the  case  of  many  species,  for  any  natural  lack,  of 
disseminule  mobility. 

1  Where  similar  habitats  are  discontinuous,  as  in  oceanic  islands,  the  flora  may  be 
made  up  for  a  much  longer  time  than  elsewhere  of  plants  with  mobile  disseminules; 
the  preponderance  of  ferns  in  many  such  places  probably  is  thus  explained.  Yet  even  on 
Krakatoa,  a  quarter  of  a  century  has  been  long  enough  for  the  invasion  of  a  number  of 
species  with  apparently  immobile  disseminules,  whose  mode  of  migration  is  unknown. 
It  is  to  be  noted  that  one  seed,  however  extraordinary  its  mode  of  migration,  may  be 
sufficient  to  populate  a  new  area  with  an  abundant  vegetation. 


REPRODUCTION   AND   DISPERSAL 


929 


Seeds  that  pass  through  the  alimentary  tracts 
of  large  animals,  such  as  cattle,  are  planted  most 
advantageously  in  their  excrements,  where,  upon 
germination,  the  young  seedlings  find  an  excellent  sup- 
ply of  food  materials.     Nuts  buried  by  animals,  if  they 
chance  to  escape  being  eaten,  often  are  favorably  placed 
for  germination;  it  is  to  be  recalled  also  that  some  fruits 
mature  in  the  ground  (as  in  the  peanut  and  the  violet),  so  that 
favorable  planting  is  sure  to  result.     Some  seeds  and  fruits 
have  features  enabling  them  to  remain  attached  to  their  posi- 
tion on  the  ground,  notably  in  such  hooked  fruits  as  those  of 
the  cocklebur  and  the  burdock;  in  the  seeds  of  flax  and  mustard 
the  outer  layer  becomes  mucilaginous  when  moistened,  facili- 
tating adherence  to  the  substratum. 

A  remarkable  seed-planting  mechanism  is  seen  in  certain 
hygroscopic  fruits,  notably  in  the  porcupine  grass  (Stipa,  fig. 
1224).  Here  the  fruit  is  prolonged  below  into  a  sharp  spine 
that  is  clothed  except  at  the  tip  with  hairs  that  point  upward, 
while  above  there  is  a  long  awn  whose  basal  portion  coils  into  a 
close  spiral  when  exposed  to  desiccation,  and  uncoils  when 
moistened,  the  tissues  being  so  constructed  that  the  evaporation 
and  the  absorption  of  water  are  unequally  distributed.  If  the 
spine-tipped  base  sticks  into  the  ground,  the  repeated  twisting 
and  untwisting  of  the  awn  serve  to  bury  it  deeper  and  deeper 
in  the  soil,  the  upward-pointing  hairs  preventing  any  move- 
ment in  the  reverse  direction.  These  fruits 
are  such  efficient  penetrating  mechanisms  that 
they  work  readily  through  clothes  or  through 
envelopes  in  which  they  are  stored,  and  pene- 
trate even  into  the  flesh  of  grazing  animals. 
When  the  fruits  of  Stipa  lie  horizontally  on 
the  ground,  changes  of  moisture  result  in  a 
slow  creeping  movement  along  the  surface. 
Hygroscopic  fruits  similar  in  character  to 
those  of  Stipa  are  found  in  various  grasses  (as  Aristida  and  Avena)  and 
in  Erodium,  a  relative  of  the  geraniums. 


FIG.  1224.  —  A  mature 
fruit  of  the  porcupine  grass 
(Stipa  spartea},  showing  the 
seed-bearing  portion  (d)  and 
the  long,  spirally  twisted  awn 
(a);  the  basal  portion  or 
callus  (c)  is  stiff  and  sharp, 
and  is  clothed  with  bristles 
(6)  which  point  upward. 


CHAPTER   VI  —  GERMINATION 

Seed  characters  that  facilitate  or  retard  germination.  —  Introductory 
statement.  —  Probably  most  seeds  are  able  to  germinate  at  maturity,  if 
suitable  conditions  are  present.  However,  there  are  many  seeds  which, 
under  ordinary  natural  conditions,  require  the  lapse  of  a  longer  or 
shorter  period  before  germination  is  possible.  Such  delayed  germina- 
tion may  be  due  to  a  lack  of  actual  maturity  in  spite  of  appearances, 
or,  more  commonly,  to  enveloping  structures  that  retard  the  germinative 
processes.  There  are  some  seeds  which  germinate  in  natural  condi- 
tions the  moment  that  maturity  is  reached,  the  best  illustration  of  such 
a  habit  being  afforded  by  viviparous  plants. 

Vivipary.  —  Viviparous  plants  are  those  in  which  the  embryo  con- 
tinues in  a  state  of  uninterrupted  development  from  the  outset.  Since 
a  period  of  rest  between  two  periods  of  sporophyte  activity  is  the  chief 
distinguishing  feature  of  the  seed,  it  is  obvious  that  viviparous  plants  are 
essentially  seedless,  and  hence  do  not  in  the  usual  sense  exhibit  germina- 
ation.  The  best  examples  of  vivipary  are  the  mangroves  (especially 
Rhizophora  and  Bruguiera).  In  the  American  mangrove  (Rhizophora 
Mangle)  the  "  seedlings  "  develop  a  large,  green,  pointed  structure, 
mainly  a  greatly  enlarged  hypocotyl,  which  protrudes  from  the  fruit  (figs. 
1225,  1226),  and  which  finally  becomes  so  heavy  that  the  "seedling" 
drops  into  the  mud  beneath;  since  this  structure  is  heaviest  toward  the 
lower  end  and  is  much  more  massive  than  the  plumule,  the  "  seedling  " 
falls  right  side  up  into  the  mud  and  continues  growing,  soon  striking  root 
and  exhibiting  vigorous  plumule  development  (fig.  1227).  Vivipary  has 
been  regarded  as  advantageous  to  the  mangroves,  since  ordinary  seeds 
might  not  be  able  to  germinate  in  the  oozy  slime  beneath  the  trees. 
Currents  frequently  bear  the  fallen  "  seedlings  "  to  neighboring  shores, 
so  that  the  viviparous  habit  also  facilitates  dispersal;  as  the  young 
plants  float  in  an  erect  position,  they  readily  lodge  in  places  which 
are  suitable  for  further  growth. 

Some  alpine  plants  exhibit  vivipary,  notably  species  of  Poa  and  Polygonum, 
but  the  advantage,  if  any,  is  not  evident.  Somewhat  comparable  to  vivipary  is  the 

930 


GERMINATION 


931 


early  germination  of  seeds  within  the  fruit 
(as  in  the  lemon).  The  cause  of  vivipary  is 
unknown,  though  if  seed  formation  results 
from  increasing  xerophytism  or  from  decreas- 
ing nutrition,  vivipary  may  be  due  to  the 
continuance  of  conditions  favorable  to  vege- 
tative development  or  to  the  inception  of 
such  conditions  at  fruit  maturity.  This  idea 
seems  to  be  favored  by  the  fact  that  various 
grasses  exhibit  vivipary  in  wet  autumns,  and 
that  peas  and  beans,  when  vegetative  con- 
ditions are  favorable,  often  exhibit  uninter- 
rupted embryo  development.  Approaching 
such  vivipary  is  the  germination  of  seeds  while 
still  within  the  fallen  fruits  of  Typha  and 
A  ndropogon. 

Seed  maturity  and  germination.  —  A 
number  of  seeds  are  capable  of  germi- 
nation as  soon  as  they  are  shed ;  among 
such  are  those  of  the  willows,  the  sen- 
sitive plant,  and  many  cycads,  crucifers, 
and  grasses.1  It  is  a  matter  of  com- 
mon belief,  however,  that  most  seeds 
require  a  resting  period  of  some  weeks 
or  months  before  they  are  capable  of 
germination,  and  that  in  temperate  and 
in  cold  climates  germination  ensues 
only  after  a  period  of  rest  in  the 
ground,  coupled  with  exposure  to  low 
temperatures.  In  many  seeds  under 
ordinary  conditions  the  germi native 
capacity  may  improve  with  age,  a  cer- 
tain percentage  being  capable  of  germi- 
nating after  the  first  winter,  a  larger 
percentage  after  the  second  winter,  and 
in  a  few  instances  a  still  larger  percent- 
age after  the  third  winter;  it  is  said 
that  the  seeds  of  certain  conifers  are 

1  It  will  be  recalled  that  willow  seeds  soon  lose 
their  vitality,  especially  if  desiccated;  seeds  of 
the  sensitive  plant,  however,  have  been  known 
to  retain  their  vitality  for  sixty  years. 


FIGS.  1225—1227.  —  Vivipary  in  the 
mangrove  (Rhizophora  Mangle) : 
1225,  a  mature  fruit  attached  to  the 
tree,  the  basal  portion  of  the  embryo 
(r)  just  emerging;  1226,  a  later  stage 
in  which  the  young  plant  has  become 
so  heavy  that  it  falls  from  the  parent 
tree;  note  the  plumule  (/>)  and  the 
greatly  enlarged  basal  portion  of  the 
embryo  (r)  ;  1227,  a  stage  still  later, 
in  which  the  young  plant  has  rooted 
freely  in  the  mud  (r'),  the  plumule 
meanwhile  having  grown  vigorously 


932  ECOLOGY 

incapable  of  germination  for  several  years.  There  are  some  plants  (as 
the  cocklebur,  red  clover,  and  black  locust)  in  which  some  of  the 
seeds  appear  ordinarily  to  require  a  longer  time  than  do  others  before 
they  are  capable  of  germination.  It  is  probable  that  in  most  of  these 
cases  delay  in  germination  is  due  to  the  impermeability  of  the  testa  (see 
below).  Yet  it  is  conceivable  that  in  seeds,  as  in  buds,  various  maturing 
processes  take  place  after  the  attainment  of  apparent  maturity  ;  detach- 
ment from  the  carpel  and  apparent  rest  may  not  mean  the  cessation  of 
maturing  activities.  Possibly  the  delayed  germination  of  the  hawthorn 
(Crataegus)  is  to  be  thus  explained,  since  the  removal  of  the  testa  and 
exposure  to  good  germination  conditions  seems  for  a  certain  period  in- 
effective.1 The  most  remarkable  of  all  cases  of  delayed  germination  is 
afforded  by  the  spores  of  Lycopodium,  which  seem 
to  require  a  rest  of  three  to  fifteen  years  before  they 
are  able  to  develop. 

The  relation  of  the  testa  to  delayed  germination.  — 
The  common  cocklebur  (Xanthium  canadense)  has 
two   seeds   in   each  fruit,   differing  somewhat   in 
shape  and  in  position  (fig.  1228),  and  it  has  been 
found  that  the  seed  nearest  the  base  usually  germi- 
FIG.  1228.  _  A  coc-    nates  the  first  spring  after  maturation,  while  the 
kiebur    fruit    (Xan-    upper  seed   commonly  does    not    germinate    until 

thium)  in  longitudinal 


n   m  eg 

section,    showing    the  .  '  .     . 

position  of  the  two  germinate  long  before  others,  and  it  is  not  unlikely 
seeds  ;  note  that  the  that  in  some  cases  the  seeds  of  a  given  crop  may 
lower  seed  (/)  is  larger  germinate  over  a  period  of  tnree  or  more  years. 

than  the  upper    seed 

(«)  and  better  placed  Such  a  condition  seems  advantageous,  especially  m 
for  germination,  since  annuals,  since  it  insures  the  persistence  of  a  species, 
the  fruit  begins  to  de-  eyen  though  certam  seasons  prove  unfavorable  for 

cay  at  (c)  ;   p,  hooked  \ 

prickles  which  aid  in  see(^  development.  In  Xant/num,  it  has  been  shown 
dispersal.—  After  that  the  delayed  germination  of  the  upper  seed  is 

CROCKER  (drawn  from  due  to  the  fact  fchat  itg  tegta  jg  legs  permeable  to 
a  photographic  repro- 

duction). oxygen  than  is  that  of  the  lower  seed.     In  nature 

the  lower  seed  is  exposed  first  to  good  germinative 
conditions,  because  that  end  of  the  fruit  disintegrates  first.  In  various 
plants  (as  Abutilon,  Iris,  and  Axyris)  the  testa  (or  endosperm)  delays 
germination  because  it  excludes  the  necessary  water.  The  upper 

1  Even  in  Crataegus,  germination  has  been  brought  about  in  two  months  through  the 
removal  of  the  testa,  though  in  natural  conditions  it  usually  requires  a  year  and  a  half. 


GERMINATION  933 

seed  in  Xanthium  may  be  made  to  germinate  early  by  exposing  it  to 
high  temperatures  (32°  C.  to  34°  C.),  probably  because  the  absorp- 
tion of  oxygen  and  water  is  thus  facilitated;  if  the  testa  is  removed 
from  the  upper  seed,  it  germinates  as  readily  as  does  the  lower  seed, 
and  at  as  low  a  temperature  (22°  C.  to  24°  C.). 

It  has  been  seen  elsewhere  that  the  testa  is  chiefly  responsible  for 
prolonged  vitality  in  seeds,  and  it  is  here  seen  to  be  responsible  for  most 
cases  of  delayed  germination.  Longevity  obviously  is  advantageous, 
and  to  a  certain  extent  delayed  germination  also  may  be  advantageous, 
especially  in  annuals.  There  is  reason  to  believe,  however,  that  some 
seeds,  especially  among  xerophytes,  are  overprotected,  the  pericarp  or 
testa  being  so  impermeable  that  death  is  likely  to  occur  before  the  water 
and  the  oxygen  necessary  for  germination  have  an  opportunity  to  enter. 

The  relation  of  external  factors  to  germination.  —  It  is  a  matter  of  com- 
mon observation  that  water  and  moderately  high  temperatures  are  nec- 
essary for  the  germination  of  seeds,  and  very  simple  experiments  show 
the  equal  necessity  of  oxygen.  Nor  are  any  one  or  two  of  these  factors 
sufficient.  Seeds  on  a  dry  shelf  never  germinate,  in  spite  of  favorable 
temperatures,  nor  will  they  germinate  in  an  atmosphere  without  oxygen 
or  at  low  temperatures,  whatever  the  other  conditions.  Oxygen  and 
water  appear  to  be  directly  necessary  for  germination,1  the  oxygen  being 
necessary  to  combine  with  the  accumulated  foods,  thus  making  energy 
for  further  activity  available,  and  the  water  being  necessary  to  give  the 
requisite  dilution  to  the  cell  contents  to  permit  of  growth.  High  tem- 
peratures, however,  probably  are  of  value  only  as  they  facilitate  the 
absorption  of  water  and  of  oxygen. 

The  vigorous  respiration  of  developing  seedlings  is  in  striking  contrast  to  the 
weak  respiration  of  seeds,  germination  soon  ceasing  in  closed  chambers  from  lack 
of  oxygen.  The  favoring  influence  of  high  temperatures  is  well  shown  in  the  date, 
whose  seeds  germinate  in  a  few  days  in  a  hot  greenhouse,  otherwise  requiring  weeks 
or  even  months.  Until  recently  it  had  been  supposed  that  germination  takes  place 
equally  well  in  light  and  in  darkness.  Probably  it  is  true  that  many  seeds  are  indif- 
ferent to  the  presence  or  absence  of  light,  but  a  few  seeds  require  light  for  germina- 
tion (as  in  Viscum  and  in  several  species  of  Rhododendron)',  a  number  of  seeds  ger- 
minate better  in  light  than  in  darkness  (as  in  Poa  pratensis  and  Veronica  peregrina). 
On  the  other  hand,  there  are  some  seeds  whose  germination  is  retarded  by  light 
(as  in  Phacelia  tanaceti 'folia).  In  certain  mycophytes  and  parasites,  as  previously 
seen,  there  is  a  fourth  condition  necessary  for  germination,  namely,  contact  with  a 

1  In  a  few  cases,  as  in  rice  and  in  the  water  hyacinth,  no  oxygen  is  required  for  ger- 
mination. 


934  ECOLOGY 

suitable  host.  The  exact  factor  here  concerned  is  not  known,  though  it  may  be 
chemical  in  nature ;  in  certain  orchids  concentrated  solutions  may  replace  the  usual 
symbiotic  fungus.  Spores  in  general  require  germinative  conditions  similar  to  those 
of  seeds.  Where  the  spores  are  green,  as  in  mosses  and  ferns,  light  generally  is  re- 
quired for  germination,  though  in  most  cases  germination  may  be  induced  in  the 
darkness  by  proper  chemical  stimulation.  Many  fungus  spores,  especially  those  of 
parasites,  germinate  readily  in  water,  but  the  spores  of  many  saprophytes  require 
for  germination  the  presence  of  a  nutrient  medium. 

The  germinative  processes  of  seedlings.  —  Initiatory  activities.  — 
In  those  seeds  in  which  the  outer  layer  becomes  transformed  into  muci- 
lage upon  the  absorption  of  water  (as  in  flax  and  mustard),  this  is  the 
first  germinative  phase  to  be  observed.  Very  soon  the  seed  swells  notice- 
ably, owing  to  the  large  amount  of  water  absorbed.  The  outermost 
cells  become  active,  as  soon  as  their  contents  become  sufficiently  dilute; 
diastase  or  other  enzyms  are  secreted,  and  the  digestion  of  the  accumu- 
lated food  begins.  When  the  water  and  the  transformed  foods  reach 
the  embryo  and  incite  it  to  its  second  and  final  period  of  activity,  ger- 
mination proper  may  be  said  to  have  begun. 

The  digestion  and  the  absorption  of  foods.  —  The  digestive  processes 
are  observed  readily  in  the  grains  of  cereals,  as  in  wheat.  The  aleurone 
layer  (fig.  1211),  which  is  rich  in  protein,  first  shows  signs  of  life,  the  cells 
becoming  large  and  vacuolated  and  the  protoplasm  manifesting  activity. 
Soon  the  secretion  of  diastase  begins,  and  the  starch  next  to  the  aleurone 
layer  is  the  first  to  be  digested.  In  wheat,  maize,  and  other  grasses  there 
is  a  specialized  structure,  the  scutellum  (structually  the  cotyledon), 
which  greatly  facilitates  the  germinative  processes,  since  it  serves  as  a 
path  of  transfer  for  the  digested  foods  from  the  endosperm  to  the  devel- 
oping embryo;  often  on  the  side  next  to  the  endosperm  there  are  hair- 
like  absorptive  cells.  In  many  monocotyls  the  tip  of  the  cotyledon 
remains  in  the  seed  in  contact  with  the  food  and  may  be  regarded  as  an 
absorptive  organ  (fig.  1229).  In  the  date  the  tip  of  the  cotyledon 
enlarges  into  a  disk,  presenting  a  large  absorptive  surface  to  the  endo- 
sperm (fig.  1230).  In  those  seeds  in  which  the  accumulated  foods  are 
in  the  cotyledons,  specialized  absorptive  structures  are  less  likely  to  be 
present. 

The  amount  of  food  in  seeds  may  vary  from  almost  none  (as  in  many  parasites 
and  mycophytes)  to  such  large  quantities  as  are  found  in  the  coconut  and  the 
avocado  (Per sea  gratis sima).  In  cases  like  the  latter,  much  or  little  of  the  food  may 
be  utilized,  depending  upon  the  conditions  to  which  the  seedling  is  exposed 
upon  emergence  from  the  testa.  If  the  radicle  has  ready  access  to  moisture  and 


GERMINATION 


935 


the  plumule  to  light,  most  of  the  food  is  unnecessary  and 
may  gradually  decompose  in  the  ground ;  but  if  condi- 
tions for  autophytic  nutrition  are  less  favorable,  much  or 
even  all  of  the  food  may  be  used  by  the  seedling. 

Aspects  of  germination  external  to  the  seed.  — The 
earliest  conspicuous  external  index  of  germination 
is  the  rupture  of  the  testa  and  the  protrusion  of 
the  embryo.  The  time  necessary  for  such  pro- 
trusion, after  the  seeds  have  been  exposed  to 
proper  germinative  conditions,  varies  from  one  or 
two  days  (as  in  lettuce  or  mustard)  to  some  weeks 
or  months  (as  in  the  date).  Small  seeds  germinate 
more  quickly,  as  a  rule,  than  do  large  seeds,  prob- 
ably because  the  foods  are  digested  more  quickly 
through  easy  access  to  water  and  oxygen.  Starchy 

seeds  commonly  germinate  more  quickly  than  do     Monocotyl   seedlings: 

1229,   an  onion  seed- 
fatty    seeds,  and    much    more    quickly    than    do     ling  (AUium  Cepa)t  il- 

seeds  with  "  reserve  cellulose."  The  rupture  of  lustrating  epigaean 
the  testa,  which  usually  becomes  much  softened  germination ;  note  the 

curvature  of  the  coty- 

and  weakened  by  the  absorbed  water,  may  be  ie(ion  ^  whose  tip  re- 
effected  by  the  growing  radicle  or  by  the  coty-  mains  within  the  seed 
ledon,  as  in  many  monocotyls.  Sometimes  the  (s)'  acting  as  an  ab" 

sorptive  organ;    1230, 

embryo  emerges  through  thin  spots,  as  in  the  the  seedling  of  a  date 
coconut,  or  pushes  out  a  loosely  fastened  plug  palm  (Phoenix  dactyl- 
of  tissue.  Usually  the  radicle  is  the  first  part  of  ifera)  in  longitudinal 

section;    note  the  re- 

the  embryo  to  protrude,  and  this  is  doubtless  ad-  markabie  cotyledon  (c) 
vantageous,  since  most  seeds  contain  enough  food  whose  axis  elongates 
for  considerable  growth,  while  all  of  the  water  upon  germination;  one 

end   of   the  cotyledon 

must  come  from  without.  Often  (as  in  the  (5')  remains  within  the 
cocklebur)  the  radicle  is  so  situated  that  it  is  the  seed,  acting  as  an  ab- 
first  part  of  the  embryo  with  which  the  entering  so"ptive  °rgan;t.  the 

other    end    continues 

water  comes  in  contact,1  and  the  absorption  of  for  a  time  to  enclose 
water  from  the  soil  by  the  young  root  system  the  plumule  (/>)  and 
usually  is  well  initiated  before  the  external  de-  ^^f^'""1"10111 
velopment  of  the  plumule  becomes  prominent. 

In  some  cases  the  cotyledons  remain  in  the  soil,  especially  where 
these  organs  are  the  chief  seat  of  accumulated  food,  as  in  oaks  and 

1  The  significance  of  the  position  of  the  radicle  in  the  seed  of  the  cocklebur  is  seen  from 
the  fact  that  if  a  bit  of  the  testa  is  removed  near  the  tip  of  the  cotyledons,  growth  begins 
at  that  point,  the  radicle  then  being  the  last  part  to  react. 


ECOLOGY 


peas,  and  also  in  the  cereals,  where  the  scutellum  represents  the  coty- 
ledon (figs.  703,  706) ;  such  germination  is  termed  hypogaean.  In  other 
cases  the  cotyledons  emerge  from  the  seed  with  the  plumule  and  come 
into  the  light,  usually  turning  green,  as  in  the  beech  (fig.  1231),  pump- 
kin, and  mustard  (fig.  700);  such  germination  is  termed  epigaean. 
In  hypogaean  species  the  cotyledons  die,  as  soon  as  the  foods  are  re- 
moved, or  the  seedling  is  well  established  as  an  autophyte.  The  same 
is  true  in  some  epigaean  species,  as  in  various  beans,  but  in  many  other 
epigaean  forms  the  cotyledons  enlarge  and  doubt- 
less manufacture  considerable  food.  Cotyledons 
are  much  more  uniform  in  shape  than  are  ordi- 
nary leaves,  perhaps  because  of  the  relatively 
uniform  conditions  in  which  they  are  developed. 
Usually  they  are  undivided,  though  occasionally 
divided,  as  in  Tilia.  In  the  monocotyls  the  en- 
closure of  the  delicate  part  of  the  plumule  within- 
its  older  sheathing  leaves  prevents  injury  in 
breaking  through  the  soil.  In  many  epigaean 
dicotyls  the  cotyledons  adhere  at  the  tips,  pro- 
tecting the  plumule  until  it  emerges  from  the 
ground.  In  other  cases  (as  in  the  pumpkin)  the 
hypocotyl  elongates  considerably,  while  the  coty- 
ledons remain  within  the  seed,  resulting  in  an 
arching  of  the  young  stem  and  in  the  pulling 
FIG.  1231.  — A  seed-  of  the  delicate  tip  from  the  seed  and  through  the 
ling  of  a  dicotyl,  the  ground  instead  of  pushing.  Where  seeds  germi- 


beech *  (Fagus  grftndi- 
folia),  illustrating  epi- 
gaean germination ;  c, 


nate  too  near  the  surface,  the  contraction  or  other 
movement  of  the  growing  root  exerts  a  pull  on 


cotyledons ;  /,  first  foil-     the  shoot,  so  that  the  position  proper  to  the  species 

is  eventually  acquired. 

The  germinative  processes  of  buds.  —  The  structural  features  of  buds.  — 
Buds  commonly  are  divided  into  two  classes,  active  and  resting.  Active 
buds  are  associated  with  all  seasons  in  uniform  climates  and  with  the 
vegetative  seasons  of  periodic  climates,  while  resting  buds  are  associated 
with  the  unfavorable  seasons  of  periodic  climates.  Germinative  processes 
are  conspicuous  as  resting  buds  develop  into  active  buds.  The  resting 
buds  of  shrubs  and  trees,  commonly  called  winter  buds  in  temperate 
and  in  cold  climates,  are  protected  by  tough  and  impermeable  bud 
scales,  whose  structure  and  role  have  been  considered  elsewhere  (p.  643). 


GERMINATION 


937 


Within  the  bud  scales  are  delicate  embryonic  leaves  that  are  most  eco- 
nomically arranged  as  to  utilization  of  space,  a  particular  kind  of 
arrangement,  known  as  vernation,  often  being  characteristic  of  special 
plant  groups. 

Leaves  in  the  bud  may  be  plane  ;  conduplicale,  or  folded  inward,  as  in  the  bean ; 
plicate,  or  folded  in  pleats,  as  in  the  beech;  crumpled,  as  in  the  poppy;  involute, 
or  with  the  halves  rolled  inward,  as  in  the  violet;  revolute,  or  with  the  halves  rolled 
outward,  as  in  the  dock;  convolute,  or  with  the  leaf  rolled  from  one  margin  to  the 
other,  as  in  the  canna ;  or  circinate,  that  is,  with  the  leaf  rolled  inward  on  itself  from 
the  apex  downwards,  as  in  ferns  (fig.  382).  It  is  believed  that  the  kind  of  verna- 
tion is  due  in  part  at  least  to  the  limitations  of  space  within  the  bud ;  by  experi- 
mentally restricting  this  space,  the  leaves  of  Prunus  which  usually  are  flat  become 
crumpled,  and  by  cutting  the  stipules,  the  leaves  of  Magnolia  become  flat. 

External  factors  in  relation  to  bud  germination.  —  The  germination  of 
winter  buds  is  associated  largely  with  spring,  and,  as  with  seeds,  it  takes 
place  when  the  temperature  becomes  sufficiently  high  to  permit  water 
to  enter  the  embryonic  shoot  in  abundance  and  to  incite  it  to  activity. 
Buds  differ  from  seeds  in  being  em- 
bryonic shoots  rather  than  embryonic 
plants,  and  usually  also  in  remaining 
attached  to  the  plants  that  bear  them 
(except  in  the  winter  buds  of  water 
plants),  the  water  used  in  germina- 
tion thus  coming  from  the  plant  rather 
than  from  the  ground.  As  shown 
elsewhere,  buds  often  appear  to  be 
mature  before  they  are  capable  of  ger- 
mination, the  maturation  process  seem- 
ing to  consist  in  part  in  the  accumula- 
tion of  food.  Germination  may  be 
hastened  by  placing  a  plant  or  even 
a  branch  indoors  in  winter.  Only  local 
stimuli  are  needed  for  germination,  as 
is  shown  by  growing  a  single  branch 

detached   from   a  plant   and   placed  in      Cicuta  bulbifera;    the  first  bladeless 

water  indoors,  or  by  training  a  branch     leaves  or  phyll°des  (/>)  are  followed 

/•  .    .  , .   .    .         ,  by  foliage  leaves  ( /) :  b,  bulbil. 

from  a  tree  into  an  adjoining  house, 

or  even  by  supplying  favorable  temperatures  locally  to  a  part  of  a 

plant  outside,  as  by  bending  a  willow  branch  into  a  sheltered  position; 


1232 


1233 


FIGS.  1232,    1233. —  Bulblings    of 


93* 


ECOLOGY 


because  of  locally  favorable 
temperatures,  buds  germinate 
soonest  on  sunny  slopes,  and 
willows  on  the  tundra  come 
into  flower  while  their  roots 
are  still  in  frozen  soil.  The 
resting  buds  of  herbaceous 
plants,  represented  by  bulbs 
and  bulbils  (the  latter  de- 
veloping into  bulblings,  figs. 
1232,  1233),  and  by  the  buds 
of  tubers  (fig.  1037),  of  rhi- 
zomes, and  of  multicipital 
herbs,  germinate  under  con- 
ditions similar  to  those  that 
incite  activity  in  the  resting 
buds  of  trees  and  shrubs. 

Climate  and  bud  development.  — 
As  a  rule,  trees  and  shrubs  with 
large  buds  (as  the  poplars,  willows, 
and  alders,  fig.  1234)  develop  vig- 
orous shoots  early  in  spring,  while 
species  with  small  buds  (as  the  ca- 
talpa  and  the  honey  locust)  develop 
much  later.  Such  differences  seem 
to  follow  from  the  fact  that  in  the 
former  the  embryonic  shoots  are 
much  the  more  advanced  while  still  within  the  resting  bud.  Many  of  the  large- 
budded  forms  which  thus  develop  at  the  inception  of  spring  are  northern  species, 
and  such  habits  seem  very  advantageous  for  far  northern  plants,  owing  to  the  short- 
ness of  the  vegetative  season.  On  the  other  hand,  where  the  vegetative  season  is 
long,  small  buds  seem  advantageous,  owing  to  their  exemption  from  the  develop- 
ment of  extensive  protective  structures.  Some  large-budded  trees,  such  as  the 
hickory,  are  both  late  in  germinating  and  relatively  southern  in  distribution. 


FIG.  1234.  —  A  branch  of  the  European  alder 
(Alnus  glutinosa)  in  its  winter  aspect ;  5,  the  buds 
of  staminate  inflorescences ;  p,  the  buds  of  pistil- 
late inflorescences ;  /,  fruit  cones  of  the  preceding 
year;  /,  leaf  buds. 


CHAPTER   VII  — PLANT  ASSOCIATIONS 

Definition.  —  In  the  preceding  chapters,  plants  have  been  considered 
as  individuals  having  certain  relations  to  their  physical  surroundings, 
or  to  each  other.  While  plants  sometimes  occur  as  isolated  individuals, 
they  are  associated  far  more  commonly  in  more  or  less  definite  groups. 
So  true  is  this  that  when  one  who  is  familiar  with  nature  sees  a  given 
species  in  the  field,  he  comes  almost  instinctively  to  look  for  other  species 
that  he  has  seen  associated  with  it.  If  he  is  observant,  he  looks  a 
step  further  and  finds  that  these  associated  species  also  are  associated 
with  a  definite  kind  of  habitat.  For  example,  pitcher  plants,  sundews, 
cranberries,  and  peat  moss  grow  together  in  imperfectly  drained  swamps 
known  as  bogs  or  moors  ;  beach  peas,  sea  rocket,  and  beach  grass 
grow  together  on  sandy  coasts;  beech,  maple,  beech  fern,  and  beech- 
drops  grow  together  in  mesophytic  forests.  A  group  of  plants  in  its 
entirety  occurring  in  a  common  habitat  is  known  as  a  plant  association. 
Sometimes  the  association  of  specific  plants  is  obligate,  as  in  the  case 
of  the  beechdrops,  which  grows  parasitically  on  the  beech,  while  the 
beech  in  turn  appears  to  require  certain  fungi  in  the  soil.  More 
commonly,  however,  the  association  is  purely  facultative.  Pitcher 
plants  and  sundews  or  maples  and  beeches  grow  together,  because 
they  thrive  in  similar  conditions;  so  far  as  is  known,  the  presence 
or  the  absence  of  one  is  a  matter  of  no  particular  consequence  for 
the  other,  except  as  it  occupies  or  leaves  a  certain  amount  of  space. 

The  kinds  of  associations.  —  Plant  associations  have  been  variously 
classified,  the  simplest  grouping  being  based  on  the  water  relation,  and 
the  large  divisions  being  termed  hydrophyiic,  mesophytic,  and  xerophytic, 
while  these  in  turn  are  subdivided  into  various  groups  of  associations. 
This  classification,  though  advantageous  because  of  its  ready  applica- 
tion, has  the  great  disadvantage  of  grouping  together  associations  that  are 
entirely  unrelated  in  origin,  such  as  those  of  bogs  and  ordinary  swamps, 
while  separating  closely  related  associations,  such  as  those  of  bogs  and 
of  the  coniferous  forests  into  which  they  commonly  develop.  Though 
it  is  more  difficult  to  apply,  there  are  many  advantages  in  a  genetic  classir 

939 


940  ECOLOGY 

fication,  that  is,  one  which  groups  associations  in  a  series  in  their  order 
of  development. 

Succession.  —  The  basis  of  a  genetic  classification  is  the  principle  of 
succession,  namely,  that  in  the  physiographic  development  of  a  region  the 
various  habitats  pass  through  a  series  of  more  or  less  definite  stages, 
owing  chiefly  to  the  processes  of  erosion  and  deposition,  supplemented 
by  the  accumulation  of  humus.  The  primitive  associations,  that  is, 
those  of  new  lands  or  waters,  are  likely  to  be  either  xerophytic  or  hydro- 
phytic.  In  a  region  with  a  mesophytic  climate,  the  primitive  associa- 
tions become  displaced  by  others  that  are  slightly  more  mesophytic, 
and  they  in  turn  by  others,  until  the  series  finally  culminates  in  the 
most  mesophytic  association  of  which  the  region  as  a  whole  is  capable. 
For  example,  in  the  eastern  United  States  an  upland  of  rock,  sand,  or 
clay,  whose  original  flora  is  xerophytic,  becomes  gradually  more  and 
more  mesophytic,  either  through  land  denudation  or  humus  accumula- 
tion or  both,  until  it  becomes  clothed  with  the  ultimate  plant  association 
of  the  region,  namely,  a  deciduous  mesophytic  forest.  A  pond  in  the 
same  region  gradually  becomes  filled  through  humus  accumulation  or 
through  stream  and  shore  deposition,  or  both,  so  that  the  original  aquatic 
vegetation  becomes  displaced  by  a  swamp  vegetation,  and  this  in  turn 
through  further  humus  accumulation  becomes  displaced  by  a  forest 
quite  comparable  to  that  which  marks  the  final  stage  in  an  upland  suc- 
cession. In  arid  or  semi-arid  climates  it  is  obvious  that  the  final  stage 
could  not  be  mesophytic,  but  would  necessarily  be  an  association  which 
is  much  nearer  the  primitive  xerophytic  association  of  the  region. 

The  scope  of  this  book  forbids  any  attempt  at  a  detailed  classification 
of  plant  associations.  The  general  principles  enunciated  above  must 
suffice.  In  the  remaining  paragraphs  of  this  chapter  there  will  be  pre- 
sented some  of  the  more  striking  features  of  a  few  of  the  more  impor- 
tant plant  associations,  especially  of  those  that  are  found  in  the  United 
States,  but  no  attempt  will  be  made  to  bring  out  genetic  relationships 
or  to  make  exhaustive  analyses. 

Pond  associations.  —  Perhaps  the  most  representative  fresh- water 
associations  are  those  of  ponds,  and  these  are  among  the  most  interest- 
ing of  all  associations,  partly  because  they  are  more  likely  to  remain 
natural  than  are  most  habitats  in  densely  populated  districts,  but  espe- 
cially because  they  show  obvious  and  rapid  stages  in  succession  between 
the  primitive  aquatic  associations  and  the  various  sorts  of  swamps.  The 
vegetation  of  ponds  consists  usually  of  free-floating  forms  (including 


PLANT   ASSOCIATIONS 


941 


many  algae  and  some  higher  plants)  and  of  forms  attached  to  the  bot- 
tom ;  of  the  latter  some  forms  are  submersed,  some  have  floating  leaves, 
and  still  others  are  in  part  emersed.  Aquatic  plants  or  hydrophytes, 
especially  those  that  are  submersed,  have  many  noteworthy  structural 
peculiarities  that  have  been  separately  noted  on  previous  pages,  but 
which  may  here  be  summarized. 

The  root  systems  commonly  are  reduced,  both  in  length  and  in  amount 
of  branching,  and  root  hairs  are  absent,  at  least  in  the  water.  True 
water  roots  are  hairless,  and  may  possess  root  pockets.  The  chloren- 
chyma  is  spongy  and  but  slightly  differentiated,  and  usually  the  plastids 
are  large  and  motile.  The  leaves  of  submersed  plants  are  very  thin  and 
often  are  finely  dissected.  Air  chambers  are  capacious,  often  exceeding 
the  tissues  in  actual  volume.  Stomata  are  absent  in  submersed  leaves 
and  are  present  only  on  the  upper  surfaces  of  floating  leaves;  where 
present  in  floating  or  in  emersed  leaves,  they  have  but  slightly  cutinized 
walls  and  are  almost  always  open.  Protective  features  are  few  or  want- 
ing; for  example,  cutin  and  cork  rarely  are  developed  below  the  water 
surface,  hairs  are  scarce,  and  the  cell  sap  has  a  low  osmotic  pressure; 
the  absence  of  protective  structures  is  not  disadvantageous,  since  ab- 
sorption is  easy,  and  below  the  water  level,  transpiration  is  slight  or  even 
absent.  Leaves  equal  or  surpass  roots  in  importance  as  absorptive 
organs.  Submersed  organs  usually  are  slime-covered,  the  slime  har- 
boring commensalistic  communities  of  bacteria  and  other  low  organ- 
isms. The  aerial  surfaces  of  floating  organs  usually  are  wax-coated  and 
thus  are  not  readily  wetted.  Conductive  and  mechanical  tissues  are 
greatly  reduced.  Vegetative  reproduction  is  highly  developed,  both 
through  the  fragmentation  of  ordinary  shoots,  and  through  the  develop- 
ment of  winter  buds.  In  the  algae,  reproduction  and  dispersal  are 
facilitated  by  zoospores  and  by  motile  gametes.  Among  the  higher 
plants,  flowers  and  seeds  are  less  abundant  than  in  most  habitats. 

Swamps.  —  Various  swamp  stages  in  turn  follow  the  primitive  pond 
associations,  bulrushes,  cat-tails,  and  reeds  often  being  among  the  first 
emersed  plants,  and  sedges  are  often  prominent  later.  In  mesophytic 
climates,  thickets  (as  of  willows  and  alders)  soon  appear,  and  they  in 
turn  are  replaced  by  forest  vegetation.  The  structural  features  of  swamp 
plants  are  in  part  like  those  noted  above,  especially  in  the  matter  of  re- 
duced root  systems  and  prominent  air  chambers,  but  in  general  they  are 
not  unlike  those  seen  in  mesophytes;  particularly  is  this  true  of  leaf 
thickness,  stomata,  chlorenchyma,  and  protective  structures,  The 


942  ECOLOGY 

roots  frequently  are  horizontal  or  even  ascending  rather  than  descend- 
ing. Rhizomes  are  greatly  developed,  accounting  in  large  part  for  the 
rapid  invasion  of  ponds  by  swamp  plants.  A  somewhat  remarkable 
feature  is  the  abundant  development  of  vertical  chlorophyll-bearing 
organs,  whether  leaves  (as  in  the  flags)  or  stems  (as  in  the  rushes). 
Among  the  most  plastic  of  plants  as  to  leaf  form  and  structure  are  the 
amphibious  plants;  in  view  of  the  rapid  transformation  of  ponds  into 
swamps,  such  plasticity  permits  certain  species  to  dominate  in  two  dis- 
tinct successional  stages. 

Bogs  or  moors.  —  The  mature  vegetation  of  a  peat  bog  contrasts  most 
strikingly  with  that  of  an  ordinary  swamp,  although  the  early  stages  may 
be  quite  the  same  in  both  cases.  While  a  number  of  plants  are  common 
to  swamps  and  bogs,  there  are  many  kinds  of  plants  which  are  more  or  less 
peculiar  to  bogs,  the  most  noteworthy  being  those  with  such  xerophytic 
features  as  prominent  palisade  tissues  and  cutin,  dwarfness  of  habit, 
and  high  osmotic  pressure.  Among  the  bog  xerophytes  are  many  ericads 
(such  as  the  cranberry,  leather  leaf,  and  Labrador  tea)  and  conifers; 
that  some  of  the  bog  plants  are  true  xerophytes  is  shown  by  the  fact  that 
a  number  of  species  are  common  to  bogs  and  to  dry  rocky  cliffs.  The 
peat  moss  (Sphagnum)  is  especially  characteristic  of  bogs,  as  are  many 
orchids,  and  it  is  in  bogs  that  most  carnivorous  plants  are  found.  As 
compared  with  mesophytic  habitats  or  with  ordinary  swamps,  bogs 
present  conditions  that  are  deleterious  for  the  majority  of  plants ;  indeed, 
some  of  the  plants  which  are  characteristic  of  bogs  (notably  the  tamarack) 
thrive  much  better  elsewhere,  suggesting  that  they  "  tolerate  "  bogs 
rather  than  "  select  "  them.  An  analysis  of  the  bog  problem  is  beyond 
the  scope  of  this  book,  but  some  points  that  bear  on  the  matter  have  been 
suggested  elsewhere  (p.  537). 

Maritime  associations.  —  Plants  that  grow  in  salt  water  or  in  salty  soil 
have  been  denominated  halophytes.  The  submersed  halophytes  are 
chiefly  algae,  which  sometimes  reach  gigantic  size,  and  which  differ  in 
color,  being  green,  red,  or  brown.  Most  of  the  larger  algae  are  attached 
to  rocks  by  anchoring  organs,  namely,  the  holdfasts  or  rhizoids;  some 
rise  and  fall  with  the  tide,  bladders  filled  with  air  often  facilitating  their 
buoyancy.  Salt  marshes  show  stages  in  succession  comparable  to  those 
of  ponds,  but  the  species  involved  are  very  different.  Emersed  halophytes 
are  strikingly  xerophytic  in  their  characteristic  features,  palisade  tissue 
being  prominently  developed,  and  often  the  epidermis  is  highly  cutinized. 
The  most  striking  feature  of  salt  marsh  halophytes,  taken  as  a  class, 


PLANT   ASSOCIATIONS  943 

is  their  succulence,  which  is  accompanied  by  a  very  high  osmotic  pres- 
sure. In  temperate  regions  the  most  representative  salt  marsh  plants 
are  herbaceous,  but  in  the  tropics  extensive  mangrove  forests  are  found 
in  similar  conditions ;  few  plants  show  more  marked  xerophytic  features 
than  do  the  mangroves,  which  have  evergreen  leaves  with  water  tissue, 
prominent  palisade  cells,  and  thick  cutin.  Often  there  is  a  network  of 
prop  roots  above  the  water  line,  and  in  some  cases  there  are  ascending 
"knees"  (fig.  726). 

Xerophytic  associations.  —  The  characteristic  features  of  xerophytes.  — 
In  most  respects  xerophytes  are  the  reverse  of  hydrophytes  in  their 
structural  features.  The  roots  frequently  are  strongly  developed  (though 
not  in  cacti),  possessing  either  considerable  length  or  great  size;  roots  of 
the  latter  class  accumulate  large  amounts  of  water  and  food.  In  some 
extreme  xerophytes  the  root  hairs  extend  to  the  root  tips,  and  in  certain 
cases  they  possess  rigid  thickened  walls.  Palisade  tissue  is  strongly 
developed,  and  the  chlorenchyma  in  the  leaves  and  stems  commonly  is 
deeply  sunken,  giving  them  a  pale  tint  as  viewed  from  without;  usually 
the  plastids  are  small  and  relatively  immotile. 

Protective  features  are  remarkably  developed  both  in  amount  and  in 
kind,  and  their  advantage  is  undoubted,  owing  to  the  great  exposure  of 
xerophytes  to  transpiration.  The  transpiring  surface  usually  is  rela- 
tively reduced,  the  leaves  being  small  and  thick.  Many  species  are 
leafless,  the  cylindrical  stems  exposing  a  relatively  small  surface  to  trans- 
piration, while  their  vertical  orientation  affords  some  protection  from 
the  intense  rays  of  light  at  midday ;  species  with  vertical  leaves  are  sim- 
ilarly protected.  In  many  cases  there  is  a  temporary  reduction  of  sur- 
face, as  in  the  involute  leaves  of  grasses,  as  in  those  legumes  whose  leaves 
close  in  dry  weather,  and  as  in  the  "  resurrection  plants."  Temporary 
reduction  of  surface  is  exhibited  also  by  plants  which  shed  their  leaves 
or  stems  during  dry  periods ;  annuals,  which  die  at  the  beginning  of  dry 
periods,  represent  the  culminating  form  of  such  behavior.  Dwarf- 
ness  of  habit  is  a  prominent  xerophytic  feature,  the  resulting  compactness 
in  arrangement  of  branches  and  leaves  and  the  closeness  to  the  ground 
affording  considerable  protection. 

The  more  minute  structural  features  of  xerophytes  are  no  less  signifi- 
cant than  are  the  more  obvious  characters.  Commonly  the  epidermis 
is  thick  and  highly  cutinized  (except  in  succulent  xerophytes),  and  often 
it  is  superficially  coated  with  wax,  resin,  or  varnish.  In  woody  stems 
there  is  a  prominent  bark  development,  the  cork  in  particular  being  of 


944  ECOLOGY 

high  significance  in  checking  transpiration.  The  leaf  and  stem  sur- 
faces frequently  are  covered  with  hairs;  spinescence  also  is  common, 
though  its  protective  significance  may  not  be  important.  The  stomata 
occur  mainly  on  the  more  protected  (chiefly  the  under)  surfaces,  and 
often  are  at  the  bases  of  pits  and  specially  protected  by  hairy  coats  or  by 
cutinized  walls ;  as  a  rule,  they  are  not  wide  open.  Many  xerophytes  are 
succulent,  containing  large  amounts  of  colorless  sap  or  of  latex ;  oils  and 
resins  often  are  abundantly  developed.  The  osmotic  pressure  of  the  cell 
sap  often  is  very  high,  especially  in  shrubs  and  in  plants  of  alkaline  soil. 
The  conductive  tracts  are  prominent,  the  vessels  being  larger  and  longer 
and  the  walls  thicker  than  in  most  plants ;  lignification  is  prominent, 
and  annual  rings  are  well  developed.  Bast  fibers  and  other  mechanical 
elements  reach  their  highest  development  in  xerophytes. 

Some  xerophytes,  particularly  the  lichens,  appear  wanting  in  prominent 
xerophytic  structures,  seeming  able  to  withstand  prolonged  desiccation 
without  injury.  Apart  from  the  lichens  and  mosses,  absorption  through 
aerial  organs  is  relatively  rare  in  xerophytes,  though  some  of  the  epi- 
phytic leaf-absorbing  bromelias  grow  in  dry  climates.  Tubers,  corms, 
and  bulbs  especially  characterize  arid  climates,  and  it  is  obvious  that 
their  ability  to  develop  rapidly  at  the  inception  of  a  rainy  season  is  a 
character  of  great  advantage.  Xerophytic  conditions  usually  are  be- 
lieved to  favor  the  formation  of  flowers  and  fruits. 

Characteristic  xerophytic  associations.  —  Perhaps  the  most  repre- 
sentative xerophytic  region  is  the  desert,  and  it  is  here  that  the  features 
above  mentioned  reach  their  most  pronounced  development.  In  general 
the  severity  of  the  desert  conditions  increases  as  the  rainfall  decreases, 
it  being  common  to  distinguish  half  deserts,  such  as  the  sagebrush  plains, 
from  the  more  extreme  deserts,  such  as  those  in  which  such  plants  as 
the  cacti,  or  the  creosote  bush,  are  dominant  forms.  Still  more  extreme 
are  the  alkali  deserts,  in  which  excessive  climatic  aridity  is  supplemented 
by  a  soil  in  which  concentrated  salts  make  absorption  difficult.  Suc- 
culents with  sap  of  high  osmotic  pressure  seem  best  fitted  for  existence 
under  such  conditions.  There  are  habitats  where  the  alkalinity  is  so 
great  that  plant  life  is  almost  if  not  quite  excluded. 

Many,  but  not  all,  alpine  and  arctic  habitats  have  plants  whose  struc- 
tures are  chiefly  xerophytic.  Even  though  there  is  an  abundant  supply 
of  water,  the  soil  often  is  so  cold  that  absorption  is  difficult;  conse- 
quently plants  without  xerophytic  structures  are  poorly  fitted  for  such 
habitats,  except  in  alpine  meadows  or  in  similar  situations,  where  the 


PLANT   ASSOCIATIONS  945 

protective  mantle  of  snow  prevents  the  loss  of  water  by  transpiration  in 
the  seasons  during  which  absorption  is  impossible.  The  leaves  of  alpine 
and  arctic  xerophytes  are  largely  sclerophyllous,  hairy  leaves  being  rather 
less  abundant  than  in  deserts.  The  perennial  habit  is  almost  universal, 
the  shortness  of  the  season  scarcely  permitting  annuals  to  complete  their 
life  cycle.  Anthocyans  are  prominently  developed,  resulting  in  showy 
leaves  and  flowers.  Palisade  tissue  is  more  pronounced  in  alpine  than 
in  arctic  xerophytes. 

In  climates  where  the  rainfall  and  the  temperature  are  such  as  to  facili- 
tate their  development,  there  are  many  habitats  where  the  local  condi- 
tions prevent  for  a  time  the  appearance  of  mesophytes.  Among  such 
habitats  rocky  and  sandy  areas  are  of  first  importance.  In  rocky  regions 
the  pioneer  plants  often  are  lichens,  which  are  able  to  grow  on  the  bare 
rock  surface ;  with  these  come  many  crevice  plants.  By  the  accumula- 
tion of  humus,  the  growth  of  other  plants  is  made  possible,  and  after  a 
time  a  thicket  develops,  and  later  a  forest,  in  which  pines  and  junipers 
may  play  an  important  part.  In  sandy  regions  the  instability  of  the 
soil  usually  inhibits  the  development  of  a  pioneer  lichen  stage,  whereas 
xerophytic  herbs  and  shrubs  make  their  appearance  sooner  than  on 
rocks.  The  subsequent  stages  on  rock  and  sand  are  much  the  same. 

Mesophytic  associations.  —  Mesophytes  in  their  structural  charac- 
teristics are  in  many  respects  intermediate  between  hydrophytes  and 
xerophytes.  There  is  a  prominent  development  of  progeotropic  roots 
with  abundant  root  hairs.  The  foliage  reaches  a  maximum  develop- 
ment, and  the  leaves  are  relatively  large  and  thin,  while  the  thin  trans- 
parent epidermis  and  the  abundant  chlorophyll  together  cause  the  leaves 
to  appear  dark  green.  Stomata  usually  occur  on  both  leaf  surfaces,  ex- 
cept in  trees,  and  the  guard  cells  possess  a  maximum  capacity  for  move- 
ment. Cutinization  is  moderate,  except  in  such  evergreens  as  the  hem- 
lock and  the  India-rubber  tree.  The  under  epidermis  commonly  has 
wavy  lateral  walls,  contrasting  thus  with  the  straight  walls  in  hydrophytes 
and  xerophytes. 

The  most  representative  mesophytic  associations  are,  on  the  one  hand, 
various  forests,  and,  on  the  other,  certain  grasslands.  The  mesophytic 
forests  as  a  class  are  the  culminating  vegetation  of  any  region  where  they 
grow,  since  they  represent  not  only  the  most  luxuriant  kind  of  plant 
association,  but  also  because  they  form  the  terminal  member  of  the  suc- 
cessional  series  in  such  climates  as  are  humid  enough  to  support  them  over 
extensive  areas.  It  is  in  the  mesophytic  forests  that  humus  accumula- 


946  ECOLOGY 

tion  above  the  water  level  reaches  its  maximum,  and  on  this  account  the 
soil  is  more  uniformly  moist  than  in  other  land  habitats.  Furthermore, 
the  rich  supply  of  humus  makes  possible  a  wealth  of  saprophytic  fungi 
and  bacteria,  leading  to  mycosymbiosis  and  to  other  symbiotic  relations. 

The  most  luxuriant  of  mesophytic  forests  is  the  rain  forest  of  the 
tropics,  which  is  characterized  by  the  dense  crowding  of  individual 
plants,  resulting  in  the  maximum  occupation  of  space.  Not  only  are 
there  ordinary  trees,  shrubs,  and  herbs,  but  lianas  often  are  abundant, 
while  epiphytes  cover  the  limbs  and  even  develop  on  the  leaves  of  many 
trees.  The  trees  often  are  slender  and  smooth-barked,  and  the  leaves 
are  characteristically  evergreen.  The  epiphytes  include  ferns  and 
orchids,  the  latter  with  characteristic  absorptive  roots  and  xerophytic 
leaves  or  stems ;  the  leaves  of  many  of  the  trees  appear  xerophytic  also. 
In  north  temperate  regions  there  are  extensive  mesophytic  forests  that 
either  are  deciduous,  as  in  the  eastern  United  States,  Europe,  and  Japan, 
or  evergreen,  as  in  the  northwestern  United  States.  In  these  forests  the 
tree  species  are  relatively  few  in  number,  as  compared  with  the  tropical 
forests,  and  the  trees  often  are  large  and  rough-barked.  The  epiphytic 
vegetation  consists  chiefly  of  lichens,  liverworts,  and  mosses.  Fre- 
quently mesophytic  areas  are  treeless,  as  in  some  prairies  and  in  alpine 
meadows,  where  grasses  and  herbage  dominate  the  landscape. 

The  influence  of  man  upon  vegetation.  —  Man  is  the  most  destructive 
of  animals.  He  has  cleared  vast  tracts  of  forest  for  lumber,  and  for  the 
building  of  cities  and  the  development  of  farms,  and  has  destroyed  other 
tracts  through  forest  fires.  Man  also  is  responsible  for  distributing 
through  the  world  most  of  the  "weeds  "  which  burden  the  farmer  and 
throng  the  roadsides.  Such  plants  as  the  Russian  thistle,  cocklebur, 
burdock,  and  Canada  thistle  once  were  somewhat  restricted  in  area, 
and  they  owe  their  present  widespread  distribution  directly  or  in- 
directly to  man.  Plants  of  this  sort  that  inhabit  fields  and  waste 
places  are  known  as  ruderals.  Often  there  are  ruderal  associations,  such 
as  those  that  develop  on  cultivated  land  that  is  left  fallow.  The  pioneer 
associations  that  follow  in  man's  destructive  train,  such  as  the  ruderal 
associations  of  fallow  land  or  the  "  fire  weed  "  associations  of  a  burned 
forest  tract,  usually  are  comparable  to  pioneer  associations  of  xerophytic 
tracts,  and  often  they  contain  xerophytic  species.  If  man  leaves  such 
areas  to  their  natural  course,  there  is  a  succession  of  associations  com- 
parable to  those  previously  noted,  culminating  finally  in  the  plant  asso- 
ciation that  originally  dominated  in  the  region. 


CHAPTER    VIII  —  ADAPTATION 

The  problem  of  adaptation.—  In  the  preceding  chapters  it  has  been 
seen  that  most  plant  structures  are  more  or  less  perfectly  suited  for  the 
conditions  in  which  they  live,  and  that  their  behavior  is  in  most  instances 
advantageous.  There  have  been  various  theories  to  account  for  the 
origin  of  the  obvious  harmony  between  plants  and  their  surroundings. 
Originally  it  was  supposed  that  plants  were  specially  created  with  the 
structures  and  behavior  that  they  now  possess,  but  it  has  long  been  known 
that  their  characters  are  the  result  of  evolution. 

The  theory  of  adaptive  response.  —  A  common  theory,  prevalent  es- 
pecially in  the  past  century,  has  been  that  plants  possess  an  inherent 
capacity  to  adapt  themselves  to  their  surroundings,  being  able  as  con- 
ditions change  to  change  their  structure  or  behavior  or  both  in  an  ad- 
vantageous manner.  Indeed,  this  doctrine  has  been  formulated  into  a 
so-called  law,  namely,  that  the  cause  of  a  need  is  at  the  same  time  the 
cause  of  its  satisfaction ;  new  conditions  create  new  needs,  and  the  new 
needs  are  supposed  to  result  in  new  organs.1  For  example,  the  abundant 
development  of  root  hairs  in  a  moist  air  culture  of  maize  seedlings  has 
been  regarded  as  an  adaptive  response,  the  hairs  being  believed  to  grow 
abundantly  because  they  are  needed,  whereas  water  cultures  of  the  same 
species  are  thought  to  be  hairless,  because  hairs  are  not  needed  in  a  water 
medium.  Such  a  philosophy,  in  which  need  is  supposed  to  control  re- 
sponse, and  in  which,  therefore,  the  pursuit  of  material  causes  is  replaced 
by  purposive  expressions,  is  denominated  teleological. 

The  theory  of  adaptive  response  has  had  two  aspects;  some  investi- 
gators hold  that  plant  species  remain  plastic  and  thus  able  to  adapt 
themselves  directly  to  changed  conditions,  while  others  hold  to  a  theory 
of  original  plasticity,  with  the  subsequent  fixation  of  adaptive  structures, 
the  so-called  inheritance  of  acquired  characters.  The  continued  plasticity 
of  amphibious  plants  and  of  beer  yeast  might  be  cited  in  favor  of  the 
former  view,  while  the  facts  of  progressive  variability  (p.  759),  the  re- 
tention of  short  vegetative  cycles  in  plants  from  northern  grown  seeds, 

1 A  less  crude  modern  statement  of  this  theory  is  that  new  conditions  result  in  new 
functions,  which  in  turn  result  in  modified  organs. 

947 


948  ECOLOGY 

and  the  rigidity  of  many  apparently  adaptive  characters,  as  in  the  xero- 
phytic  structures  of  conifers,  may  be  cited  in  favor  of  the  latter  view. 

Apparently  favoring  the  theory  of  adaptive  response  are  the  facts 
previously  cited  in  connection  with  the  development  of  cutin,  cork,  and 
air  spaces,  all  of  which  are  best  developed  where  most  useful.  Cutin 
and  cork  are  wanting  in  submersed  aquatics,  and  they  develop  increas- 
ingly as  the  atmosphere  becomes  more  desiccated,  where  their  protec- 
tive advantages  become  greater.  Air  spaces  are  best  developed  in  sub- 
mersed aquatics,  where  the  difficulties  in  oxygenation  are,  perhaps, 
greatest,  while  they  are  most  poorly  developed  in  xerophytes,  where  large 
air  spaces  would  tend  to  facilitate  excessive  transpiration. 

Associated  with  the  adaptation  theory  is  the  doctrine  of  use  and 
disuse,  it  being  held  that  an  organ  develops  most  when  most  used,  and 
least  when  least  used.  The  best  illustration  that  may  be  given  of  this 
is  in  the  conductive  tissues,  where  an  abundant  flow  of  material  occa- 
sions maximum  development,  and  where  a  slight  flow,  as  in  hydrophytes, 
occasions  minimum  development.  There  is  no  necessary  association, 
however,  between  the  adaptation  theory  and  the  theory  of  use  and  dis- 
use ;  the  conductive  tracts  that  develop  under  the  stimulus  of  parasitism 
might  illustrate  development  through  use,  but  they  are  very  far  from 
being  adaptations ;  air  spaces,  on  the  other  hand,  might  be  cited  as  illus- 
trating adaptation,  but  they  develop  most  where  they  are  least  used  and 
least  where  they  are  most  used. 

The  theory  that  plants  are  able  to  adapt  themselves  to  new  conditions 
is  no  longer  tenable,  being  invalid  a  priori  and  disproven  empirically. 
The  hypothesis  of  adaptive  response  rests  upon  the  same  foundation 
as  does  the  doctrine  of  vitalism,  which  postulates  that  there  is  something 
inherently  different  between  lifeless  and  living  matter.  Each  year  the 
list  of  "  vitalistic  activities  "  of  plants  becomes  more  and  more  restricted 
through  the  establishment  of  a  definite  physical  or  chemical  cause  for 
what  had  been  thought  to  have  a  vitalistic  explanation,  while  never  in 
the  history  of  science  has  any  phenomenon  once  explained  on  a  physi- 
cal or  chemical  basis  been  found  later  to  be  vitalistic.  The  same  is  true 
of  adaptations;  for  example,  tubers  at  one  time  were  thought  to  be  a 
provision  made  by  plants  for  their  vegetative  offspring,  but  now  it  is 
known  that  they  arise  as  a  definite  reaction  to  specific  external  conditions 
(p.  744).  Similarly,  many  organs  and  structures  now  are  known  to 
result  from  definite  conditions,  and  comparable  explanations  may  be 
expected  in  the  case  of  various  organs  whose  cause  is  still  unknown. 


ADAPTATION  949 

Many  phenomena  are  out  of  harmony  with  the  theory  of  adaptive 
response.  Perhaps  the  best  illustration  of  this  is  afforded  by  the  re- 
actions of  plants  that  are  attacked  by  parasites,  where  the  conductive 
tissues  are  stimulated  to  increased  development ;  obviously  this  is  a  most 
disadvantageous  reaction,  and  the  same  may  be  said  for  the  increased 
development  of  such  tissues  in  xerophytes.  In  other  cases  galls  are 
formed  which  accumulate  quantities  of  food  that  may  be  very  useful  to 
the  parasite,  but  certainly  are  harmful  to  the  host.  Nor  does  the  adap- 
tation theory  account  for  the  reactions  of  bast  fibers,  which  develop  best 
under  xerophytic  conditions,  but  are  of  little  if  any  value  in  checking 
transpiration;  they  seem  not  to  be  stimulated  by  mechanical  agents, 
though  their  chief  value  is  mechanical.  Conductive  tissues  and  the 
velamen  of  orchid  roots  are  most  useful  only  when  they  are  dead,  and 
it  is  not  easy  to  see  how  adaptation  can  arise  through  dying,  nor  how 
function  here  determines  form,  as  adaptationists  suppose. 

Even  instances  commonly  cited  by  teleologists  as  demonstrating  adap- 
tation often  prove  fallacious  when  analyzed.  For  example,  root  hairs, 
though  lacking  in  a  water  medium  where  they  are  said  not  to  be  needed, 
are  present  in  the  same  species  in  saturated  soil,  where  their  need  is  no 
more  obvious ;  in  dry  soil  or  in  concentrated  media,  where  they  are  needed 
most  of  all,  they  fail  to  develop.  Both  in  water  and  in  dry  soil,  root  hairs 
are  absent,  not  because  they  are  not  needed,  but  because  the  conditions 
necessary  for  their  development  are  absent.  Similarly,  palisade  tissue, 
which  has  been  thought  to  develop  where  the  plastids  need  protection 
from  intense  sunlight,  now  is  believed  to  have  but  little  significance  in 
this  respect,  since  the  palisade  plastids  of  xerophytes  have  relatively 
slight  motility.  Nor  are  useless  structures  lost  because  they  are  useless, 
but  only  because  the  factors  which  induce  them  fail  to  operate.  There 
are  many  obviously  useless  structures  in  plants,  as  the  stamens  of  Balano- 
phora  (p.  917)  and  the  cork  wings  of  the  sweet  gum  and  of  various  elms; 
many  other  structures  seem  inconsequential,  for  example,  many  hairs 
(including  stinging  hairs)  and  spines.  If  the  adaptation  hypothesis 
is  inadequate  to  explain  the  cases  above  noted,  it  is  unnecessary  else- 
where, since  there  are  theories  of  causation  which  account  equally  well 
for  the  origin  and  survival  of  useless  or  moderately  harmful  structures 
and  for  the  origin  and  survival  of  structures  that  are  advantageous. 

The  theory  of  adaptive  response  is  contrary  to  the  current  physical 
and  chemical  conceptions  of  the  behavior  of  matter.  Plants  are  made 
up  of  substances  that  react  definitely  to  definite  conditions,  whether  or 


950  ECOLOGY 

not  such  reactions  happen  to  be  useful  or  harmful.  If  a  new  structure 
arises,  it  must  be  through  some  chemical  or  physical  influence  within 
the  plant  or  in  its  environment.  The  adaptation  theory,  in  implying 
that  a  plant  responds  only  in  an  advantageous  direction  and  in  advan- 
tageous amount,  endows  the  plant  with  a  power  of  choice,  and  almost 
imagines  it  to  survey  the  situation  and  to  determine  upon  a  course  of 
action.  It  implies  the  possession  of  an  inherent  power  to  contravene  the 
ordinary  laws  of  nature.  It  presupposes  a  vital  mechanism  that  holds 
adaptations  in  readiness  for  conditions  that  have  not  as  yet  occurred. 
And  yet  man  himself  possesses  no  such  power  of  adaptation ;  he  cannot 
"  by  taking  thought  add  one  cubit  to  his  stature,"  though  he  can  (as  a 
plant  cannot)  study  the  laws  of  nature  and  place  himself  in  such  con- 
ditions as  to  facilitate  desired  reactions. 

The  theory  of  fortuitous  variation.  —  The  preceding  considerations 
appear  to  show  that  protoplasm  is  not  inherently  adaptive.  Disad- 
vantageous structures  (such  as  the  food  layers  of  galls  or  the  enlarged 
conductive  tracts  of  parasitized  plants)  or  indifferent  structures  (such 
as  cork  wings  and  many  hairs  and  spines)  are  quite  as  normal  expres- 
sions of  protoplasmic  behavior  as  are  the  more  numerous  advantageous 
structures.  The  theory  of  fortuitous  variation,  which  is  based  upon  the 
laws  of  chance,  postulates  that  newly  developing  structures  are  of  all 
kinds :  some  advantageous,  some  disadvantageous,  and  some  indifferent. 
The  supporters  of  this  theory  are  aligned  in  two  general  schools;  the 
one  school  holds  that  new  structures  arise  chiefly  through  the  influence 
of  external  factors,  while  the  other  holds  that  factors  residing  within  the 
plant  itself  are  more  important.  Many  investigators  maintain  that 
external  factors  are  more  important  in  some  instances  and  internal  fac- 
tors in  others;  this  composite  view,  embracing  opinions  of  the  two 
opposing  schools,  seems  best  able  to  explain  the  facts  as  they  are  now 
understood. 

In  the  first  place,  variations  are  of  frequent  occurrence,  though  their 
supposed  rarity  once  was  given  as  an  argument  against  the  theory  of 
evolution.  Scarcely  any  species  or  any  structure  that  has  been  studied 
carefully  has  been  found  to  be  invariable,  and  in  some  cases  the  amount 
of  divergence  from  a  supposed  type  is  enormous,  Not  only  do  the  indi- 
viduals of  a  species  as  found  in  nature  often  differ  from  each  other  in 
many  particulars,  but  the  same  is  true  of  individuals  whose  ancestry 
is  known  to  be  identical.  An  excellent  illustration  of  such  variation  is 
seen  in  water  cultures  of  various  seedlings;  while  in  such  cultures  maize 


ADAPTATION  951 

roots  commonly  are  hairless  and  wheat  roots  hairclad,  certain  maize 
roots  may  be  hairclad  or  wheat  roots  hairless,  though  the  conditions 
appear  to  be  the  same. 

Congenital  and  reaction  structures. —  Structures  (such  as  cork  or 
cutin)  which  arise  through  reaction  to  environmental  changes  may  be 
called  reaction  structures.1  If  a  plant,  when  placed  in  xerophytic  con- 
ditions, happens  to  develop  as  a  reaction  thereto  such  xerophytic  features 
as  cutinization,  succulence,  or  dwarfness,  it  may  be  called  a  reaction 
xerophyte;  similarly,  there  may  develop  reaction  mesophytes  or  reaction 
hydrophytes.  It  is  obvious,  however,  that  many  reaction  structures 
cannot  be  classed  as  hydrophytic,  mesophytic,  or  xerophytic ;  especially 
is  this  true  of  those  that  do  not  happen  to  be  advantageous.  Contrasting 
with  reaction  structures  are  those  structures  that  are  born  with  the 
species  in  whatever  habitat  it  is  developed  (as  in  the  case  of  mutations), 
and  which  are  not  lost  if  the  species  is  grown  in  other  habitats.  Such 
structures  may  be  termed  congenital  structures.  If  a  species  happens  to 
be  born  with  such  xerophytic  features  as  succulence  or  dwarfness,  it  may 
be  called  a  congenital  xerophyte;  similarly,  there  may  develop  congenital 
hydrophytes  or  congenital  mesophytes,  or  plants  that  cannot  be  thus 
classed.2  Thus  any  given  structure,  as  a  cutinized  epidermal  wall  or 
a  succulent  cortex,  may  be  a  reaction  structure  in  one  species  and  a  con- 
genital structure  in  another  species.  Furthermore,  any  species  may  at 
the  same  time  possess  both  variable  reaction  structures  and  fixed  con- 
genital structures,  otherwise  called  adaptation  and  organization  charac- 
ters, careful  experiment  alone  determining  which  is  which.  According 
to  this  theory,  each  plant  association  is  composed  of  certain  species  that 
are  fit  because  their  critical  features  are  the  product  of  the  habitat,  and 
of  other  species  that  happen  to  have  been  born  fit  and  thus  enabled  to 
survive. 

Even  congenital  structures  and  organs  are  influenced  by  external 
factors.  It  has  been  seen  that  most  stomata,  hairs,  and  spines,  if  present 
at  all,  have  a  definite  and  fixed  structure,  and  thus  may  be  regarded  as 
congenital  rather  than  reactive.  However,  their  presence  or  absence  is 
determined  by  external  agents,  whence  the  latter  are  called  determinative 

1  Too  much,  emphasis  cannot  be  placed  upon  the  fundamental  distinction  between  the 
word  reaction  and  such  words  as  adaptation,  adjustment,  accommodation,  or  regulation. 
The  latter  words  imply  an  inherent  power  to  change  advantageously,  while  the  word 
reaction  implies  no  such  power. 

2  Reaction  and  congenital  xerophytes,  mesophytes,  and  hydrophytes  also  may  be  termed, 
respectively,  facultative  and  obligate  xerophytes,  mesophytes,  and  hydrophytes. 


952  ECOLOGY 

factors.  Sometimes  reference  is  made  instead  to  releasing  factors,  the 
structure  in  question  being  regarded  as  potentially  present,  though  its 
manifestation  is  inhibited  until  the  proper  factor  enters  in  place  of  or 
in  addition  to  the  inhibiting  factor.  In  the  case  of  reaction  structures 
it  is  believed  that  the  form  assumed,  as  well  as  the  time  and  the  place  of 
appearance,  is  due  to  external  agents,  wherefore  the  latter  are  called 
formative  factors. 

While  there  is  no  doubt  of  the  reality  of  reaction  structures,  because 
they  are  so  readily  capable  of  experimental  production,  there  are  many 
investigators  who  disbelieve  in  the  reality  of  congenital  structures.  An 
alternative  hypothesis  is  that  all  plant  structures  are  or  have  been  plastic. 
The  so-called  rigid  or  congenital  structures  may  in  some  ancestral  form 
have  been  as  plastic  as  are  the  reaction  structures  of  to-day ;  in  this  event 
the  fixation  of  reaction  structures  has  resulted  in  structures  that  now  are 
congenital.  It  is  equally  possible  that  the  supposedly  rigid  congenital 
structures  really  are  plastic,  but  to  an  imperceptible  degree,  as  compared 
with  the  leaf  plasticity  of  amphibious  plants;  if  this  is  true,  all  plant 
structures  are  plastic,  some  obviously  and  rapidly,  and  others  so  slightly 
or  slowly  that  only  experiments  of  long  duration  can  reveal  plasticity. 
While  in  the  present  state  of  imperfect  knowledge,Jt  is  convenient  and 
not  necessarily  incorrect  to  contrast  reaction  structures  and  congenital 
structures,  it  is  possibly  more  correct  to  subdivide  plant  structures  into 
those  that  certainly  are  plastic  and  those  that  apparently  are  rigid. 

The  survival  of  advantageous  structures  through  natural  selection. — 
While  the  origin  of  structures  through  reaction  or  through  mutation  can- 
not account  for  the  present  preponderance  of  advantageous  structures 
and  advantageous  behavior,  the  theory  of  natural  selection  is  in  this  re- 
spect as  satisfactory  as  it  has  proven  unsatisfactory  from  the  standpoint 
of  causation.1  Of  the  species  with  new  congenital  structures,  only 
those  are  likely  to  survive  that  happen  to  be  suited  for  existence  in  the 
habitat  in  which  the  new  structures  develop,  or  that  are  able  to  migrate 
to  a  suitable  habitat;  a  mesophyte  that  happens  to  originate  in  a  desert 
or  a  plant  suited  to  warm  climates  that  happens  to  originate  in  a  cold 
region  cannot  survive.  Of  the  more  plastic  species  only  those  are  likely 

1  Against  natural  selection  as  a  causative  theory  there  may  be  urged:  (i)  the  existence 
in  many  species  of  a  capacity  for  advantageous  regeneration,  though  the  opportunity  for 
such  regeneration  rarely  if  ever  occurs  in  nature;  (2)  the  existence  of  complicated  struc- 
tures (such  as  stinging  hairs,  digestive  glands,  and  extra-floral  nectaries)  whose  value  to  the 
plants  possessing  them  is  slight;  and  (3)  the  existence  of  "overadaptation,"  as  in  the. 
flowers  of  orchids  and  in  the  seeds  of  certain  xerophytes. 


ADAPTATION  953 

to  survive  which  are  able  to  react  advantageously.  Thus  in  the  course  of 
time  it  is  to  be  expected  that  the  plants  that  are  congenitally  unfit  and 
the  plants  that  react  disadvantageously  will  largely  be  eliminated.  Only 
occasionally  would  there  survive  plants  with  structures  that  arc  slightly 
disadvantageous.  Somewhat  more  abundant  might  be  the  number 
of  plants  with  indifferent  or  with  only  slightly  useful  structures.  Many 
more  individuals  are  born  than  have  a  chance  to  live,  since  severe  physi- 
cal conditions  and  crowding  by  other  plants  cause  the  elimination  of  the 
unfit  and  the  survival  of  the  fit;  consequently  advantageous  structures 
must  ultimately  dominate,  whatever  the  nature  of  the  primitive  struc- 
tures. Such  an  explanation  of  the  predominance  of  advantageous  struc- 
tures seems  far  more  tenable  than  does  the  theory  of  origin  through 
adaptation. 


APPENDIX 


SUPPLEMENTARY   LITERATURE 

THE  following  references  are  inserted  in  order  to  enable  any  who  so  desire  to 
obtain  more  detailed  information  concerning  the  subjects  treated  in  Part  III.  A 
complete  bibliography  is  quite  out  of  place,  but  it  is  hoped  that  the  most  useful 
references  may  be  found  in  the  appended  list.  In  all  cases  the  references  chosen 
are  those  which  generally  will  be  found  relatively  accessible  in  libraries  containing 
the  leading  botanical  and  biological  journals.  No  attempt  is  made  to  give  the 
older  references,  chief  attention  being  paid  to  those  of  recent  date,  in  which  allu- 
sion to  older  treatises  may  generally  be  found. 

Following  each  topic  is  the  page  of  Part  III  to  which  reference  is  made,  as  (491). 
The  authors  to  whom  no  reference  is  made  other  than  by  name  are  those  men- 
tioned in  the  General  List,  as  HABERLANDT  and  GOEBEL  I.  The  arrangement 
of  authors  under  each  topic  is  alphabetical,  and  the  abbreviations  of  the  various 
journals  will  be  intelligible  to  any  librarian  or  to  any  one  acquainted  with  botanical 
and  biological  journals. 

GENERAL 

DE  BARY,  Comparative  Anatomy,  Oxford,  1884;  GOEBEL  I,  Organography 
of  Plants,  Part  I,  Oxford,  1900;  GOEBEL  II,  Experimented  Morphologic  der 
Pflanzen,  Leipzig,  1908;  HABERLANDT,  Physiologische  Pflanzenanatomie,  4th 
edition,  Leipzig,  1909;  JOST,  Lectures  on  Plant  Physiology,  Oxford,  1907;  KER- 
NER,  Natural  History  of  Plants,  New  York,  1895  J  SCHIMPER,  Plant  Geography, 
Oxford,  1903;  WARMING,  Oecology  of  Plants,  Oxford,  1909. 


CHAPTER  I 
ROOTS  AND  RHIZOIDS 

Root  hairs  (491):   HABERLANDT;    SNOW,  Bot.  Gaz.  40,  12-48,  1905. 
Absorption  (493):    FITTING,  Zeit.  Bot.  3,  209-275,  1911;   HILL,  New  Phyt.  7, 

133-142,  1908;  JOST. 
Root  excretions  (493) :    LIVINGSTON,  etc.,  Bulls.  28,  36,  U.  S.  Bur.  Soils,  1905, 

1907;  REED,  Pop.  Sci.  Mon.  73,  257-266,  1908;   SCHREINER  and  REED, 

Bull.  40,  U.  S.  Bur.  Soils,  1907;  Bull.  Torr.  Bot.  Club  34,    270-303,  1907; 

Bot.  Gaz.  45,  73-102,  1908;    STOKLASA   and  ERNST,  Jahrb.  Wiss.  Bot. 

46,  55-102,  1908. 
Root  structure  (496):   FREIDENFELT,  Flora  91,  115-208,  1902;   TSCHIRCH, 

Flora  94,  68-78,  1905;   VON  ALTEN,  Bot.  Zeit.  67,  175-199,  1909. 
Direction  of  growth  (499) :  JOST. 

954 


APPENDIX  955 

Root  contraction  (504):    RIMBACH,  Bot.  Gaz.  30,  171-188;   33,  401-420,  1900, 

1902. 
Water  and  root  form  (505):    CANNON,  Carnegie  Inst.  Publ.  113,  59-66,  1909; 

FITTING,  Zeit.  Bot.  3,  209-275,  1911;  SPALDING,  Bot.  Gaz.  38,  122-138, 

1904. 
Absorptive  air  roots  (511):    LEAVITT,  Rhodora  2,  29  ff.,  1900;    NABOKICH, 

Bot.  Cent.  80,  331  ff.,  1899. 

Anchoring  air  roots  (513):    WENT,  Ann.  Jard.  Bot.  Buit.  12,  1-72,  1893. 
Prop  roots  (514):   BESSEY,  Mo.  Bot.  Card.  Rep.  1908,  25-33. 
Rhizoids  (516):    BACHMANN,  Jahrb.  Wiss.  Bot.  44,  1-40,  1907;    BENECKE, 

Bot.  Zeit.  61,  19-46,  1903;   HABERLANDT;   PAUL,  Bot.  Jahrb.  32,  231- 

274,  1903;  SCHOENE,  Flora  96,  276-321. 


CHAPTER  II 

LEAVES 

Chloroplasts  and  chlorophyll  (521):  GRIFFON,  Ann.  Sci.  Nat.  Bot.  VIII,  10, 
1-123,  1899;  HABERLANDT;  JOST;  SCHIMPER,  Jahrb.  Wiss.  Bot. 
16,  1-247,  l885- 

Albescence  (523):  BAUR,  Ber.  Deutsch.  Bot.  Ges.  22,  453-460,  1904;  Biol.  Cent. 
3°>  497-5J4,  1910. 

Chloroplast  movements  (524):  HABERLANDT;  STAHL,  Bot.  Zeit.  38,  297  ff., 
1880;  SENN,  Leipzig,  1908. 

Synthesis  of  carbohydrates  (525):  BLACKMAN,  New  Phyt.  3,  33-38,  1904; 
GIBSON,  Ann.  Bot.  22,  117-120,  1908;  JOST;  McPHERSON,  Science  33, 
131-142,  1911;  USHER  and  PRIESTLY,  Proc.  Roy.  Soc.  77,  369-376;  78, 
318-327,  1905,  1906. 

External  factors  and  carb  hydrate  synthesis  (526):  BEIJERINCK  and  VAN 
DELDEN,  Cent.  Bakt.  10,  33-47,  1903;  BLACKMAN,  New  Phyt.  3,  237- 
242,  1904;  BROWN  and  ESCOMBE,  Proc.  Roy.  Soc.  76,  29-112,  1905; 
JOST;  STAHL,  Jena,  1909. 

Anthocyan  (528):  HABERLANDT;  KERNER;  OVERTON,  Jahrb.  Wiss. 
Bot.  33,  171-231,  1899;  WHELDALE,  Proc.  Roy.  Soc.  81,  44~6o,  1909; 
Prog.  Rei.  Bot.  3,  457-473,  1910. 

Chlorenchyma  structure  (530):  HABERLANDT;  HEINRICHER,  Jahrb.  Wiss. 
Bot.  15,  502-567,  1884. 

Chlorenchyma  plasticity  (533):  ARESCHOUG,  Flora  96,  329-336, 1906;  CLEM- 
ENTS, Trans.  Amer.  Mic.  Soc.  1905,  19-102;  HABERLANDT;  HABER- 
LANDT, Sitz.  Wien  Akad.  in,  69-91,  1902;  PICK,  Bot.  Cent,  ir,  400  ff., 
1882;  STAHL,  Bot.  Zeit.  38,  868-874,  1880. 

Leaves  and  light  (539):  BERGEN,  Bot.  Gaz.  48,  450-461;  BLACKMAN,  New 
Phyt.  6,  270-279,  1907;  KERNER;  WIESNER,  Eiol.  Cent.  23,  2095.,  1903. 

Phyllotaxy  (549):  KERNER;  WINKLER,  Jahrb.  Wiss.  Bot.  36,  1-79;  38,  501- 
544,  1901,  1903. 

Air  chambers  (551):  HABERLANDT;  SCHENCK,  Jahrb.  Wiss.  Bot.  20,  526- 
574,  1889. 


956  APPENDIX 

Structure  of  stomata  (555):  HABERLANDT. 

Movements  cf  stomata  (562):  COPELAND,  Ann.  Bot.  16,  327—364,  1902;   DAR- 
WIN, Phil.  Trans.  190,  531-621,  1898;    HABERLANDT. 
R61e  of  stomata  (563):  BLACKMAN,  Phil.  Trans.  186,  503-562,  1895;  BROWN 

and     ESCOMBE,     Phil.     Trans.     193,    223-292,    1900;    HABERLANDT; 

LLOYD,  Carnegie  Inst.  Publ.  82,  1908;    STAHL,  Bot.  Zeit.  52,  117-146, 

1894. 
Transpiration  (565):    BURGERSTEIN,  Jena,  1904;   RENNER,  Flora  no,  451- 

547,  1910. 

Epidermis  (567):  HABERLANDT;  KERNER. 

Hairs  (572):   BAUMERT,  Beitr.  Biol.  9,83-162,  1907;  HABERLANDT;  KER- 
NER; RENNER,  Flora  99,  127-155,  1908. 
Leaf  movements  (579) :    JOST;   PFEFFER,  Leipzig,  1907;  TSCHIRCH,  Jahrb. 

Wiss.  Bot.  13,  544-568,  1882. 
Leaf  fall  (582):    LEE,  Ann.  Bot.  25,  51-106,  1911;    WIESNER,  Ber.  Deutsch. 

Bot.  Ges.  22,  64  ff.,  1904;   23,  49  ff.,  1905. 
Protective  features  in  the  cell  sap  (587) :   BARTETZKO,  Jahrb.  Wiss.  Bot.  47, 

57-98,   1909;    BLACKMAN,  New  Phyt.   8,  354-363,   1909;    LIDFORSS, 

Lund,  1907. 

Variations  in  leaf  form  (589):    GOEBEL  I;    GOEBEL  II. 
Variations  in  algae  (591):    BRUNNTHALER,  Sitz.  Wien  118,  501-573,  1909; 

LIVINGSTON,  Bot.   Gaz.  30,   289-317,   1900;    32,    292-302,   1901;    Bull. 

Torr.  Bot.  Club  32,  1-34,  1905. 
Leaf  variations  in  amphibious  plants  (593):    BURNS,  Ann.  Bot.  18,  570-587, 

1904:   McCALLUM,  Bot.  Gaz.  34,  93-108,  1902;    SHULL,  Carnegie  Publ. 

36,  1905- 
Recapitulation  (596):   DIELS,  Berli   ,  1906;   DUFOUR,  Rev.  Gen.  Bot.  22,  369- 

384,  1910;    GRIGGS,  Amer.  Nat.  43,  5-30,  1909. 
Leaf  variations  in  land  plants  (598) :    BONNIER,  Ann.  Sci.  Nat.  Bot.  VII,  20, 

217-360,  1895;   GOEBEL,  Flora  82,  1-13,  1896. 
Asymmetry   and   anisophylly    (607):     FIGDOR,    Leipzig,     1909;     GENTNER, 

Flora  99,  289-300,   1909. 
Absorption  in  water  plants  (609):    POND,  U.S.  Fish   Commission  Report,  1905; 

SNELL,  Flora  98,  213—249,  1907. 
Absorption  in  lichens  and  mosses  (610):  HABERLANDT;  KERNER;  LEAVITT, 

Rhodora  2,  65-68,  1900;   MULLER,  Jahrb.  Wiss.  Bot.  46,  587-598,  1909. 
Leaf  absorption  in  land   plants  (613):    HALKET,  New  Phyt.  10,  121-139,  1911; 

HABERLANDT;    KERNER;   SPALDING,  Bot.  Gaz.  41,  262-282,  1906. 
Leaf  absorption  in  epiphytes  (615):    ASO,  Flora  100,  447-449,  1910;   HABER- 
LANDT;   MEZ,  Jahrb.  Wiss.  Bot.  40,  157-229,  1904;    SCHIMPER,  Bot. 

Cent.  17,  192  ff.,  1884. 
Carnivorous  plants   (616):    DARWIN,  Insectivorous  plants,  New  York,   1895; 

HABERLANDT;  KERNER. 
Water  exudation  (620):    HABERLANDT;    JOST;    LEPESCHKIN,  Flora  90, 

42-60,  1902;    SPANJER,  Bot.  Zeit.  56,  35-81,  1898. 

Secretion  and  excretion  (622):    DETTO,  Flora  92,  147-199,  1903;    HABER- 
LANDT; JOST. 


APPENDIX  957 

Water  accumulation  in  leaves  (627):    FITTING,  Zeit.  Bot.  3,  209-275,   1911; 

HABERLANDT;    MACDOUGAL  and  SPALDING,  Carnegie  Publ.  i4I, 

1910. 
Intumescences  (633):    COPELAND,  Bot.  Gaz.  33,  300-308,  1902;    DOUGLAS, 

Bot.  Gaz.  43,  233-250,  1907;    KU'STER,  Ber.  Deutsch.  Bot.  Ges.  21,  452- 

458,   1903;    Flora  96,  527-537,   1906;    VON  SCHRENK,  Mo.  Bot.  Gard. 

Rep.  16,  125-148,  1905. 
Reproduction  by  leaves  (636) :  GOEBEL  I;  GOEBELII;  GOEBEL,  Biol.  Cent. 

22,  385  ff.,  1902;    STINGL,  Flora  99,  178-192,  1909. 
Scale  leaves  (642):   GOEBEL  II;  LUBBOCK  (Avebury),  Buds  and  Stipules, 

London,  1899;    MACDOUGAL,  Bull.  Torr.  Bot.  Club  30,  503-512,  1903; 

MOORE,  Bull.  Torr.  Bot.  Club  36,  117-145,  1909;   THOMAS,  Rev.  Gen. 

Bot.  12,  394  ff.,  1900;   WIEGAND,  Bot.  Gaz.  41,  373-424,  1906. 


CHAPTER   III 

STEMS 

Stem  branching  (646):   KERNER;   BARANETZKY,  Flora  89,  138-239,  1901. 
Lianas  (651);    FITTING,  Jahrb.  Wiss.  Bot.  38,  545-634,  1903;    39,  424-526, 

1904;   JOST:  SCHENCK,  Jena,  1893,  189   ;   SCHIMPER;  WOODHEAD 

and  BRIERLY,  New  Phyt.  8,  284-298,  1909. 
Epiphytes  (657):    FITTING,  Ann.  Jard.  Bot.  Buit.,  Supplement  III,  505-518, 

1910;    SCHIMPER;    SCHIMPER,  Jena,  1888. 

Lenticels  (660):   DEVAUX,  Ann.  Sci.  Nat.  Bot.  VIII,  12,  1-240,  1900;  HABER- 
LANDT. 
Rhizomes  (667):    FRANCOIS,  Ann.  Sci.  Nat.  Bot.  IX,  7,  25-58,  1908;  LID- 

FORSS,  Jahrb.  Wiss.  Bot.  38,  343-376,  1903;    MASSART,  Bull.  Soc.  Bot. 

Belg.  41,  67-79,   1903;    RAUNKIAER,  Bull.  Acad.   Sci.   Denmark,   1904; 

SPERLICH,  Flora  96,  451-473,   1906. 

Runners  (672):    MAIGE,  Ann.  Sci.  Nat.  Bot.  VIII,  n,  249-364,  1900. 
Tubers  (674):   MOLLIARD,  Bull.  Soc.  Bot.  France,  56,  42-45,  1909. 
Bulbs  (674):  BLODGETT,  Bot.  Gaz.  50,  3  0-373,  r910;  RIMBACH,  Bot.  Gaz. 

30,  171—188;    33,  401—420,  1900,  1902;    ROBERTSON,  Ann.  Bot.  20,  429— 

440,  1906. 
Conduction  and  conductive  tissues  (678):    COPELAND,  Bot.  Gaz.  34,  161  ff., 

1902;    DE  BARY;   EWART,  Ann.  Bot.  24,  85-105,  1910;    GROOM,  Ann. 

Bot.  24,  241-269,  1910;  HABERLANDT;    JOST;   MANGHAM,  Sci.  Prog. 

5,  256  ff.,  1910,  1911;   OVERTON,  Bot.  Gaz.  51,  28  ff.,  1911;  WIEGAND, 

Amer.  Nat.  40,  409-453,  1906. 
Conduction  in  bryophytes  (685):    TANSLEY  and  CHICK,  Ann.  Bot.  15,  1-38, 

1901. 
Vascular  variations  in  primary  tissues  (686):    CANNON,  Bot.  Gaz.  39,  397—408, 

1905;   FREUNDLICH,  Jahrb.  Wiss.  Bot.  46,  137-206,  1908;   MOLLIARD, 

Compt.  Rend.  144,  1063-1064,  1907;    SCHUSTER,  Ber.  Deutsch.  Bot.  Ges. 

26,  194-237,  1908;    SIMON,  Jahrb.  Wiss.  Bot.  45,  351-478,  1907;  WINK- 

LER,  Jahrb.  Wiss.  Bot.  45,  1-82,  1907. 


958  APPENDIX 

Variations  in  growth  rings  (689):    BONNIER,  Compt.  Rend.  135,  1285-1289, 

1902;    HABERLANDT;    SIMON,  Ber.  Deutsch.  Bot.  Ges.   20,   229-249, 

1902;   URSPRUNG,  Bot.  Zeit.  62,  189-210,  1904. 
Tyloses  (695):  VON  ALTEN,  Bot.  Zeit.  67,  1-23,  1909. 
Stereids  (696):  DE  BARY;  HABERLANDT. 
Influence  of  external  factors  on  mechanical  tissues  (699):    BALL,  Jahrb.  Wiss. 

Bot.  39,  305-341,  1903;   BORDNER,  Bot.  Gaz.  48,  251-274,  1909;  HABER- 
LANDT; HIBBARD,  Bot.  Gaz.  43,  361-382,  1907;  METZGER,  Nat.  Zeit. 

6,  249-273,  1908;    PENNINGTON,  Bot.  Gaz.  50,  257-284,  1910;  SONN- 

TAG,  Jahrb.  Wiss.  Bot.  39,  71-105,  1903;   WIEDERSHEIM,  Jahrb.  Wiss. 

Bot.  38,  41—69,  1902. 
Role  of  mechanical  tissues  (700):    AMBRONN,  Jahrb.  Wiss.  Bot.  12,  473-541, 

1881;    COHN,    Jahrb.   Wiss.  Bot.    24,    145-172,    1892;     HABERLANDT; 

SCHWENDENER,  Leipzig,  1874;    SONNTAG,  Flora  99,  203-259,  1909; 

TSCHIRCH,  Jahrb.  Wiss.  Bot.  16,  303-335,  1885. 
Cork  (705) :  HABERLANDT. 
Water  accumulation  in  stems  (718):   MACDOUGAL  and  SPALDING,  Carnegie 

Publ.  141,  1910;   SPALDING,  Bull.  Torr.  Bot.  Club  32,  57-68,  1905. 
Food  accumulation  in  stems  (719):   COMBES,  Rev.  Gen.  Bot.  23,  129-164,  1911 ; 

BARTER,  Plant  World  13,  144-147,  1910;   LECLERC  DU  SABLON,  Rev. 

Gen.  Bot.  16,  341,  ff.;    18,  5  ff.,  1904,  1906;   NIKLEWSKI,  Bei.  Bot.  Cent. 

19,  68-117,  1905;    SCHELLENBERG,  Ber.  Deutsch.  Bot.  Ges.  23,  36-45, 

1905. 
Latex  (720):  BRUSCHI,  Annali  di  Botanica  7,  671-701,  1909;   GAUCHER,  Ann. 

Sci.  Nat.  Bot.  VIII,  12,  241-260,  1900;  HABERLANDT;   KNIEP,  Flora  94, 

129-205,  1905;   MOLISCH,  Jena,  1901;   PARKIN,  Ann.  Bot.  14,  193—214, 

1900. 

Resin  ducts,  etc.  (723):   HABERLANDT;   TSCHIRCH,  Leipzig,  1908. 
Stem  variation  (725) :    GOEBEL  I;   GOEBEL  II. 
Stem  elongation  (725):   BRENNER,  Flora  87,  387-439,  1900;   LAURENT,  Rev. 

Gen.  Bot.  19,  129-160,  1907;  MACDOUGAL,  Mem.  N.  Y.  Bo  .  Gard.,  1903; 

SELBY,  Bull.  Torr.  Bot.  Club  33,  67-76,  1906. 

Stem  dwarfing  (730):   BONNIER,  Ann.  Sci.  Nat.  Bot.  VII,  20,  217-360,  1895. 
Spinescence  (741):  LOTHELIER,  Rev.  Gen.  Bot.  5,  480  ff.,  1893;  LOTHELIER, 

Lille,  1893;  ZEIDLER,  Flora  102,  87-95,  1911. 
Tuberization  (744):    BERNARD,  Rev.  Gen.  Bot.  14,  5  ff.,  1902;    JUMELLE, 

Rev.  Gen.  Bot.  17,  49-59,  ^05;    MOLLIARD,  Bull.  Soc.  Bot.  France  50, 

631-633,  1903,  and  Rev.  Gen.  Bot.  19,  241  ff.,  1907;   VOCHTING,  Organ- 

bildung,  Bonn,  1878,  1884,  also  Jahrb.  Wiss.  Bot.  34,  1-149,  1889,  and  Bot. 

Zeit.  60,  87—114,  1902. 
Regeneration   (748):     COULTER   and   CHRYSLER,   Bot.    Gaz.   38,   452-458, 

1904;  GOEBEL  I  and  II,  also  Biol.  Cent.  22,  385  ff.,  1902,  and  Bull.  Torr. 

Bot.  Club  30,  197-205,   1903;    McCALLUM,  Bot.    Gaz.  40,  97  ff.,   1905; 

MORGAN,  New  York,  1901. 
Polarity  (749):    KLEBS,  Jena,  1903;    KUSTER,  Jahrb.  Wiss.  Bot.  40,  279-302, 

1904;    KUPFER,  Mem.  Torr.  Bot.  Club  12,   195-241,   1907;    MORGAN, 

Bull.  Torr.  Bot.  Club  30,  206-213,  1903;  31,  227-230,  1904,  and  Science  20, 


APPENDIX  959 

742-748,   1904;    VOCHTING,  Organbildung,  Bonn,   1878,   1884,  and  Bot. 
Zeit.  64,  101-148,  1906;  WINKLER,  Jahrb.  Wiss.  Bot.  35,  449-469,  1900. 


CHAPTER  IV 
SAPROPHYTISM  AND  SYMBIOSIS 

Symbiosis  (752):  JOST;  KERNER;  WARMING. 

Myrmecophytes  (753):  BUSCALIONI  and  HUBER,  Bei.  Bot.  Cent.  9,  85-88, 
1900;  FIEBRIG,  Biol.  Cent.  29,  i  ff.,  1909;  VON  IHERING,  Bot.  Jahrb. 
39,  666-714,  1907;  RETTIG,  Bei.  Bot.  Cent.  17,  89-122,  1904;  RIDLEY, 
Ann.  Bot.  24,  457-483,  1910;  SCHIMPER;  ULE,  Bot.  Jahrb.  37,  335-352, 
1906. 

Saprophytism  in  fungi  (754):    KUNZE,  Jahrb.  Wiss.  Bot.  42,  357-393,  1906. 

Saprophytism  in  algae  (756):  ARTARI,  Ber.  Deutsch.  Bot.  Ges.  20,  172-201, 
1902;  Jahrb.  Wiss.  Bot.  40,  593-613,  1904;  TREBOUX,  Ber.  Deutsch.  Bot. 
Ges.  23,  432-441,  1905. 

Saprophytism  in  seed  plants  (757):    LAURENT,  Rev.  Gen.  Bot.  16,  14  ff.,  1904. 

Parasitism  (761):  HABERLANDT;  JOST;  KERNER. 

Parasitic  fungi  (762):  BULLER,  Jour.  Econ.  Biol.  i,  101-138,  1906,  and  Sci. 
Prog.  3,  361-378,  1909;  DUGGAR,  Plant  Diseases,  Boston,  1909;  DUYSEN, 
Hedwigia  46,  25—56,  1906;  SMITH,  Bot.  Gaz.  33,  421—436,  1902. 

Heteroecious  fungi  (764):  ERIKSSON,  Ann.  Sci.  Nat.  Bot.  VIII,  15,  1-160,  1902, 
also  Ann.  Bot.  19,  55-59,  1905,  and  Biol.  Cent.  30,  618-623,  1910;  JAC- 
ZEWSKI,  Zeit.  Pflanzenkrank  20,  321-359,  1910;  KLEBAHN,  Berlin,  1904; 
•WARD,  Ann.  Bot.  19,  1—54,  1905. 

Physiological  species  and  specialization  (765):  FISCHER,  Berne,  1907,  and 
Geneva,  1908;  KLEBAHN,  Berlin,  1904;  NEGER,  Flora  88,  333-370, 
1901;  90,  221—272,  1902;  REED,  Trans.  Wis.  Acad.  Sci.  15,  135—162,  1905; 
15,  527-547,  1907;  and  Bull.  Torr.  Bot.  Club  36,  353-388,  1909;  SALMON, 
Bei.  Bot.  Cent.  14,  261—315,  1903,  also  Phil.  Trans.  Roy.  Soc.  London,  197, 
107-122,  1904;  198,  87-97,  I9°S;  New  Phyt.  3,  55  ff.,  1904;  4,  217-221, 
1905;  Ann.  Bot.  19,  125—148,  1905;  WARD,  Ann.  Bot.  16,  233—315,  1902. 

Origin  of  parasitism  in  fungi  (766) :  FULTON,  Bot.  Gaz.  41,  81-108,  1906;  MAS- 
SEE,  Phil.  Trans.  Roy.  Soc.  London,  197,  7—24,  1904. 

Immunity  (768):   BROOKS,  Ann.  Bot.  22,  479-487,  1908. 

Parasitic  seed  plants  (769):  HABERLANDT;  KERNER;  MIRANDE,  Bull. 
Sci.  34,  1-280,  1901;  PEIRCE,  Ann.  Bot.  7,  291-327,  1893;  8,  53-118,  1894. 

Partially  parasitic  seed  plants  (772):  BRAY,  Bull.  166,  U.  S.  Bur.  PI.  Ind.,  1910; 
GAUTIER,  Rev.  Gen.  Bot.  20,  67-84,  1908;  HEINRICHER,  Jahrb.  Wiss. 
Bot.  31,  77-124,  1898;  32,  389-452,1898;  36,665-752,  1901;  37,  264-337, 
1902;  46,  273-376,  1909;  47,  539-587,  1910;  YORK,  Bull.  Univ.  Tex.  120, 
1909.  f 

Origin  of  parasitism  in  seed  plants  (775):  MACDOUGAL,  Carnegie  Inst.  Publ. 
129,  1910;  Plant  World  13,  207-214,  1910;  PEIRCE,  Bot.  Gaz.  38,  214-217, 
1904;  WHITE,  Amer.  Nat.  42,  98-108,  1908. 


960  APPENDIX 

Reciprocal  influence  of  stock  and  scion  (778):  DANIEL,  Compt.  Rend.  Paris 
Acad.  Sci.  141,  214—215,  1905;  148,  431—433,  1909;  GRAFE  and  LINS— 
BAUER,  Ber.  Deutsch.  Bot.  Ges.  24,  368-371,  1906;  GRIFFON,  Bull.  Soc. 
Bot.  France,  57,  517  ff.,  1910;  GUIGNARD,  Ann.  Sci.  Nat.  Bot.  IX,  6,  261- 
305,  1907;  McCALLUM,  Plant  World  12,  281-286,  1909;  MEYER  and 
SCHMIDT,  Flora  100,  317-397,  1910;  RAVAZ,  Compt.  Rend.  Paris  Acad. 
Sci.  150,  712,  1910. 

Graft  hybrids  and  chimeras  (779):  BAUR,  Zeit.  Abst.  Vererb.  i,  330-351,  1909; 
Ber.  Deutsch.  Bot.  Ges.  27,  603-605,  1910;  Biol.  Cent.  30,  497-514,  1910; 
BUDER,  Ber.  Deutsch.  Bot.  Ges.  28,  188-192,  1910;  CAMPBELL,  Amer. 
Nat.  45,  41-53,  1911;  COWLES  and  CHAMBERLAIN,  Bot.  Gaz.  51,  147- 
153,  1911;  GRIFFON,  Bull.  Soc.  Bot.  France  55,  397-405,  1908;  56,  203  ff., 
1909;  STRASBURGER,  Jahrb.  Wiss.  Bot.  44,  482-555,  1907;  Ber.  Deutsch. 
Bot.  Ges.  27,  511-528,  1909;  WINKLER,  Ber.  Deutsch.  Bot.  Ges.  25,  568- 
576,  1907;  26a,  595-608,  1908;  28,  116-118,  1910;  Zeit.  Bot.  i,  315-345, 
1909;  2,  1-38,  1910. 

Galls  (780):  HOUARD,  Ann.  Sci.  Nat.  Bot.  20,  289-384,  1904;  KERNER; 
KUSTER,  Flora  87,  117-193,  1900;  Biol.  Cent.  20,  529-543,  1900;  Jena, 
1903;  Biol.  Cent.  30,  116-128,  1910;  MOLLIARD,  Bull.  Soc.  Bot.  France 
57,  24-31,  1910;  TUBEUF,  Nat.  Zeit.  8,  349-351,  1910. 

Fasciation  (786):  HUS,  Amer.  Nat.  42,  81-97,  1908;  Plant  World  14,  88-96,  1911; 
KNOX,  Carnegie  Inst.  Publ.  98,  1909;  WORSDELL,  New  Phyt.  4,  55-74, 
1905. 

Root  tubercles  (787) :  BUCHANAN,  Cent.  Bakt.  23,  59-91,  1909;  FISCHER, 
Ber.  Deutsch.  Bot.  Ges.  28,  10-20,  1910;  GAGE,  Cent.  Bakt.  27,  7-48,  1910; 
MOORE,  Bull.  71,  U.S.  Bur.  PI.  Ind.,  1905;  PEIRCE,  Proc.  Cal.  Acad. 
Sci.  Ill,  2,  295-328,  1902. 

Nitrogen  fixation  and  nitrification  (789):  BLACKMAN,  New  Phyt.  3,  125-129, 
1904;  HALL,  Science  22,  449-464,  1905;  JACOBITZ,  Cent.  Bakt.  7,  783  ff., 
1901;  JAMIESON,  Aberdeen,  1905;  LIPMAN,  Pop.  Sci.  Mon.  62,  137- 
144,  1902;  LOHNIS,  Cent.  Bakt.  14,  582  ff.,  1905;  PRINGSHEIM,  Biol. 
Cent.  31,  65-81,  1911;  REINKE,  Ber.  Deutsch.  Bot.  Ges.  21,  371  ff.,  1903; 
22,  95-100,  1904;  STEVENS  and  WITHERS,  Cent.  Bakt.  27,  169-186, 
1910;  VOGEL,  Cent.  Bakt.  15,  33  ff.,  1905. 

Mycosymbiosis  (791):  BERNARD,  Rev.  Gen.  Bot.  16,  405  ff.,  1904;  Ann.  Sci. 
Nat.  Bot.  IX,  9,  1-196,  1909;  BURGEFF,  Jena,  1909;  GALLAUD,  Rev. 
Gen.  Bot.  17,  i  ff.,  1905;  GRUENBERG,  Bull.  Torn  Bot.  Club  36,  165- 
169,  1909;  MACDOUGAL,  Ann.  Bot.  13,  1-47,  1899;  PEKLO,  Ber.  Deutsch. 
Bot.  Ges.  27,  239-247,  1909;  SARAUW,  Rev.  Myc.  25,  157  ff.,  1903;  STAHL, 
Jahrb.  Wiss.  Bot.  34,  539-668,  1900. 

R61e  of  root  fungi  (794):  ARZBERGER,  Mo.  Bot.  Gard.  Rep.  21,  60-102,  1910; 
BERNARD,  Bull.  Pasteur  Inst.,  1909;  FROHLICH,  Jahrb.  Wiss.  Bot.  45, 
256-302,  1908;  HEINZE,  Ann.  Mycol.  4,  41-63,  1906;  LATHAM,  Bull. 
Torr.  Bot.  Club  36,  235-244,  1909;  MAGNUS,  Jahrb.  Wiss.  Bot.  35,  205- 
272,  1900;  PEKLO,  Cent.  Bakt.  27,  451-579,  1910;  PENNINGTON,  Bull. 
Torr.  Bot.  Club  38,  135-139,  1911;  SHIBATA,  Jahrb.  Wiss.  Bot.  37,  643- 
684,  1902;  TERNETZ,  Jahrb.  Wiss.  Bot.  44,  353-408,  1907;  ZACH,  Sitz 


APPENDIX  961 

Wien  Akad.  117,  973-983,  1908;    119,  307-330,  1910;    Oest.  Bot.  Zeit.  60, 

49-55.  I9Io- 
Lichens  (800):    ELENKIN,  St.  Petersburg,  1906;    FITTING,  Ann.  Jard.  Bot. 

Buit.  1910,  505-518;    PEIRCE,  Proc.  Cal.  Acad.  Sci.  Ill,  i,  207-240,  1899; 

TOBLER,  Ber.  Deutsch.  Bot.  Ges.  29,  3-12,  1911. 
Green-celled  animals  (803):   KEEBLE  and  GAMBLE,  Quar.  Jour.  Mic.  Soc.  51, 

167-219,  1907;  52,  431-479,  1908. 


CHAPTER  V 
REPRODUCTION  AND  DISPERSAL 

Reproduction  (805):    JOST;   KUSTER,  Leipzig,  1906;    MOBIUS,  Jena,  1897. 
Fairy  rings  (807):  MASSART,  Ann.  Jard.  Bot.  Buit.  1910,  583-586;  MOLLIARD 

Bull.  Soc.  Bot.  France  57,  62-69,  1910. 
Fungus  spores   (8n):    BECQUEREL,   Compt.   Rend.   Paris  Acad.    150,    1437- 

1439,  1910;    BULLER,  Researches  in  Fungi,  London,  1909. 
Sexual  reproduction  (816) :  BLAKESLEE,  Science  25,  366-372,  1907;  GUILLIER- 

MOND,  Bull.  Sci.  44,  109-196,  1910;    HARPER,  Amer.  Nat.  44,  533-546, 

1910;   HOYT,  Bot.  Gaz.  49,  340-370,  1910;  LILLIE,  Science  25,  372-376, 

1907;    WILSON,  Science  25,  376-379,  1907. 
Significance  of  sexual  reproduction  (819):   BLACKMAN,  New  Phyt.  3,  149-158, 

1904;   BUHLER,  Biol.  Cent.  24,   65  ff.,  1904;    DANGEARD,  Le  Botaniste 

n,  1-311,  1910;   SCHULTZ,  Biol.  Cent.  25,  465-473,  1905;   WOODRUFF, 

Amer.  Nat.  42,  520-526,  1908;   Biol.  Bull.  17,  287-308,  1909. 
Apogamy  and  parthenogenesis  (822):    BLARINGHEM,  Bull.  Sci.  43,  113-170, 

1909;    KIRCHNER,  Ber.  Deutsch.  Bot.   Ges.   22,  83-97,   1904;    TREUB, 

Ann.  Jard.  Bot.  Buit.  15,  1-25,  1898;   WINKLER,  Prog.  Rei.  Bot.  2,  293- 

454,  1908. 

Flowers  (825):  KERNER. 

Anther  dehiscence  (829):   BURCK,  Rev.  Gen.  Bot.  19,  104-111,  1907. 
Pollen  (830):   LIDFORSS,  Jahrb.  Wiss.  Bot.  29,  1-38,  1896;   33,  232-312,  1899; 

PFUNDT,  Jahrb.  Wiss.  Bot.  47,  1-40,  1909. 
Stigma  (831):    LUTZ,  Zeit.  Bot.  3,  289-348,  1911. 
Pollen  tube  (832):    JOST,  Ber.  Deutsch.  Bot.  Ges.  23,  504-515,  1906;   Bot.  Zeit. 

65,  77-116,  1907;    LIDFORSS,  Zeit.  Bot.  i,  443-496,  1909. 
Wind  pollination  (834) :  KERNER. 
Insect   pollination   (838):     DARWIN    (various   works);    KERNER;     KNUTH, 

Oxford,  1906-1909;   MUELLER,  Fertilization  of  Flowers,  London,  1883. 
Pollinating  insects  (840) :  KNUTH,  Oxford,  1906-1909;  MUELLER  (as  above). 
Nectaries  (843):  HABERLANDT. 
Role  of  color  and  odor  in  flowers  (845):   ANDREAE,  Bei.  Bot.  Cent.  15,  427-470, 

1903;    DETTO,  Flora  94,  424-463,  1905;    GILT  AY,  Jahrb.  Wiss.  Bot.  40, 

368-402,  1904;  43,  468-499,  1906;   LOVELL,  Amer.  Nat.  35,  197-212,  1901; 

36,  203-242,  1902;   37,  365  ff.,  1903;   43,  338-349,  1909;   44,  673-692,  1910; 

PLATEAU,  Bull.  Roy.  Acad.  Belg.   1895-1910;    Biol.  Cent.   16-23,   l89°- 

1903;    Bull.  Soc.  Bot.  Belg.  46,  339-369,  1909. 


962  APPENDIX 

Extrafloral  nectaries  (858):  VON  UEXKULL-GULDENBANDT,  Ann.  Jard. 
Bot.  Buit.  II,  6,  195-327,  1907. 

Pollination  in  figs.  (860):  LECLERC  DU  SABLON,  Rev.  Gen.  Bot.  20,  14  ff., 
1908;  22,  65-69,  1910;  TSCHIRCH,  Ber.  Deutsch.  Bot.  Ges.  29,  83-96, 
1911. 

Cleistogamy  (864):  GOEBEL,  Biol.  Cent.  24,  673  ff.,  1904;  HACKEL,  Oest.  Bot. 
Zeit.  56,  81  ff.,  1906;  LECLERC  DU  SABLON,  Rev.  Gen.  Bot.  12,  305- 
318,  1900;  RITZEROW,  Flora  98,  163-212,  1908. 

Significance  of  cross  pollination  (866):  BURCK,  Biol.  Cent.  28,  177-195,  1908; 
DARWIN,  Cross-and  Self-Fertilisation,  London,  1876;  SHULL,  Proc.  Amer. 
Breeder's  Assoc.,  1908. 

Flower  duration  (870):    FITTING,  Zeit.  Bot.  i,  1-86,  1909;    2,  225-267,  1910. 

Closing  of  flowers  (871):  BURGERSTEIN,  Oest.  Bot.  Zeit.  51,  185-193,  1901; 
FARMER,  New  Phyt.  i,  56-58,  1902;  STOPPEL,  Zeit.  Bot.  2,  369-453, 
1910. 

Origin  of  floral  structures  (875):   ROBERTSON,  Bot.  Gaz.  37,  294-298,  1904. 

Reproductive  variation  in  the  algae  and  fungi  (878) :  COPELAND,  Bot.  Gaz.  47, 
9-25,  1909;  DANFORTH,  Mo.  Bot.  Gard.  Rep.  21,  49-59,  1910;  FREUND, 
Flora  98,  41-100,  1908;  HOYT,  Bot.  Gaz.  43,  383-392,  1907;  KAUFFMAN, 
Ann.  Bot.  22,  361-387,  1908;  KLEBS,  Jena,  1896;  Jahb.  Wiss.  Bot.  35,  80- 
.  203,  1900;  KNIEP,  Jahrb.  Wiss.  Bot.  44,  635-724,  1907;  LEWIS,  Bot. 
Gaz.  50,  59-64,  1910;  MORGENTHALER,  Cent.  Bakt.  27,  73-92,  1910; 
WAKEFIELD,  Nat.  Zeit.  7,  521-551,  1909;  WILLIAMS,  Ann.  Bot.  19, 
531-560,  1905. 

Artificial  parthenogenesis  (882) :  DAUDIN,  Bull.  Sci.  43,  297-372,  1909;  DE- 
LAGE,  Compt.  Rend.  Paris  Acad.  147,  553  ff.,  1908;  DONCASTER,  Sci. 
Prog.  3,  40-52,  1908;  4,  90-104,  1909;  LOEB,  Am.  Jour.  Physiol.  1899, 
1900;  New  York,  1906;  Arch.  Ges.  Physiol.  118,  181  ff.,  1907;  Arch.  Entwickl. 
23,  479-486,  1907;  Boston  Zool.  Congress,  1908. 

Reproductive  variation  in  pteridophytes  (884):  MOTTIER,  Bot.  Gaz.  50,  209- 
213,  1910;  SHATTUCK,  Bot.  Gaz.  49,  19-40,  1910;  WORONIN,  Flora 
98,  101-162,  1907;  WUIST,  Bot.  Gaz.  49,  215-219,  1910. 

Vegetative  and  reproductive  periods  in  seed  plants  (885):  DIELS,  Berlin,  1906; 
FISCHER,  Flora  94,  478-490,  1905;  GOEBEL  I  and  II;  HOWARD,  Halle, 
1906;  JOHANNSEN,  Jena,  1906;  KLEBS,  Jena,  1903;  Biol.  Cent.  24, 
257  ff.,  1904,  Halle,  1906;  SELIBER,  Rev.  Gen.  Bot.  21,  420  ff.,  1909, 
Jahrb.  Wiss.  Bot.  42,  155-320,  1905. 

Sex  determination  (895):  BITTER,  Ber.  Deutsch.  Bot.  Ges.  27,  120-126,  1909; 
BLARINGHEM,  Bull.  Sci.  41,  1-248,  1907;  CORRENS,  Jahrb.  Wiss.  Bot. 
44,  124-173,  1907;  45,  661-700,  1908;  Berlin,  1907;  Amer.  Nat.  42,  8n- 
824,  1908;  Ber.  Deutsch.  Bot.  Ges.  26a,  686-701,  1908;  DARLING,  Bull. 
Torr.  Bot.  Club  36,  177-199,  1909;  DONCASTER,  Sci.  Prog.  3,  40-52, 
1908;  4,  90-104,  1909;  GOEBEL,  Biol.  Cent.  30,  657  ff.,  1910;  HARPER 
Science  25,  379-382,  1907;  IORNS,  Science  28,  125-126,  1908;  JOHNSON, 
Jour.  Exper.  Zool.  9,  715-749,  1910;  JORDAN,  Pop.  Sci.  Mon.  74,  540  ff., 
1909;  Amer.  Nat.  44,  245-252,  1910;  McCLENDON,  Amer.  Nat.  44,  404- 
411,  1910;  MOBIUS,  Biol.  Cent.  20,  561-572,  1900;  MOLLIARD,  Rev, 


APPENDIX  963 

Gen.  Bot.  21,  1-7,  1909;  MORGAN,  Amer.  Nat.  44,  449-496,  1910;  SCHAFF- 
NER,  Proc.  Ohio  Acad.  Sci.  5,  327-350,  1910;  STRASBURGER,  Biol.  Cent. 
20,  657  ff.,  1900;  Jena,  1909;  Zeit.  Bot.  i,  507-524,  1909;  Jahrb.  Wiss.  Bot. 
48,  427-520,  1910;  THOMSON,  Jour.  Roy.  Mic.  Soc.  1911,  141-159;  WIL- 
SON, Science  25,  382-384,  1907;  29,  53-70,  1909;  WUIST,  Bot.  Gaz.  49, 
215-219,  1910. 

Variations  in  flower  color  (898) :  FISCHER,  Flora  98,  380-385,  1908;  KRAEMER, 
Science  23,  699,  700,  1906;  29,  828,  1909;  WHELDALE,  Proc.  Roy.  Soc. 
81,  44-6o,  1909. 

Variations  in  flower  form  (900):  GOEBEL  I  and  II;,  Biol.  Cent.  24,  673  £f.,  1904; 
MOLLIARD,  Compt.  Rend.  Paris  Acad.  133,  548-551,  1901 ;  ORTLEPP,  Flora 
98,  406-422,  1908;  VOCHTING,  Jahrb.  Wiss.  Bot.  17,  297-346,  1886. 

Bud  variation  (904):  CRAMER,  Haarlem,  1907;  EAST,  Plant  World  11,77-83, 
1908. 

Fruits  and  seeds  (904):  AVEBURY  (Lubbock),  Jour.  Roy.  Mic.  Soc.,  1909,  137- 
166;  KERNER. 

Seed  vitality  (908):  BEAL,  Bot.  Gaz.  40,  140-143,  1905;  BECQUEREL,  Ann. 
Sci.  Nat.  Bot.  IX,  5,  193—311,  1907;  Compt.  Rend.  Paris  Acad.  148,  1052- 
1054,  1909;  COKER,  Amer.  Nat.  43,  677-681,  1909;  DIXON,  Ann.  Bot. 
16,  590-591,  1902;  DUVEL,  Bull.  U.S.  Bur.  PI.  Ind.  58,  83,  1904,  1905; 
EWART,  Proc.  Roy.  Soc.  Victoria,  1908;  SCHNEIDER-ORELLI,  Flora 
100,  305-311,  1910. 

Starch  (912):  FISCHER,  Bei.  Bot.  Cent.  18,  409-432,  1905;  HABERLANDT; 
KRAEMER,  Bot.  Gaz.  34,  341-354,  1902;  REINHARD  and  SUSCHKOFF 
Bei.  Bot.  Cent.  18,  133-140,  1904. 

Fruit  and  seed  variations  (916):  FITTING,  Biol.  Cent.  29,  193  ff.,  1909;  GUIG- 
NARD,  Ann.  Sci.  Nat.  Bot.  VII,  4,  202-240,  1886;  LECLERC  DU  SABLON, 
Rev.  Gen.  Bot.  20,  14-24,  1908;  MOLLIARD,  Bull.  Soc.  Bot.  France  50, 
135  ff.,  1903. 

Fruit  and  seed  dispersal  (919):  BIRGER,  Bei.  Bot.  Cent.  21,  263-280,  1907; 
KERNER;  OSTENFELD,  Svensk.  Bot.  Tid.  2,  i-n,  1908;  RIDLEY, 
Ann.  Bot.  19,  351-363,  1905;  SERNANDER,  Stockholm,  1906. 


CHAPTER  VI 
GERMINATION 

Delayed  germination  (932):    CROCKER,  Bot.  Gaz.  42,  265—291,  1906;   44,  375- 

380,  1907. 
External  factors  and  germination  (933) :   HEINRICHER,  Ber.  Deutsch.  Bot.  Ges. 

26a,  298-301,  1908;    KINZEL,  Ber.  Deutsch.  Bot.  Ges.  25,  269-276,  1907; 

26a,  105-115,  1908;    27,  536-545,  1909;   LAAGE,  Bei.  Bot.  Cent.  21,  76-115, 

1907;    LIFE,  Mo.  Bot.  Gard.  Rep.  18,  109-122,  1907;    SCHULZ,  Bei.  Bot. 

Cent,  n,  81-97,  1901. 

Seedlings  (934) :  LUBBOCK  (AVEBURY),  New  York,  1892. 
Buds  (936) :  ARNOLDI,  Flora  87,  440-478,  1900. 


964  APPENDIX 

CHAPTER  VIII 
ADAPTATION 

Adaptation  (947):  BLARINGHEM,  Paris,  1908;  BORDAGE,  Bull.  Sci.  44,  51- 
88,  1910;  BOURNE,  Science  32,  729-742,  1910;  BUTSCHLI,  Leipzig,  1901; 
DETTO,  Jena,  1904;  DRIESCH,  Leipzig,  1901;  FARMER,  New  Phyt.  2, 
193  ff.,  1903;  GANONG,  Science  19,  493-498,  1904;  HENSLOW,  London, 
1908;  KLEBS,  Heidelberg,  1909;  MACDOUGAL,  Science  33,  94-101, 
1911;  Amer.  Nat.  45,  5-40,  1911;  Bot.  Gaz.  51,  241—257,  1911;  MORGAN, 
Science  14,  235-248,  1901;  31,  201-210,  1910;  New  York,  1903;  SCOTT, 
Nature  81,  115-118,  1909;  WENT,  Biol.  Cent.  27,  257-271,  1907;  WETT- 
STEIN,  Ber.  Deutsch.  Bot.  Ges.  18,  184-200,  1900. 


INDEX 


[Figures  in  italics  indicate  pages  upon  which  illustrations  occur.] 


Abies,  648. 

Absciss  layer,  582,  583. 

Absinthin,  626. 

Absorption,  epiphytes,  614,  615;  external 
factors,  493;  foods,  616,  752  ff.,  934; 
hairs,  613,  615,  617;  land  plants,  610; 
leaves,  608,  609,  613,  616;  rhizoids,  517, 
518;  roots,  4Qi,  493,  510,  511 ;  transpi- 
ration, 565;  water  and  salts,  491,  493, 
517,  518,  565,  608,  614,  615;  water 
plants,  609. 

Abutilon,  albescence,  524,  536. 

Acacia,  thorns  and  food  bodies,  754. 

Acceleration,  period  of,  690. 

Accommodation,  487,  760,  951. 

Accumulation,  foods,  500,  627,  628,  719, 
720,  911,  912,  913,  914,  915;  leaves,  627, 
628,629,630,631,  632,  633;  roots,  500; 
seeds,  911,  912,  913,  914,  915;  stems,  718, 
719,  720,  721,  723,  724,  725;  waste,  623, 
624,  625,  626,  718,  723,  724,  725;  water, 
627,  628,  629,  630,  631,  632,  633,  718. 

Acer,  flowers,  833;  fruit,  921 ;  galls,  7^2  ; 
habit,  584;  hairs,  572;  leaves,  542;  stem 
section,  685;  twigs,  646. 

Achene,  857,  919,  921,  924. 

Achillea,  flowers,  846. 

Acorn,  919,  925. 

Acquired  characters,  947. 

Actinomorphic  flowers,  825,  842,  859. 

Active  buds,  936. 

Adapt;  adaptation;  adaptation  characters ; 
adaptive  response,  487,  947,  949,  951. 

Adhesive  disks,  652,  653. 

Adjustment,  487,  951. 

Adult  stages,  596,  601,  725. 

Advantage;  advantageous  reactions,  487, 
488. 

Adventitious  roots,  507,  502,  503,  5/0,  512, 
514,  637,  668,  671,  673,  675,  730,  744. 

Aecidial  stage;   aecidiospores,  764,  813. 

Aeranthus,  root,  5/2. 

Aeration,  551,  660. 

Aerenchyma,  553. 

Aerial  rhizoids,  518. 

Aerial  roots,  510,  511,  512,  513,  514,  515. 

Aerial  stems,  719,  725. 

Agave,  bast  fibers,  696;  epidermis,  568; 
habit,  628;  palisade  cells,  551. 


Agelaea,  656. 

Aggregate  fruit,  924. 

Air  cavities,  stomatal,  555,  556,  558,  559; 
chambers,  551,  552,  553,  557,  561,  687, 
688,  703,  718;  leaves,  590,  592,  503,  505; 
pore,  557;  reservoirs,  554  (see  also  Air 
chambers) ;  roots,  510,  511,  512,  5/3,  514, 
515;  spaces,  534,  557,  552,  553,  554,  621. 

Albescence,  523,  524,  536. 

Albino,  845. 

Albugo,  haustoria,  763. 

Alburnum,  685. 

Alder,  buds,  938. 

Aleurone,  914,  975. 

Algae,  absorption,  609,  610;  asexual  spores, 
810;  chlorenchyma,  533;  conductive 
tissues,  685 ;  form  variations,  591,  593 ; 
lichens,  800,  801,  802 ;  reproductive  varia- 
tion, 878,  881 ;  rhizoids,  519,  520;  sapro- 
phytism,  756;  sexual  reproduction,  816, 
#77;  vegetative  reproduction,  806. 

Alkaloids,  626. 

Allium,  bulbils,  903;  flowers,  #74;  seed- 
ling, 935- 

Alnus,  buds,  938. 

Aloin,  626. 

Alpine  plants,  599,  648,  730,  737,  732,  73$, 
890,  944. 

Alternation  of  generations,  814,  822. 

Alternative  parasitism,  787. 

Ammophila,  leaf  section,  581. 

Amphibious  plants,  hairs,  574,  575,'  leaf 
variation,  590,  592,  593,  594,  595,  597. 

Amphigastria,  608. 

Amphimixis,  820. 

Amphivasal  bundles,  683. 

Anaptychia,  800. 

Anastomosing  veins,  638. 

Anatropous  ovules,  906. 

Anchoring  roots,  497,  498,  572,  573. 

Andromeda,  leaf,  578. 

Anemophilous  flowers,  834. 

Angelica,  bud  protection,  641. 

Angiosperms  (see  also  Seed  plants),  vessels, 
681. 

Animals  (see  also  Ants,  Birds,  Carnivorous 
plants,  Galls,  Insects),  dispersal  by,  923 ; 
green-celled  A.,  803;  parthenogenesis, 
882;  protection  from,  743,  9°8;  repro- 


ductive  variation,  88 1 ;  sex  determina- 
tion, 897. 

Anisophylly,  608. 

Annual,  501,  714,  715. 

Annual  rings,  685,  689,  690,  691. 

Annular  vessels,  680,  68 i, 

Annulus,  815. 

Anther,  825,  826,  829,  830,  833,  835,  840, 
851,  853,  854,  857,  871. 

Anthericum,  stomata,  560. 

Antheridia,  817. 

Anthesis,  887. 

Anthocyan,  522,  528,  529,  571,  845,  898. 

Ant  plants,  753,  754. 

Ants,  753,  754,  858. 

Aplectrum,  mycorhiza,  793. 

Apogamy,  822. 

Apospory,  822. 

Apostrophe,  524. 

Apothecia,  880;  659,  800. 

Aquatic  plants,  509,  609,  676,  678,  715,  719, 
728,  810,  837,  838,  940. 

Arabis,  root  contraction,  504. 

Aralia,  546. 

Arbor  vitae,  leaf  variation,  601. 

Archegonia,  817. 

Arctic  plants,  733,  890,  944. 

Arid  climates,  889. 

Aril,  907. 

Aristolochia,  pollination,  853. 

Arm  palisades,  532. 

Arrowhead,  leaf  variation,  590. 

Artemisia,  770. 

Artichoke,  bud,  642. 

Arum,  pollination,  860. 

Asarum,  leaf  section,  537. 

Ascocarp,  8n. 

Asexual  reproduction,  809,  810,  811,  812, 
813,  814,  £75. 

Ash,  twigs,  736. 

Aspidium,  sporangia,  815. 

Asplenium,  reproduction,  636. 

Association,  plant,  939,  940,  942,  943,  944, 
945- 

Asymmetry,  607,  734,  735. 

Atrophy,  782. 

Atropin,  626. 

Autecology,  485. 

Autoecious  parasites,  764. 

Autogamy,  829,  863  (see  also  Close  pollina- 
tion). 

Automixis,  820. 

Autoparasitism,  786. 

Autophyte,  752. 

Autotrophic  plants,  752. 

Autumn  wood,  689,  690. 

Auxospore,  809. 

Awn,  924,  929. 

Azotobacter,  789. 


Bacillus,  787,  788,  790. 

Bacteria,  carbohydrate  synthesis,  526;  in- 
fection threads,  787,  788;  parasitism, 
762,  766;  root  tubercles,  787,  788,  789, 
790;  saprophytism,  754;  vegetative  re- 
production, 806. 

Bacteroids,  787. 

Balanophora,  parthenocarpy,  917. 

Bald  cypress,  508. 

Banana,  585. 

Banyan,  prop  roots,  515. 

Barb,  654,  924. 

Barberry,  flower,  851 ;   leaf  variation,  604. 

Bark,  685,  704,  708,  709. 

Bartramia,  611. 

Basidiospores,  764,  811. 

Bast  fibers,  696,  699,  701. 

Beak,  921. 

Bean,  regeneration,  748;  root  hairs,  497; 
seed  section,  905;  twining,  651. 

Beech,  mycorhiza,  797;   seedling,  936. 

Bees,  840,  848. 

Beet,  500. 

Beetles,  842. 

Begonia,   collenchyma,   697;    leaf  section, 

533- 

Behavior,  488. 

Berberis,  flower,  851 ;   leaf  variation,  604. 
Berry,  919. 
Beta,  500. 

Betula,  lenticels,  662. 
Bicollateral  bundles,  683. 
Bidens,  fruit,  924;  pollen,  #37. 
Biennials,  501,  714. 
Bifacial  leaves,  629. 
Bilabiate  flowers,  829,  846,  853,  875. 
Birch,  lenticels,  662. 
Birds,  842,  924. 
Black  knot,  784. 
Bladder,  555,  579,  618,  619,  678. 
Bladderwort,  618,  619,  678. 
Blade,  521. 
Blastophaga,  86 i. 
Bleeding,  622. 
Bloom,  568,  570. 
Bog  plants,  486,  537,  734,  942. 
Boneset,  cork,  705. 
Bordered  pits,  680,  690. 
Botrychium,  mycorhiza,  793. 
Botrydium,  520. 

Box  elder,  flowers,  #33;   stem  section,  685. 
Bract,  827,  737,  829,  872. 
Branch  fall,  726. 
Branching,  646,  647,  649. 
Brasenia   glands,  623. 
Brassica,  intumescences,  633;    root  hairs, 

491- 

Bridging  hosts,  765. 
Bristles,  927,  929. 


INDEX 


Bromeliaceae,  614,  615,  657,  658. 

Brooms,  witches',  783. 

Bryophyllum,  lenticels,  660;   regeneration, 

637. 

Bryophytes  (see  also  Liverworts,  Mosses), 
absorption,  609,  610,  611,  612,  613,  614; 
asexual  reproduction,  814;  reproductive 
variation,  884;  vegetative  reproduction, 
807,  808. 

Bryopsis,  reversal  of  polarity,  750. 

Bud,  646,  668,  669,  674,  678,  736,  874,  888, 
8Q3,  938;  active,  936;  germination,  936, 
937)  938;  lateral,  646;  pollination,  866; 
protection,  640;  resting,  936,  938; 
scales,  642,  643,  644,  646,  833;  terminal, 
646;  variation,  904;  winter,  555,  572, 
646,  678,  736,  936,  938. 

Budding,  776,  777. 

Buffer  cells,  563. 

Bulb,  674,  675,  676,  746. 

Bulbil,  636,  675,  902,  903,  937. 

Bulbling,  636,  937,  938. 

Bulrush,  665. 

Bundle,  arrangement,  695,  701,  702,  703; 
bicollateral,  683;  collateral,  683;  com- 
pound, 682;  concentric,'  683;  conduc- 
tive, 530,  531,  533,  534,  536,  680,  682, 
683,  684,  686,  687,  688,  695 ;  fibrovascu- 
lar,  682;  radial,  683;  sheath,  530,  581, 
639,  683;  vascular  (see  conductive). 

Buoyancy,  554. 

Bur  fruit,  923,  924;   oak,  649;   reed,  545. 

Buttercup,  leaf  variation,  590. 

Butterflies,  841. 

Butterwort,  618. 

Buttressed  trunks,  508,  509. 

Cactus,  711,  725,  740. 

Caducous  bud  scales,  642. 

Calamovilfa,  497. 

Calamus,  656. 

Callus,  682,  706,  929. 

Calystegia,  849. 

Calyx,  731,  825,  826,  828,  829,  833,  835, 
846,  853,  854,  857,  859,  869;  water,  845. 

Cambium,  680,  684,  769;  cork,  66 i,  705; 
fascicular  684;  interfascicular,  684 ;  ring, 
684. 

Campanula,  leaf  section,  598 ;  leaf  varia- 
tion, 598,  600,  601,  602. 

Campy lotropous  ovules,  905. 

Canna,  seed  section,  905. 

Cap,  root,  497. 

Capitate  hairs,  623. 

Caprification  ;  caprifig,  86 1. 

Capsule,  666,  814,  874,  919,  920. 

Carbohydrates,  521,  525,  912  (see  also 
Starch,  Sugar) ;  conduction,  694 ;  syn- 
thesis, 525,  526,  527,  563,  588,  660,  665. 


Carbon  dioxid,  527. 

Carnation,      mechanical     cylinder,      701; 

stoma,  558. 

Carnivorous  plants,  616,  617,  618,  619. 
Carotin,  522. 
Carpel,  826,  924. 
Carpophore,  924. 
Carpospore,  811. 
Castalia,  540. 

Castor  bean,  endosperm,  914. 
Catalpa,  twigs,  736. 
Catkin,  828,  834. 
Cauliflory,  839. 

Cauliflower,  intumescences,  633. 
Cave  moss,  549. 

Cavity,  stomatal,  555,  556,  558,  559,724- 
Cecidia,  781. 
Cecidomyia  galls,  784. 
Cellulose  layer,  568;  reserve,  913. 
Celtis,  leaves,  607. 
Century  plant,  bast  fibers,  696;  epidermis, 

568;  habit,  628;  palisades,  551. 
Cephaloneon  gall,  782. 
Ceratophyllum,  leaf  section,  688. 
Cereus,  711. 
Chalaza,  905. 
Chalk  gland,  621. 
Chamber,  air,  551   (see  also  Air  chamber, 

Air  space) ;   larval,  782. 
Chasmogamous  flowers,  864. 
Chemotropism,  767. 
Cherry,  gall,  784;  shoot,  643. 
Chimera,  780. 
Chlorenchyma,  530,  531,  532,  533,  534,  535, 

536,  538,  557,  567,  574,  58i,  621,  629, 

630,  631,  632,  639,  661,  688,  724. 
Chlorophyll,  512,  521,  523,  525,  660. 
Chlorophyllin,  522. 
Chloroplast,  511,  517,  521,  522,  524,  531, 

533,  534,  538,  555,  556,  557,  559,  612, 

688,  705. 
Chlorosis,  523. 
Choke  cherry,  shoot,  643. 
Chromatophore,  522. 
Chromoplast,  522. 
Cicuta,  bulblings,  937. 
Cilia,  810,  817. 

Cineraria,  habit,  541 ;  hairs,  573. 
Circinate  vernation,  937. 
Cistern  epiphyte,  659. 
itrus,  oil  gland,  624. 
Cladonia,  algal  cell,  802.;  habit,  611. 
laviceps,  sclerotium,  808. 
left  grafting,  777. 
Cleistogamy,  864,  865,  901. 

limate,  691,  889,  938. 
Climbing  roots,  512,  513,  653;    stems,  651, 

652,  653,  654,  655,  656. 
Close  pollination,  829,  852,  863,  866,  869. 


INDEX 


Closing  of  flowers,  871,  872,  873. 

Clover,  root  tubercles,  788. 

Coat,  seed,  905,  907,  909,  915,  932. 

Cobaea,  tendrils,  652. 

Cocain,  626. 

Cocklebur,  fruit,  924;  fruit  section,  932. 

Codiaeum,  pollen,  831. 

Coenocyte,  520,  697,  750. 

Coleus,  epidermis,  577;   flowers,  829,  846. 

Collateral  bundles,  683. 

Collenchyma,  697,  699. 

Colony,  rhizome,  671. 

Color,  527,  528,  708,  845,  847,  898,  925. 

Columella,  811. 

Columnar  strength,  704. 

Commensal;   commensalism,  752,  753. 

Companion  cells,  681,  682. 

Compass  plants,  547. 

Competition,  487. 

Complementary  tissue,  661. 

Compositae,  flowers,  856,  862. 

Compound  bundles,  682;  leaves,  544,  546, 
550,  605,  652,  676,  783. 

Compression  strength,  704. 

Concentric  bundles,  683. 

Conduction  of  carbohydrates,  694. 

Conductive  bundles,  530,  531,  533,  534,  536, 
581,  617,  621,  680,  682,  683,  684,  686, 
687,  688,  695,  701,  702,  703;  elements, 
679,  680,  68 1 ;  parenchyma,  682;  tissue, 
513,  55i,  630,  638,  639,  678,  679,  680, 
681,  682,  683,  684,  685,  686,  687,  688, 
689,  690,  692,  693,  769. 

Conduplicate  vernation,  937. 

Cone,  fruit,  938;  tree,  648. 

Congenital  structures,  951. 

Conidia,  811. 

Conifers,  648. 

Connective,  840. 

Contact  pollination,  862. 

Contraction,  root,  504,  505. 

Convoluta,  803. 

Convolute  vernation,  937. 

Cord  grass,  rhizomes,  669. 

Cork,  661,  705,  706,  707;  cambium,  661, 
705;  cortex,  705;  wound,  707. 

Corm,  675,  676. 

Cornus,  stem,  730. 

Corolla,  731,  825,  826,  828,  829,  835,  840, 
841,  846,  854,  857,  859,  875. 

Corona,  902. 

Correlation,  486,  509,  747. 

Cortex,  769,  782;  cork,  705. 

Corylus,  flowers,  834. 

Corymb,  828,  846. 

Cosmopolitan  plants,  927. 

Cottonwood,  absciss  layer,  583;  twigs, 
736. 

Cotyledon  (plant),  leaf  section,  629. 


Cotyledon  (seed  leaf),  491,  601,  905,  906, 

935,  936. 

Crab  apple,  spines,  739. 
Crataegus,    655;    root    propagation,    505; 

spines,  739. 
Creation,  special,  947. 
Creeping  stems,  507,  672,  673. 
Creosote  bush,  roots,  506. 
Cross  pollination,  829,  836,  852,  854,  859, 

866,  867. 

Croton,  pollen,  831. 
Crown,  902. 
Crustose  lichens,  615. 
Crumpled  vernation,  937. 
Crystal  aggregates,  625;    sand,  625. 
Crystals,  625,  723;    protein,  912,  915. 
Cucumber,  tendrils,  652. 
Cup,  acorn,  925. 
Cuphea,  pollen,  #37. 
Cupule,  808. 
Currant,  leaf  veins,  639. 
Cuscuta,  habit,  768;  haustoria,  769. 
Cushion  plants,  611,  738. 
Cuticle,  532,  535,  55^,  567,  568,  569,  570, 

623,  639,  642,  697. 

Cuticular  layer,  568;  transpiration,  564. 
Cutin;  cutinization,  557,  568,  569. 
Cuttings,  propagation  by,  637. 
Cyanic  colors,  845. 
Cyclanthera,  tendril  sections,  699. 
Cycloloma,  922. 
Cyme,  825,  828,  829. 
Cynara,  bud,  642. 
Cynipid  galls,  782. 

Cypress  knees,  507,  508,  509;   swamp,  508. 
Cystolith,  626. 

Dandelion,  flower,  #57,'  inflorescence  move- 
ments, 872;  roots  and  stems,  677;  varia- 
tion, 599. 

Date,  seedling,  935. 

Dead  tissues,  advantage,  693. 

Death,  587,  910. 

Deciduous  herbs,  712,  713;  trees,  583,  584, 
649,  710. 

Decussate  phyllotaxy,  542,  543,  549,  641, 

647- 
Dehiscence,  anthers,  829,  830;   fruits,  919, 

920. 

Delayed  germination,  932. 
Deliquescent  trees,  584,  649,  650. 
Dependent  plants,  752. 
Desert    plants,    505,    635,    944     (see    also 

Xerophytes) . 
Desiccation,  587,  700. 
Determinate  inflorescence,  825,  828. 
Determinative  factors,  951. 
Dextrorse  twiners,  652. 
Dextrose,  914. 


INDEX 


Dianthus,  mechanical  cylinder,  707;  stoma, 

55*. 
Diaphototropism,  539,  540,  542,  544,  546, 

547- 

Diaphragm,  552. 
Dichogamy,  818,  836,  852. 
Dicliny,  827,  856. 
Dicotyls,  chlorenchyma,  550;  seedling,  936 

(see  also  Seed  plants). 
Differentiation,  488. 
Digestion,  934. 
Digestive  hairs,  617. 
Digitalin,  626. 

Dioecious  plants,  818,  827,  833,  895. 
Diospyros,  endosperm,  913. 
Disease  resistance,  768. 
Disk,  adhesive,  652,  653;    flowers,  846. 
Dispersal,  805,  910*,  920,  922,  923,  925. 
Display,  stem,  542,  543,  546,  548,  645,  646, 

648,  649,  650,  666,  667. 
Disseminule,  805. 
Distichous  phyllotaxy,  510,  549. 
Disuse,  948. 

Divided  leaves,  544,  546,  550. 
Dodder,  habit,  768;   haustoria,  769. 
Dogwood,  stem,  730. 
Domatia,  753. 

Dorsal  walls,  555,  556,  558,  562. 
Dorsiventral  symmetry,  489,  629,  630. 
Double  flowers,  901,  902. 
Douglas  spruce,  735. 
Dripping  point,  636. 
Dropper,  505,  675. 
Drosera,  digestive  hairs,  616,  617. 
Drupe,  919. 
Duckweed,   vegetative   reproduction,    678; 

water  roots,  509. 
Ducts,   conductive,   679,  680,  68 1;    resin, 

723,  724. 

Dune  plants,  729,  730. 
Duramen,  685,  725. 
Duration,  flowers,  870;  leaves,  582;   roots, 

501;  stems,  717. 

Dwarf;   dwarfing,  730,  731,  732,  733,  734. 
Dyes,  723. 

Earth  star,  812. 
Echinocactus,  740. 
Echinocystis,  tendrils,  652. 
Ecological  variation,  486. 
Ecology,  485. 
Ectoparasite,  763. 
Ectotrophic  mycorhiza,  791,  792. 
Edema,  633. 
Egg,  816,  817,  827. 
Eichhornia,  root,  570. 
Elaeagnus,  hairs,  573. 
Elaioplast,  721. 
Elater,  814. 


Elder,  lenticel,  66 i. 

Elecampane,  inulin,  914. 

Elm,  655. 

Elodea,  vascular  bundle,  687. 

Elongation,  646,  648,  725,  726,  727,  728, 

729,  736. 

Embryo,  816,  826,  905,  931 ;   sac,  826. 
Emergence,  654,  742. 
Endemic  plants,  927. 
Endodermis,  511,  617,  683,  696,  703. 
Endogenous  origin,  489. 
Endoparasite,  763. 
Endosaprophyte,  787,  802. 
Endosperm,  906,  913,  914. 
Endotrophic  mycorhiza,  792,  793,  796. 
Entomophilous  flowers,  487,  838. 
Ephedra,  664. 
Epidendrum,  air  roots,  5/0. 
Epidermal  cork,  705;  gland,  622,  623. 
Epidermis,  491,  493,  511,  530,  531,  532, 

535,  534,  535,  536,  55&,  560,  567,  568, 

570,  571,  572,  573,  574,  576,  577,  581, 

615,  617,  621,  623,  632,  642,  661,  688, 

704,  724,  769,  781,  782. 
Epigaean  germination,  491,  935,  936. 
Epigynous  flowers,  828,  857. 
Epilithic  lichens,  615. 
Epinasty,  582. 
Epiphyll,  659. 
Epiphyte,  657,  658,  659,  660;    absorption, 

614,  615;  dispersal,  927;  roots,  510,  511, 

512,  514- 
Epistrophe,  524. 
Epithem,  621. 

Equilateral  leaves,  629,  630,  631. 
Equisetum,    reproductive    variation,    885; 

stem  variation,  733. 
Erectness,  stem,  647. 
Ergot,  sclerotium,  808. 
Ericaceae,  leaves,  57$. 
Erythronium,  dropper,  505. 
Etiolation;    etiolin,  522. 
Eupatorium,  cork,  705. 
Euphorbia,     habit,     647;      nectary,     844; 

pollen,  831. 

Euphrasieae,  parasitism,  772,  773,  774,  775. 
Everbloomer,  887. 
Evergreen,  herbs,  712;  trees,  583,  584,  585, 

586,  709,  710. 
Evergrower,  737. 

Excretion,  620,  622,  626,  722,  724;  root,  493. 
Excurrent  trees,  584,  647,  648. 
Exfoliation,  708,  709. 
Exine,  830,  831. 
Existence,  struggle  for,  487. 
Exodermis,  492,  511. 
Exogenous  origin,  489. 
External  factors,  486,  etc. 
Extrafloral  nectaries,  844,  858. 


INDEX 


Extrorse  anthers,  851. 
Exudation,  water,  620,  622. 
Eyes,  674. 

Factors,  488;  correlative,  486;  determina- 
tive, 951;  external,  486,  etc.;  formative, 
952;  inherent,  486;  internal,  486;  re- 
leasing, 952. 

Facultative  dwarfs,  734;  epiphytes,  660; 
hydrophytes,  951;  mesophytes,  951; 
parasites,  762;  saprophytes,  762;  xero- 
phytes,  951. 

Fagus,  mycorhiza,  791;  seedling,  936. 

Fairy  rings,  807. 

False  Solomon's  seal,  rhizome,  668. 

Fasciation,  786. 

Fascicular  cambium,  684. 

Fats,  521,  913. 

Felt,  woolly,  572,  573,  574- 

Ferns,  asexual  reproduction,  815;  climb- 
ing, 6 S3;  foliage,  550;  reproduction, 
636;  rhizoids,  516. 

Fescue  grass;  Festuca,  flowers,  835. 

Fiber,  bast,  696,  699,  701 ;  wood,  697. 

Fibrovascular  bundle,  682. 

Ficus,  leaf,  636,  638;  pollination,  860,  861 ; 
roots,  502,  514,  515;  stoma,  558. 

Fig,  pollination,  860,  861 ;  strangling,  514, 
515;  wasp,  86 1. 

Figwort,  flowers,  853. 

Filament,  825,  826,  835,  851. 

Fir,  648. 

Fission,  591,  806. 

Fixation  of  nitrogen,  789,  797. 

Fleshy  fruits,  924;  roots,  500. 

Flexile  strength,  702. 

Flies,  842. 

Flower-forming  substances,  750,  891. 

Flowers,  599,  731,  768,  825,  826,  829,  833, 
834,  835,  837,  838,  840,  841,  843,  846, 
849,  851,  853,  854,  857,  859,  861,  865, 
869,  870,  871,  872,  873,  874,  875,  893, 
902,  903;  closing,  871,  872,  873;  color, 
845,  847,  898;  double,  901,  902;  dura- 
tion, 870;  form,  900;  plastids,  522;  pol- 
lination, 829,  834,  838;  protection,  857, 
869,  870,  871,  872,  873;  single,  902; 
structure,  827,  828,  875;  variation,  898, 
900. 

Foliage,  539,  544  (see  Leaves). 

Foliose  lichens,  615,  659,  800. 

Food  absorption,  616,  752  ff.,  934;  accu- 
mulation, 500,  627,  719,  720,  911,  912, 
913,  914,  915;  bodies,  754;  digestion, 
934;  manufacture,  521,  525,  528;  sur- 
plus, 718. 

Foreign  pollen,  855. 

Forests,  945;  tropical,  656,  946;  under- 
growth, 545,  546,  550. 


Formaldehyde,  525. 

Formative  factors,  952. 

Fortuitous  variation,  950. 

Fragmentation,  677. 

Fraxinus,  twigs,  736. 

Freezing,  587. 

Frigid  climates,  889. 

Fruit,  865,  874,  875,  904,  905,  916,  917,  919, 
920,  921,  924,  925,  929,  938;  dehiscence, 
919;  dispersal,  919;  dots,  636,  659;  pro- 
tection, 874. 

Fuchsia,  leaf  teeth,  62 1. 

Fucus,  sexual  reproduction,  817. 

Function,  487. 

Fungi,  asexual  reproduction,  811,  812,  813; 
galls,  784;  gardens,  754;  parasitic,  762, 
763,  766;  reproductive  variation,  878, 
880;  root,  791,  792,  793,  794,  796; 
saprophytic,  754;  sexuality,  883;  tuberi- 
zation,  745,  746;  vegetative  reproduction, 
806. 

Funiculus,  905. 

Galls,  576,  780,  781,  782,  783,  784,  785; 

flowers,  86 i ;   hairs,  574,  576. 
Gametangium,  817. 
Gamete,  816,  817,  826. 
Gametophyte,  666,  814,  826. 
Garlic,  wild,  bulbils,  903. 
Gas  exchanges,  551,  554,  563. 
Geaster,  812. 
Geitonogamy,  829,  862. 
Gemmae;  gemmation,  807,  808. 
Generalized  parasites,  765. 
Generations,  alternation  of,  814,  822. 
Geophilous  plants,  487. 
Geotropism,  499,  668,  670. 
Geranium,  hairs,  623. 
Germination,  buds,  936,  937,  938;  delayed, 

932;    epigaean,   491;    hypogaean,   492, 

494;   pollen,  832;    seeds,  906,  930,  931, 

932,  933,  934,  935,  936. 
Geum,  leaf  variation,  602. 
Gigantism,  734. 
Ginger,  wild,  leaf  section,  531. 
Ginseng,  dwarf,  corm,  676. 
Girdling,  694. 
Gland,   chalk,   621;    epidermal,   617,   622, 

623,  625 ;  hair  (see  epidermal) ;  internal, 

623,  624;  mucilage,  632;  oil,  624;  salt, 

613,  621;   slime,  561,  623. 
Glaucous  bloom,  568,  570. 
Gleditsia,  771. 
Globoid,  914,  915. 
Glochidia,  818. 
Gluten  layer,  915. 
Glycerid,  913. 
Goldenrod,  galls,  784. 
Gonidia,  800,  80 1. 


INDEX 


Gourd,  sieve  plates,  68 1. 

Graft  hybrids,  779. 

Grafting,  776,  777. 

Grain,  919 ;    pollen,  826,  830,  831,  834. 

Grape,  gall,  576;   habit,  655. 

Grass,  flowers,  835;  foliage,  544,  650;  leaf 

section,  531 ;  rhizome,  669;  stoma,  556. 
Grasslike  foliage,  544,  545. 
Gravity,  499;  pollination,  863. 
Green-celled  animals,  803. 
Greening,  902. 

Growth  ring,  685,  689,  690,  691,  Qi2. 
Guard  cell,  555,  556,  558,  550,  562,  571,  621, 

724. 

Gutter  point,  636. 
Gymnosperm  tracheids,  680. 

Hackberry,  leaves,  607. 

Hadrocentric  bundle,  683. 

Hadrome,  530,  630,  638,  682,  683,  692. 

Hairs,  absorptive,  609,  615,  617 ;  capitate, 
623;  digestive,  617;  glandular,  617,  622, 
623,  625;  nutritive,  782;  protective, 
536,  567,  572,  573,  574,  575,  57$,  577, 
578,  623,  661,  781,  858;  root,  4Qi,  4Q2, 
493,  494,  495,  496,  512,  513;  stinging, 

577- 

Halophytes,  537,  632,  942. 
Haptera,  519,  520. 
Harebell,  leaf  section,  598;  leaf  variation, 

598,  600,  601,  602. 

Haustorium,  763,  768,  769,  771,  772. 
Hawthorn,    655;    root   propagation,   505; 

spines,  739. 
Hazel,  flowers,  834. 
Head,  599,  828,  846,  872. 
Heart- wood,  685. 

Helianthemum,  leaf  variation,  731. 
Heliophobe,  487. 
Helleborus,  stoma,  562. 
Helotism,  787,  802. 
Hemicellulose,  719,  913. 
Hemi-epiphyte,  659. 
Hemlock,  dwarfed,  732. 
Herb,  646,  650,  712,  713,  714,  715,  716. 
Hermaphroditic  plants,  818. 
Heterocyst,  806. 

Heteroecious  parasites,  764,  767. 
Heterogamy,  817. 
Heterophyte,  752. 
Heterospory,  815,  819. 
Heterostyly,  854. 
Heterotrophic  plants,  752. 
Hexarch  bundles,  683. 
Hibernaculum,  678. 
Hibiscus,  pollen,  831 ;   stigma,  831. 
Hilum,  919. 

Hinge,  stomatal,  556,  558,  562. 
Holdfast,  519. 


Holly,  leaf,  570. 

Holoparasite,  761,  770. 

Holosaprophyte,  754. 

Homogamy,  852. 

Homospory,  814. 

Honey  dew,  813. 

Honeysuckle,  leaves,  543. 

Hook  climber,  654. 

Hop,  barbs,  654. 

Hop  tree,  crystals,  625;  fruit,  921;  vas- 
cular bundle,  639. 

Horizontal  leaves,  539,  540. 

Hormogonia,  806. 

Hornwort,  leaf  section,  688. 

Host,  752,  769,  780;   bridging,  765,  766. 

Hoya,  sclereid,  697. 

Humulus,  barbs,  654. 

Husk,  915. 

Hyacinth;  Hyacinthus,  bulb,  675;  water, 
5X0. 

Hybrid,  904;  graft,  779. 

Hybridization,  904. 

Hydathode,  620,  62 1,  622. 

Hydrocotyle,  507. 

Hydrodictyon,  zoospore,  810. 

Hydroid,  679. 

Hydrome,  682. 

Hydrophile,  487. 

Hydrophytes,  486,  715,  719,  728,  939,  940, 
941;  absorption,  609;  chlorenchyma, 
530,531,534;  congenital,  951;  faculta- 
tive, 951 ;  leaf  sections,  531,  534,  561, 
688;  obligate,  951;  pollination,  837, 
838;  reaction,  951;  roots,  509;  stem 
reproduction,  676,  678;  stomata,  559, 
560,  561,  564. 

Hydrotropism,  499. 

Hymenium,  800. 

Hyperchimera,  780. 

Hyperplasy,  781. 

Hypertrophy,  633,  781. 

Hyphae,  755,  763. 

Hypocotyl,  906. 

Hypodermis,  705,  724. 

Hypogaean  germination,  492,  494,  936. 

Hypogynous  flowers,  825,  826,  828,  843, 
851- 

Hyponasty,  582. 

Hypoplasy,  782. 

Idioblast,  682. 

Ilex,  leaf,  570. 

Immunity,  768. 

Impatiens,  pollen,  831. 

Impotent  pollen,  854. 

Inbreeding,  867. 

Indehiscent  fruit,  919.  * 

Independent  plants,  752. 

Indeterminate  inflorescence,  828,  82g. 


g 


INDEX 


Indian  pipe,  mycorhiza,  702. 

India  rubber  tree,  roots,  502;  stoma,  558. 

Indusium,  815. 

Infection  thread,  787,  788. 

Inflorescence,  628,  825,  827,  829,  833,  834, 

835,  837,  846,  865,  871.  872,  874,  875. 
Infolded  leaves,  581. 
Inherent  factors,  486;    rhythm,  737. 
Inheritance  of  acquired  characters,  947. 
Insect  galls,   782,   783,   784;    pollination, 

838,  839,  840,  841,  842,  847,  850,  857. 
Insectivorous  plants,  616,  617,  618,  619. 
Instinct,  849. 
Integument,  905. 
Interfascicular  cambium,  684. 
Internal  factors,  486. 
Internal  glands,  623,  624,  723,  724. 
Internode,  489. 
Intine,  831. 
Introrse  anthers,  851. 
Intumescence,  633. 
Inula;  inulin,  914. 
Involucre,  642,  827,  846,  872,  921. 
Involute  leaves,  581;    vernation,  937. 
Ipomoea,  roots,  500. 
Isoetes,  chloroplasts,  524. 
Isogamy,  816,  817. 
Ivy,   Japan,   leaf   variation,   605;    leaves, 

544;  tendrils,  652. 

Juncus,  diaphragms,  552;  mechanical 
cylinder,  699;  rhizome,  670,  674;  rhi- 
zome section,  703. 

Juniper;  Juniperus,  habit,  672;  shoot, 
582. 

Jussiaea,  aerenchyma,  553;  cork,  705; 
roots,  508. 

Juvenile  stages,  596,  601,  725. 

Key  fruit,  921. 

Knees,  cypress,  507,  508,  509. 
Knot,  black,  784. 
Krummholz,  731,  732. 

Labiate  flowers,  829,  846,  853,  875. 

Labrador  tea,  leaves,  578. 

Lactuca,  achene,  921;   conductive  vessels, 

679;  latex  tubes,  721,-  leaf  section,  535; 

leaves,  547;    root  hairs,  493, 
Lacunae;    lacunar  tissue,   530,   534,   552, 

553,  56i. 

Lagenaria,  sieve  plates,  68 i. 
Laminae,  maxillary,  841. 
Lamium,  palisade  cells,  538. 
Land   plants,    leaf   absorption,   610;    leaf 

variation,  598,  599,  600,  601,  602,  603, 

604,  605,  606,  607,  608. 
Laportea,  stinging  hairs,  577. 
Larrea,  roots,  506. 


Larval  chamber,  782. 

Lateral  branches,  647;    buds,  646,  736. 

Latex,  720,  722;   sac,  720;   tubes,  727. 

Lathyrus,  stipules,  641;   tendrils,  640. 

Laticiferous  vessel,  721. 

Layer,  separation,  709. 

Leaf,  488,  489,  521;  absorption,  608,  609, 
613,  616;  absorptive  hairs,  613,  615,  617; 
air,  590,  592,  593,  595:  air  spaces,  534, 
55i,  552,  553,  554,  557,  561,  688;  al- 
bescence,  523,  524,  536;  anisophylly, 
608;  anthocyan,  528,  529;  asymmetry, 
607;  carnivorous,  617,  618,  619;  chloren- 
chyma,  530,  531,  532,  533,  534,  535,  536, 
538,  567,  581,  629,  630,  631,  632,  688, 
724;  chlorophyll,  521,  523  ;  chloroplasts, 
521,  524,  531,  533,  534,  538,  688;  com- 
pound, 546,  550,  652,  676,  783;  conduc- 
tive tissues,  530,  531,  533,  534,  536,  630, 
638,  639;  correlation,  509;  crystals,  625  ; 
cuticle,  532,  535,  558,  567,  568,  569,  570, 
639;  epidermis,  530,  531,  532,  533,  534, 

535,  536,  567,  568,  570,  571,  632,  688, 
724;     evergreen,    585;     excretion,    620, 
622;  -fall,  582,  583,  585,  586;    food  ab- 
sorption, 616;    food  accumulation,  627; 
food  manufacture,  521,  525,  528;    galls, 
781;     gas    exchanges,    551;     glandular 
hairs,  617,  622,  623,  625  ;  hairs,  536,  572, 
573,    574,   575,    57$,    577,    578;     hori- 
zontal, 539,  540;  hydathodes,  620,  621, 
622;  internal  glands,  623,  624;  intumes- 
cences, 633;  involute,  581 , •  lacunae,  552, 
561 ;  light,  501,  539,  540,  541,  542,  543, 
544,  546,  547,  548,  549,  55O,  603,  607,  647; 
mechanical  tissue,  639;  mosaic,  543,  544, 
607;   motile,  579,  580,  582;  orientation, 
540,  541,  542,  543,  544,  547,  548;   pali- 
sade cells,  530,  532,  533,  534,  535,  536, 
538,  561,  567,  574,  630,  639,  642;  petiole, 
507,  540,  541,  542,  640,  641;   phototro- 
pism,    539,    540,    542,    544,    546,    547,' 
phyllotaxy,  510,  542,  543,  549,  641,  647; 
pigments,    522,    528;     plasticity,    596; 
position,  549;  protection,  565,  588;  pro- 
tective hairs,  536,  572,  573,  574,  575,  576, 

577,  578;  regeneration,  636,  637;  repro- 
duction, 636,  637;  roots,  504,  637;  run- 
off, 636;   scale,  641,  642,  643,  644,  669, 
670,  675,  733,  736,  768,  834;   scar.  583,  - 
644,  657,  660,  708,  736;  sclerophyll,  570, 

578,  588;   secretion,  620,  622,  858;   sec- 
tion, 521,  530,  531,  532,  533,  534,  535, 

536,  561,  567,  574,  581,  612,  629,  630, 
631,  632,  639,  642,  688,  724;  sinus,  637, 
639;    small,    550;    spines,    604;   sponge 
tissue,    530,    533,    534,  535,  536;    stem 
display,  542,  543,  546,  548,  645,  646,  650 ; 
stipules  640,  641;  stomata,  530,  531, 534, 


INDEX 


535,  536,  555,  556,  55-?,  559,  560,  561, 
562;  subterranean,  642;  teeth,  621;  ten- 
drils, 640,  641 ;  transpiration,  565,  577, 
578,  588;  variation,  544,  575,  589,  590, 
592,  593,  594,  595,  597,  59$,  599,  600, 601, 

602,  603,   604,   605,   606,   607,    608,   673, 
726;   727,    731,    741,    784;    veins,    638, 
639;  vertical,  546,  547,  549,  578;   water, 
590, 592,  593,  595,  837;  water  accumula- 
tion, 627,  628,  629,  630,  631,  632,  633; 
water  exudation,  620,  622;  water  tissue, 
533,  629,  630,  631,  632,  635. 

Leafless  stem,  664,  665,  725,  733,  740;  tree, 
710,  777. 

Leaflet,  579,  640,  652,  783. 

Leaner,  654. 

Ledum,  leaves,  578. 

Legume,  920. 

Leguminosae,  root  tubercles,  787,  788,  790. 

Lenticel,  660,  661,  662,  663,  736;  water, 
663. 

Leonurus,  leaf  variation,  606. 

Lepidium,  leaf  section,  530;  roots,  498; 
rosette,  582. 

Leptocentric  bundle,  683. 

Leptome,  530,  630,  639,  682,  683,  689. 

Lettuce,  achene,  921;  conductive  vessels, 
679;  latex  tubes,  721 ;  leaf  section,  535; 
leaves,  547;  root  hairs,  493. 

Leucobryum,  leaf  section,  612. 

Leucoplast,  528,  721. 

Level,  law  of,  668,  670. 

Lianas,  651,  652,  653,  654,  655,  656;  con- 
ductive tissue,  689;  mechanical  tissue, 
703  ;  roots,  512,  513. 

Libriform,  697. 

Lichens,  absorption,  610,  611,  615  ;  chloren- 
chyma,  533;  epiphytism,  659;  protec- 
tion, 588;  rhizoids,  520;  symbiosis,  800, 
801,  802;  vegetative  reproduction,  806. 

Lid,  814. 

Light,  carbohydrate  synthesis,  526;  chloro- 
phyll, 523;  elongation,  726;  flower  color, 
898;  leaf  form,  594,  599;  leaf  position, 

603,  607;  leaves,  501,  539,  540,  541,  542, 
543,  544,  546,  547,  54$,  549,  55O,  647; 
palisade  cells,  535 ;   stomata,  563. 

Lignification  ;   lignin,  679,  693. 

Ligustrum,  lenticel,  661. 

Lilium;  lily,  bulbils,  675;  leaf  section, 
532;  stoma,  555. 

Linear  migration,  669,  670,  671. 

Lip,  829,  846. 

Lithophyte,  659. 

Liverworts,  chlorenchyma,  533,  557;  con- 
ductive tissue,  685 ;  gemmae,  808;  rhi- 
zoids, 516,  517. 

Locust,  honey,  777. 

Longevity  of  seeds,  908. 


Long  moss,  614. 

Lonicera,  leaves,  543. 

Loosestrife,  leaf  section,  642;   leaves,  641. 

Lopseed,  flowering  shoot,  875. 

Lunularia,  gemmae,  808;    rhizoids,  517. 

Lupine ;   Lupinus,  pod,  920. 

Lycopodium,  tuber,  746. 

Lysimachia,  leaf  section,  642;  leaves,  641. 

Maize,  roots,  494,  499,  514. 

Man,  946. 

Mangrove,  roots,  575;  vivipary,  930,  931. 

Mantle,  water,  533,  631,  632. 

Maple,  fruit,  921;  gall,  782;  habit,  584; 
leaves,  542 ;  scale  hairs,  572;  twigs,  646. 

Marchantia,  gemmae,  808;  rhizoids,  517; 
thallus  section,  557, 

Margo,  681. 

Maritime  associations,  942. 

Marshes,  salt,  942. 

Marsilea,  reproductive  variation,  885. 

Maturity,  period  of,  690. 

Maxillary  laminae,  841. 

Meadow  fescue,  flowers,  835. 

Mechanical  tissues,  573,  581,  639,  696,  697, 
698,  699,  700,  707,  702,  703,  705. 

Medullary  ray,  684,  685. 

Megaspore,  815,  826. 

Melilotus,  root  tubercles,  788. 

Memory,  849. 

Mermaid  weed,  leaf  sections,  534;  leaf 
variation,  592,  593,  595- 

Mesophyll,  521,  642,  688. 

Mesophytes,  486,  939,  945  ;  chlorenchyma, 
530,  53i,  534;  congenital,  951;  faculta- 
tive, 951;  forest,  656;  leaf  sections, 
530,598;  obligate,  951 ;  reaction,  951. 

Mestome,  682,  683. 

Meteorological  factors,  887. 

Micellae,  697. 

Micropyle,  832,  905. 

Microspore,  815,  825,  826,  830. 

Midrib,  638. 

Migration,  927;  linear,  669,  670,  671; 
radial,  669,  672. 

Milfoil,  water,  stem  section,  551. 

Mimosa,  leaves,  579. 

Mint,  mountain,  stem  section,  702. 

Mistletoe,  777. 

Mixophyte,  754. 

Mobility,  927. 

Moisture  and  stomatal  movement,  563. 

Mold,  habit,  755;   sporangium,  Sil. 

Monocarpic  plants,  886. 

Monocliny,  825,  826,  827,  828,  835,  838. 

Monocotyls,  chlorenchyma,  530,  537;  seed- 
lings, 935;  stomata,  560  (see  also  Seed 
plants). 

Monoecious  plants,  818,  827,  834. 


INDEX 


Monotropa,,  mycorhiza,  792. 

Moors,  942. 

Morning  glory,  flowers,  840. 

Morphological  ecology,  485. 

Morus,  fruit,  924;  leaf  variation,  603. 

Mosaic,  leaf,  543,  544,  607. 

Mosses,    absorption,    610,   611,    612,    613; 

chlorenchyma,   533;     chloroplasts,   521; 

conductive   tissue,    685,    686;    rhizoids, 

517,  518;   vegetative  reproduction,  807. 
Motherwort,  leaf  variation,  606. 
Moths,  841. 

Motile  leaves,  570,  580. 
Mountain  sheep,  738. 
Movements,  chloroplasts,  524;  leaves,  57 9, 

580;   stomata,  562;   zoospores,  810. 
Mucilage;   mucilage  glands,  517,  622,  624, 

632,  723- 

Mucor,  habit,  755;    sporangium,  811. 
Mulberry,  fruit,  024;   leaf  variation,  603. 
Mullein,  hairs,  572. 
Multicipital  stems,  504,  676,  677. 
Musa,  leaves,  585. 
Mustard,  root  hairs,  4Qi. 
Mutualism,  786. 
Mycelium,  755. 
Mycophyte,  794. 
Mycoplasm,  764. 
Mycorhiza,  791,  702,  703,  796;  ectotrophic, 

791,  702;   endotrophic,  792,  793,  796. 
Mycosymbiosis,   791,  792,   793,   794,   796, 

798,  799. 

Myriophyllum,  stem  section,  551. 
Myrmecophily ;    myrmecophyte,  753,  754. 

Nanism,  734. 

Narcissus,  flowers,  902. 

Nasturtium,    flower,    843;     phototropism, 

540;  plastids,  522 ;  pollen,  831;  stomata 

620. 

Natural  selection,  743,  876,  952. 
Nectar,  843,  858. 

Nectary,  843,  844;    extrafloral,  844,  858. 
Nematus,  gall,  782. 
Neottia,  mycorhiza,  796. 
Nepenthes,  618. 
Nephrolepis,  653. 
Nereocystis,  519. 
Nerium,  leaf  section,  567. 
Nettle,  stinging  hairs,  577. 
Nicotiana,  548. 
Nightshade,  pollen,  831 . 
Nine-bark,  exfoliation,  709. 
Nipa,  stoma,  558. 
Nitrification,  789. 
Nitrogen  fixation,  789,  797. 
Nitrogenous  food,"  914. 
Node,  489,  501,  660. 
Non-nitrogenous  food,  913. 


Normal  plants,  892,  894. 

Nucellus,  905. 

Nut,  919,  925. 

Nutrition  and  leaf  form,  594,  605. 

Nutritive  hair,  782;  layer,  782,  783;  root, 

497,  513- 

Nyctitropic  movements,  579. 
Nymphaea,  leaf  section,  561. 
Nyssa,  508. 

Oak,  acorn,  925;  cork,  705;  gall,  7^2  ; 
habit,  649;  leaf  variation,  602. 

Obligate  dwarfs,  734;  epiphytes,  660; 
hydrophytes,  951;  mesophytes,  951; 
parasites,  762  ;  saprophytes,  762  ;  xero- 
phytes,  951. 

Oblique  palisades,  535,  536. 

Ocelli,  531. 

Odor,  847. 

Oenothera,  capsule,  020;   rosette,  714. 

Offset,  672,  837. 

Offspring,  805. 

Oil,  522,  623,  624,  723;   gland,  624. 

Oleander,  leaf  section,  567. 

Olive,  sclereid,  639. 

Onion,  inflorescence,  874 ;   seedling,  935. 

Oogonium,  817. 

Oospore,  817,  826. 

Opening  of  flowers,  872. 

Operculum,  814. 

Opuntia,  725. 

Orange,  oil  gland,  624. 

Orchids,  mycorhiza,  793,  795,  796;  polli- 
nation, 860;  roots,  5/0,  5/z,  5/2. 

Organization  characters,  951. 

Orobanche,  770. 

Orthostichy,  549,  595,  642. 

Orthotropic  organs,  748. 

Orthotropous  ovules,  905. 

Osmanthus,  sclereid,  639. 

Osmorhiza,  fruit,  024. 

Osmotic  pressure,  493,  627. 

Osmunda,  550. 

Ovary,  826,  828,  854. 

Ovule,  826,  905. 

Own  pollen,  855. 

Oxalis,  leaves,  570;  pollen,  831. 

Paeonia,  flower,  826. 

Palisade  cells,  530,  532,  533,  534,  535,  536, 
538,  539,  55i,  56i,  567,  574,  630,  631, 
639,  642,  724,  781;  arm,  532;  oblique, 
535,  536- 

Palmate  leaves,  676,  858. 

Palmetto,  habit,  645,  653. 

Panax,  corm,  676. 

Panicle,  828,  £35. 

Pappus,  557,  921. 

Paramoecium,  821. 


j 


INDEX 


Parasite;  parasitism,  752,  761,  762,  763, 
764,  766,  768,  760,  770,  771,  775,  780, 
786 ;  alternative,  787  ;  conduction,  688 ; 
facultative,  762  ;  obligate,  762  ;  partial, 

761,  771;    reciprocal,  752,  786;    water, 

762,  771. 

Parenchyma,  conductive,  682;  sheath,  683. 
Parthenocarpy,  917. 
Parthenogenesis,  823  ;    artificial,  882. 
Partial  parasites,   761,  771;    saprophytes, 

754- 

Passage  cells,  683. 

Passiflora ;    passion  vine,  nectaries,  858. 
Pathogenic  bacteria,  762. 
Pea,    leaves,    640;     root    tubercles,    788; 

stipules,  641. 
Peat  bogs,  942. 
Pedicel,  833,  874. 
Peduncle,  874,  921. 
Peg  rhizoids,  516. 
Pelargonium,  glandular  hairs,  623. 
PeHionia,  cystolith,  626. 
Pendulous  organs,  657,  703. 
Pentarch  bundles,  683. 
Penthorum,  rhizomes,  670. 
Peony,  flower,  826. 
Peperomia,  leaf  section,  632. 
Peppergrass,  leaf  section,  530;  roots,  408; 

rosettes,  582. 

Perennial,  501 ;   herbs,  713,  716. 
Perianth,  825,  826,  827,  833,  871,  902,  903. 
Pericambium,  683. 
Pericarp,  906,  915. 
Periclinal  chimera,  780. 
Pericycle,  683,  696. 
Periderm,  705,  706. 
Peridium,  812. 

Perigynous  flowers,  828,  854. 
Period,  acceleration,  690;    maturity,  690; 

retardation,  690. 
Periodicity,  735,  736,  737,  881,  885,  889, 

890. 
Peripheral  chlorenchyma,  630,  631 ;   water 

tissue,  533,  632,  635. 
Perisperm,  905,  906. 
Peristome,  814. 
Peronospora,  haustoria,  763. 
Persimmon,  endosperm,  913. 
Petal,  825,  826,  853. 
Petalization ;   petalody,  901,  902. 
Petiole,  501,  521,  540,  541,  542,  579,  640, 

641,  728,  833. 

Petunia,  flower,  859;  pollination,  841. 
Phaeophyll,  522. 
Phagocytosis,  798. 
Phaseolus,  regeneration,  748;  seed  section, 

905;   twining  stem,  651 . 
Phelloderm,  705. 
Phellogen,  553,  661,  705. 


Phenology,  887. 

Philadelphus,  flowers,  825. 

Philodendron,  roots,  512,  513. 

Phlegethonius,  841. 

Phloem,  530,  630,  682,  683,  684;  second- 
ary, 684 ;  sheath,  683. 

Phloeoterma,  683. 

Phoenix,  seedling,  935. 

Phoradendron,  777. 

Phosphorescence,  760. 

Photeolic  movements,  579. 

Phototropism,  539,  540,  542,  544,  546,  547. 

Phryma,  flowering  shoot,  875. 

Phylloclade,  666. 

Phyllode,  590,  640,  937. 

Phyllotaxy,  510,  542,  543,  549,  595,  641, 
642,  647,  714. 

Physcia,  800. 

Physiographic  ecology,  485. 

Physiological  ecology,  485  ;   species,  765. 

Physocarpus,  exfoliation,  709. 

Picea,  648. 

Pigments,  522,  528. 

Pigweed,  winged,  922. 

Pilea,  stoma,  559. 

Pine,  habit,  584;  leaf  section,  724;  second- 
ary wood,  690.  '+ 

Pinguicula,  618. 

Pinna,  754. 

Pinnule,  636. 

Pinus,  habit,  584;  leaf  section,  724; 
secondary  wood,  690. 

Pistil,  825,  826,  851,  853. 

Pistillate  flowers,  833,  834,  837,  86 1. 

Pisum,  root  tubercle,  788. 

Pit,  679,  680,  68 1 ;  bordered,  680,  690; 
stomatal,  557,  558,  567,  724. 

Pitcher  plant,  618. 

Pith,  680,  685,  769. 

Pitted  vessel,  68 i. 

Plagiotropic  organs,  748. 

Plane  rhizoids,  516;  vernation,  937. 

Plank  roots,  509. 

Plantago;   plantain,  flowers,  835. 

Plant  associations,  939,  940,  942,  943,  944, 
945- 

Planting  of  seeds,  928. 

Plastid,  522  (see  also  Chloroplast). 

Platanus,  bud  protection,  640. 

Plate,  sieve,  681,  682. 

Plicate  vernation,  937. 

Plowrightia,  gall,  784. 

Plumule,  905,  906,  931,  935. 

Plurivore,  765. 

Poa,  leaf  section,  537;  stoma,  556. 

Pocket,  root,  510. 

Pod,  865,  919,  920. 

Poinsettia,  nectary,  844. 

Polarity,  749,  750;  reversal,  750,  751. 


INDEX 


Pollen,  826,  830,  831,  834,  836,  839,  842, 
850,  852;  foreign,  855;  impotent,  854; 
own,  855  ;  prepotent,  854,  855 ;  sac,  £30; 
tube,  826,  830,  832. 

Pollination,  829,  845  ;  bud,  866 ;  close,  829, 
852,  863,  866,  869;  contact,  862;  cross, 
829,  836,  852,  854,  859,  866,  867  ;  gravity 
863 ;  insect,  838,  839,  840,  841,  842,  847, 
850,857;  self,  829,  863 ;  water,  837,  838 ; 
wind,  833,  834,  835,  837,  838. 

Pollinium,  830,  851. 

Poly  arch  bundles,  683. 

Polycarpic  plants,  886. 

Polyembryony,  820. 

Polygala,  865. 

Polygonatum,  rhizome,  671. 

Polygonum.  leaf  variation,  575, 

Polypetalous  flowers,  825,  826. 

Polystichum,  sporangia,  815. 

Polytrichum,  habit,  613;  vascular  bundles, 
686. 

Pond  associations,  540,  940. 

Pondweed,  leaf  section,  537. 

Poplar;  Populus,  absciss  layer,  583;  leaf 
section,  574;  twigs,  736 . 

Porcupine  grass,  fruit,  929. 

Pore,  612,  68 1 ;   air,  557. 

Portulaca,  leaf  section,  533. 

Potamogeton,  leaf  section,  537. 

Potato,  starch,  912;  tuber,  674,  720;  varia- 
tion, 727. 

Predisposition,  768. 

Prepotent  pollen,  854,  855. 

Pressure,  osmotic,  493,  627 ;  turgor,  566, 
621. 

Prickle,  742,  783,  932. 

Prickly  pear,  725. 

Primary  conductive  tissue,  680,  682,  683, 
686 ;  root,  491,  496,  498,  500. 

Primrose,  flowers,  854;  hydathode,  621; 
evening,  capsules,  920;  rosettes,  714. 

Primula,  flowers,  854;   hydathode,  621. 

Privet,  lenticel,  661. 

Proboscis,  841. 

Procambium,  684. 

Profile  position,  546,  547,  578. 

Progeotropism,  499. 

Progressive  variability,  759. 

Prohydrotropism,  499. 

Pronuba,  864. 

Propagation,  502,  503,  505,  805 ;  root,  505. 

Propagule,  805. 

Prop  root,  514,  515. 

Propulsion,  919. 

Prosenchyma,  679,  686,  696. 

Proserpinaca,  leaf  section,  534;  leaf  varia- 
tion, 592,  593,  595. 

Prostrate  plants,  647. 

Protandry,  837,  840,  853. 


Protection,  bud,  640;  flower,  857,  869,  870, 

£77,  872,  873;  fruit,  874;  leaf,  565,  566, 

567,  572,  587,  588;   seed,  906,  907,  908; 

stem,  704. 
Protective  hairs,  567,  572,  573,  574,  575, 

5?6,  577,   578,  623;    sheath,  617,  683; 

tissue,  704,  705,  709. 
Protein,  521,  912,  914,  915;    conduction, 

694;  grains,  914,  915;  synthesis,  528. 
Proteinoplast,  721. 
Proteolytic  enzym,  616. 
Protogyny,  835,  836,  853. 
Protonema,  807. 
Prunus,  gall,  784;   shoot,  643. 
Psedera,  leaf  variation,  605;    leaves,  544; 

pendulous  stems,  657 ;    tendrils,  652. 
Pseudoepiphyte,  659. 
Pseudomixis,  821. 
Pseudotsuga,  735. 
Ptelea,  crystals,  625;   fruit,  921 ;   vascular 

bundle,  639. 
Pteridophytes,  asexual  reproduction,  814, 

815;    reproductive    variation,    884    (see 

also  Ferns). 
Puccinia,  #73. 
Pulvinus,  579. 
Purslane,  leaf  section,  533. 
Pycnanthemum,  stem  section,  702. 
Pyrenoid,  522,  915. 
Pyrus,  spines,  739. 

Quercus,  acorn,  925;  cork,  705;  galls,  782; 
habit,  649;  leaf  variation,  602. 

Raceme,  828,  829,  833,  865. 

Radial  bundles,  683;    migration,  669,  672; 

symmetry,  489,  629,  630. 
Radicle,  906,  935. 
Ranunculus,  leaf  variation,  590. 
Raoulia,  73$. 
Raphe,  906. 
Raphides,  625. 
Rattan  palm,  656. 

Ray  flower,  846;   medullary,  684,  685. 
Reaction,   487,   951;    advantageous,   487; 

hydrophyte,     951;      mesophyte,     951; 

xerophyte,  951. 
Recapitulation,  596. 
Receptacle,  826,  827,  828,  921. 
Reciprocal  parasitism,  752,  786. 
Regeneration,   733,   748;    leaf,   636,   637; 

root,  503. 

Regulation,  487,  951. 
Reindeer  lichen,  611. 
Rejuvenescence,  597,  601. 
Releasing  factors,  952. 
Repetition,  597. 
Reproduction,  505,  636,  637,  667,  805,  806, 

807,  808,  809,  810,  811,  812,  813,  815, 


INDEX 


816,  817,  819 ;  asexual,  809,  810,  811, 812, 
813,  814,  815;  leaf,  636,  637 ;  root,  505; 
sexual,  816,  817,  819;  stem,  667,  668, 
669,  670,  671,  672,  673,  674,  675,  676, 
677,  678;  vegetative,  505,  509,  516,  591, 
636,  637,  667,  805,  806,  807,  808,  809, 
(see  also  Reproduction,  stem). 

Reproductive  organs,  488,  666,  667,  809  ff . ; 
periods,  885,  890;  variation,  878,  884, 
885,  890,  803. 

Reserve  cellulose,  013;    food,  487,  718. 

Resin,  571,  622,  624,  723. 

Respiration,  563. 

Response,  487 ;  adaptive,  947,  949. 

Resting  buds,  936,  938. 

Restitution,  748. 

Resurrection  plant,  581. 

Retardation,  period  of,  690. 

Reticulated  vessel,  680. 

Reversal  of  polarity,  750,  751. 

Reversibility  of  stage,  893. 

Revolute  leaves,  578;   vernation,  937. 

Rhipsalis,  chloroplast,  522. 

Rhizine,  520. 

Rhizobium,  787. 

Rhizoid,  516,  517,  518,  519,  520,  750; 
aerial,  518;  peg,  516;  plane,  516. 

Rhizome,  667,  668,  66g,  670,  671,  674,  744; 
climber,  653,  654;  colony,  671;  section, 

703- 

Rhizophora,  roots,  515;  vivipary,  930,  931. 

Rhizophore,  512,  516,  608. 

Rhodites,  gall,  783. 

Rhythm,  737. 

Ribes,  leaf  veins,  639. 

Riccia,  rhizoids,  516,  517. 

Ricinus,  endosperm,  914. 

Ridge,  stomatal,  555,  556,  558,  562,  574. 

Ring,  annual,  685,  689,  690,  691 ;  cam- 
bium, 684;  fairy,  807;  growth,  685,  689, 
912. 

Rock  plants,  945. 

Role,  487- 

Root,  488,  489,  491,  492,  494,  496,  497,  498, 
499,  500,  501,  502,  503,  504,  505,  506, 
509,  510,  511,  5/2,  513,  514,  515,  520; 
absorption,  491,  493,  510,  511 ;  adventi- 
tious, 501,  502,  503,  510,  512,  514,  637, 
668,  671,  673,  675,  730,  744;  air,  510, 
511,  5/2,  513,  514,  515;  anchor,  497,  498, 
5/2,  513;  cap,  497;  chlorophyll,  511, 
512;  climbing,  5/2,  513,  653;  contrac- 
tion, 504,  505;  correlation,  509;  dura- 
tion, 501 ;  excretion,  493  ;  fleshy,  500; 
food  accumulation,  500;  fungi,  791,  792, 
793,  794,  796;  hairs,  491,  492,  493,  494, 

495,  496,  512,  513;   nutritive,  497,  513; 
plank,  509;   pocket,  5/0;   primary,  491, 

496,  498,   500;    prohydrotropism,   499; 


prop,  5/4,  5/5;  propagation,  502,  503, 
505;  regeneration,  503 ;  secondary,  496, 
498;  soil,  491,  492,  496,  497,  498,  500, 
501 ;  tap,  496,  677;  tubercles,  787,  788, 
790;  variation,  505,  506,  507,  508,  513; 
water,  509,  510,  610. 

Root-forming  substances,  750. 

Rootstock,  667,  668,  669,  670,  671,  674. 

Rosa;    rose,  galls,  783;   prickles,  742. 

Rosette,  582,  714,  726;  tree,  645. 

Rosin- weed,  pollen,  830. 

Rubber  plant,  roots,  502;   stoma,  55$. 

Ruderal,  946. 

Runner,  672,  673. 

Run-off,  636. 

Rush,  diaphragms,  552;  mechanical  cylin- 
der, 699;  rhizome,  670,  674;  rhizome 
section,  703. 

Russian  thistle,  leaf  section,  630. 

Rust,  wheat,  813. 

Sabal,  645,  653. 

Sac,  embryo,  826;  latex,  720;  pollen, 
830. 

Saccharose,  914. 

Sagittaria,  leaf  variation,  590. 

Salicin,  724. 

Salicornia,  palisades,  536. 

Salix,  epidermis,  570  ,•  habit,  584;  polarity, 
749;  roots,  503. 

Salsola,  leaf  section,  630. 

Salt  gland,  613,  621;  marsh  plants,  486, 
537,  632,  942. 

Salts,  523,  565. 

Salvia,  pollination,  840. 

Salvinia,  609. 

Samara,  921. 

Sambucus,  lenticel,  66 r. 

Sand-binder,  497,  499. 

Sand  plants,  729,  730,  945;  reed,  leaf  sec- 
tion, 581. 

Sansevieria,  leaf  cutting,  637. 

Saprolegnia,  sexuality,  883. 

Saprophyte;  saprophytism,  752,  754,  755, 
750,  757,  758,  759,  760;  distribution, 
760;  facultative,  762;  obligate,  762; 
partial,  754 ;  progressive  variability,  759 ; 
symbiotic,  757,  798. 

Sap  wood,  685. 

Sarracenia,  leaf,  618. 

Satureja,  673. 

Saxif raga ;  Saxifrage,  flowers,  853. 

Scabrous  hairs,  573. 

Scalariform  vessel,  679,  680. 

Scale,  835,  925;  bud,  642,  643,  644,  646, 
833;  hair,  573,  615;  leaf,  572,  641,  642 
643,  644,  669,  670,  675,  733,  736,  768, 
834;  ventral,  516,  517. 

Scape,  599,  83?- 


INDEX 


Scar,  leaf,  583,  644,  657,  660,  708,  736; 
stem,  671. 

Schistostega,  phyllotaxy,  549. 

Schizocarp,  919,  924- 

Scion,  776,  777,  778. 

Scirpus,  66$. 

Sclereid,  630,  697,  698. 

Sclerenchyma,  697,  698,  724,  782. 

Sclerophyll,  570,  585,  639,  709. 

Sclerosis,  698. 

Sclerotic  cells,  513. 

Sclerotium,  808. 

Scouring  rush,  733. 

Scrophularia,  flowers,  853. 

Scutellaria,  rhizome,  744. 

Scutellum,  786,  934. 

Secondary  conductive  tissue,  684,  685,  689 ; 
phloem,  684;  roots,  496,  408;  wood, 
684,  685,  689,  690;  xylem,  684,  685. 

Secretion;  secretory  cells,  617,  620,  622, 
623,  624,  625,  626,  724,  843,  844,  858. 

Seed,  904,  905,  906,  907,  908,  911,  9/5,  916, 
917,  918,  919,  Q20,  928;  coat,  905,  909, 
915,  932;  death,  910;  dispersal,  919; 
food  accumulation,  911;  germination, 
930,  93 ! ;  protection,  906,  907,  908 ;  sec- 
tion, 005,  913,  915;  vitality,  908,  923. 

Seedless  plants,  reproduction,  805  (see  also 
Algae,  Bacteria,  Fungi,  Lichens,  Liver- 
worts, Mosses,  Pteridophytes). 

Seedling,  491,  4Q2,  4Q4,  499,  506,  934,  935, 
936. 

Seed  plants,  leaf  reproduction,  637;  para- 
sitism, 769,  775  ;  reproductive  variation, 
885;  saprophytism,  757  (see  also 
Dicotyls,  Monocotyls,  etc.). 

Selaginella,  chloroplasts,  521 ;  conductive 
bundle,  68 3;  habit,  608;  rhizophore, 
512,  516. 

Selection,  natural,  743,  876,  952. 

Self-parasitism,  786. 

Self-pollination,  829,  863. 

Self-pruning,  726. 

Sempervivum,  reproductive  variation,  900 ; 
stem  variation,  726. 

Senecio,  habit,  541;  hairs,  573;  leaf  sec- 
tion, 631;  leaves,  629. 

Sensitive  plant,  leaves,  579. 

Sepal,  825,  826,  859. 

Sepalody,  902. 

Separation  layer,  709. 

Seta,  667. 

Sex  determination,  895. 

Sexuality,  fungi,  883;   origin,  881. 

Sexual  reproduction,  816,  817,  819. 

Shade  plants,  chlorenchyma,  530,  537,  555. 

Sheath,  bundle,  530,  581,  639,  683 ;  paren- 
chyma, 683 ;  phloem,  683 ;  protective, 
617,  683 ;  starch,  683. 


Shield-budding,  777. 

Shoot-forming  substances,  750. 

Shrubs,  646,  650,  717. 

Sieve  plate,  681,  682 ;   tube,  681,  694. 

Signal  theory,  847. 

Silene,  pollination,  859. 

Silky  hairs,  572. 

Silphium,  pollen,  830. 

Simple  leaves,  605. 

Single  flowers,  902. 

Sinistrorse  twiners,  651,  656,  768. 

Sinus,  leaf,  637,  639. 

Sleep  movements,  579. 

Slime  gland,  561,  623. 

Slit,  stomatal,  555,  556,  562,  571. 

Small  leaves,  550. 

Smilacina,  rhizome,  668. 

Snow  protection,  588. 

Soil  exhaustion,  493 ;   roots,  491,  492,  496, 

497,  498,  $00,  soi ;  toxins,  494. 
Solanin,  626. 
Solanum,  pollen,  831 ;  stamen,  830;  starch, 

912;   tuber,  674,  720;  variation,  727. 
Solidago,  galls,  784. 
Solomon's  seal,  rhizome,  671. 
Soredia,  801. 
Sorus,  636,  815. 
Spadix,  860. 
Sparganium,  545. 
Spartina,  rhizomes,  669. 
Spasmodic  habit,  737,  887. 
Spathe,  #37,  838,  860,  874,  903. 
Special  creation,  947. 
Specialization,  765. 
Sperm,  816,  817. 

Sphagnum,  cells,  612;  habit,  666. 
Spherite,  914. 
Spherocrystal,  913,  914. 
Spike,  828,  835,  837. 
Spikelet,  835. 
Spikenard,  wild,  546. 
Spine ;  spinescence,  604,  725,  739,  740,  741, 

742,  743- 

Spiral  thickening,  612;  vessel,  680,  681. 
Spiranthes,  mycorhiza,  793. 
Spirodela,  $09. 
Sponge  tissue,  530,  533,  534,  555,  536,  561, 

567,  574,  642,  781. 
Sporangium,  755,  809,  811,  815. 
Spore,  asexual,  809,  810,  8n,  813,  814,  815. 
Sporeling,  814. 
Sporocarp,  816. 
Sporophore,  755. 
Sporophyte,  666,  814. 
Springwood,  689,  690. 
Spruce,  648;  Douglas,  735. 
Spur,  840,  843,  844. 
Stamen,  825,  826,  828,  829,  830,  833,  835, 

846,  851,  853,  854,  871,  874. 


INDEX 


Staminate  flowers,  833,  834,  837,  038. 

Starch,  522,  528,  687,  912,  915;  sheath,  683. 

Stellate  cells,  561 ;   hairs,  572. 

Stem,  488,  489,  645 ;  aeration,  660 ;  aerial, 
7iQ,  725;  air  chambers,  557,  553,  703, 
718;  aquatic,  719;  asymmetry,  734,735," 
bark,  704,  708,  700;  branching,  646,  647, 
640;  chlorophyll,  660,  66 i ;  climbing, 
651,  652,  6 S3,  654,  655,  656;  conductive 
tissue,  678,  670,  680,  681,  682,  683,  684, 
685,  686,  687,  688,  689,  600,  692,  693; 
cork,  705,  706,  707 ;  correlation,  747 ; 
creeping,  501,  672,  673;  display,  542, 
543,  546,  548,  645,  646,  648,  649,  650, 
666,  667;  duration,  717;  dwarfing,  730, 
737,  732,  733,  734 ;  elongation,  646,  648, 
725,  726,  727,  728,  72Q,  736;  epidermis, 
704;  erectness,  647 ;  excretion,  722,  723, 
724;  fall,  713;  food  accumulation,  719; 
habit,  584,  585,  588,  709,  711,  715;  len- 
ticels,  660,  661,  662,  663 ;  mechanical 
tissue,  696,  697,  699,  700,  701,  702,  703; 
multicipital,  504,  676,  677;  pendulous, 
657;  periodicity,  735,  736,  737  ;  polarity, 
749;  protection,  704 ;  regeneration,  74^; 
reproduction,  667,  668,  669,  670,  671, 
672,  673,  674,  675,  676,  677,  678;  running, 
672,  673;  section,  551,  685,  701,  702,  703, 
769;  spines,  739,  740,  741,  742,  743; 
subterranean,  667,  668,  669,  670,  671, 
674,  675,  676,  719,  744,  74<5,  865;  syn- 
thesis, 660,  665 ;  twisting,  543,  647,  648, 
673;  variation,  575,  725,  726,  727,  731, 
732,  733,  735,  736,  739,  741,  742 ;  water 
accumulation,  718;  waste  accumulation, 
723- 

Stereid,  561,  696,  697,  700. 

Stereome,  639,  682,  698,  705. 

Stigeoclonium,  591. 

Stigma,  825,  826,  831,  833,  834,  835,  840, 
846,  851,  852,  853,  854,  857,  871. 

Stinging  hairs,  577. 

Stipa,  fruit,  929. 

Stipe,  667,  812. 

Stipule,  502,  575,  579,  640,  641,  643,  783. 

Stock,  776,  777,  778. 

Stolon,  672,  #37. 

Stomata,  530,  531,  534,  535,  536,  555,  556, 
55#,  550,  560,  561,  562,  563,  564,  566, 
567,  577,  574,  581,  620,  629,  66 1,  724; 
water,  620,  621, 

Stone  cell,  697,  698;  fruit,  919. 

Stonecrop,  ditch,  670. 

Storage,  487 ;   tracheid,  536,  631. 

Strand,  conductive,  680,  682,  683,  684,  686, 
687,  688,  695. 

Strangling  fig,  5/4,  515. 

Strength,  columnar,  704 ;  compression, 
704;  flexile,  702;  tensile,  700,  703. 


Strigula,  habit,  659. 

Strobilus,  733,  814,  825. 

Stroma,  521,  7^4. 

Structure,  488. 

Struggle  for  existence,  487. 

Strychnin,  626. 

Style,  £25,  826,  829,  831,  840,  846,  853,  854, 

857: 

Suberin,  706. 
Submersed  stems,  703. 
Subsidiary  cells,  555,  556,  558,  724. 
Substitute  hydathodes,  622. 
Subterranean  flowers,  864,  865  ;  leaves,  642  ; 

stems,  667,  668,  669,  670,  671,  674,  675, 

676,  719,  744,  74<5,  865. 
Succession,  940. 
Succulence,  533,  627,  628,  629,  630,   631, 

632,  633,  634,  635,  747. 
Sugar,  528,  914;    conduction,  694. 
Sundew,  glandular  hairs,  616,  617. 
Sun    plants,    chlorenchyma,    535;    leaves, 

547- 

Surplus  food,  718. 
Swamp  plants,  506,  507,  508,  509,  941  (see 

also  Bog  plants). 
Swarm  spores,  810. 
Sweet  cicely,  fruit,  924. 
Sweet  pea,  leaves,  640. 
Switch  plants,  664,  666. 
Sycamore,  bud  protection,  640. 
Symbiont,  752. 
Symbiosis,  752. 

Symbiotic  saprophytism,  757,  798. 
Symmetry,    489;     dorsiventral,    489,    629, 

630;   radial,  489,  629,  630. 
Sympetalous  flowers,  829,  846,  853,  857, 

85Q- 

Synconium,  860,  861. 
Syncyte,  679. 
Synecology,  485. 
Syngenesious  anthers,  #57. 
Synthesis,  carbohydrate,  525,  526,  527,  563, 

588,  660,  665 ;  protein,  528. 
Syringa,  flowers,  #25. 
Systrophe,  524. 

Tactile  spot,  653. 

Tannin,  724. 

Tape  grass,  pollination,  #37. 

Tap  root,  496,  677. 

Taraxacum,  flower,  £57 ;  flower  movements, 

#72;   roots,  677;  variation,  599. 
Taxodium,  508. 
Taxonomic  variation,  486. 
Teeth,  leaf,  621. 
Telentospore,  764,  813,  814. 
Teleology,  947. 
Temperature,  523,  527,  587. 
Tendril,  640,  641,  652,  699. 


INDEX 


Tensile  strength ;   tension,  699,  700,  703. 

Terminal  bud,  646,  736. 

Termo,  486. 

Testa,  005,  907,  909,  915,  932. 

Tetrad,  830. 

Tetrarch  bundles,  683. 

Tetraspore,  811. 

Thallophytes,  absorption,  614;   conductive 

tissues,  685 ;    form  variations,   591    (see 

also  Algae,  Bacteria,  Fungi,  Lichens). 
Thallus,  488,  516,  557,  650,  678,  800. 
Thistle,  Russian,  leaf  section,  630. 
Thorn,  739,  74*,  754- 
Thorn  climber,  654. 
Thread,  infection,  787,  788. 
Thuja,  leaf  variation,  601. 
Tillandsia,  absorptive  scales,  615;    habit, 

614,  658. 
Tomentum,  578. 
Torsion,  dehiscence  by,  QSO. 
Torus,  680. 
Toxins,  soil,  494. 
Trachea,  679,  680,  681,  692. 
Tracheid,  617,  621,  631,  639,  679,  680,  692; 

storage,  536,  631. 
Transfusion  cells,  511. 
Transpiration,    536,    564,    565,    566;     leaf 

form  and,  595  ;  protection  from,  565,  577, 

578,  588. 

Transplanting  and  root  form,  498,  501. 
Transverse   phototropism,    539,   540,   542, 

544,  546. 
Traumatism,  896. 
Trees,  645,  648,  649,  650,  709,  710,  717; 

deciduous,  583.  584,  649,  710;  evergreen, 

709,  710;   leafless,  710,  711;  origin,  737; 

rosette,    645;     stomata,    560;     tropical, 

709;  variation,  725. 
Triarch  bundles,  683. 
Trichome  hydathode,  622. 
Triticum,  grain  section,  915;    root  hairs, 

492. 
Tropaeolum,    flower,   843;    phototropism, 

540;    plastids,  522;     pollen,  831;    sto- 
mata, 620. 

Tropical  forest,  656,  946;   trees,  709. 
Tsuga,  dwarfed,  732. 
Tube,   bacterial,   787,   788;    corolla,   859; 

latex,  721;   pollen,  826,  830,  832;  sieve, 

681,  694. 
Tuber;    tuberization,  671,  674,  675,  720, 

744,  745,  746. 

Tubercle,  root,  787,  788,  790. 
Tumbleweed,  922. 
Tupelo,  508. 

Turgor  pressure,  566,  621. 
Twiners,  651,  656,  768. 
Twisting,  stem,  543,  647,  648,  673. 
Tyloses,  559,  695. 


Ulex,  variation,  741. 

Ulmus,  655. 

Ulothrix,  reproductive  organs,  817. 

Umbel,  676,  828,  874,  903. 

Umbelliferae,  geitonogamy,  862. 

Underground  stem,  667,  668,  669,  670,  671, 

674,  675,  676,  719,  744,  746,  865. 
Undergrowth,  forest,  545,  546,  550. 
Univore,  765. 
Uredospore,  764,  813,  814. 
Urtica,  stinging  hairs,  577. 
Use,  948. 
Usnea,  801. 
Utricularia,    bladders,    618,    619;    winter 

buds,  678, 

Vaccinium,  stamen,  830. 

Vallisneria,  pollination,  837. 

Valves,  814,  920. 

Variation,  486 :  air  spaces,  553 ;  bud,  904 ; 
chlorenchyma,  533,  534,  535,  538;  con- 
ductive tissues,  686,  689,  690,  692 ;  cork, 
706 ;  ecological,  486 ;  fortuitous,  950 ; 
fruit,  916,  917 ;  hairs,  573,  574,  575,  576; 
leaf  form,  589,  590,  592,  593,  594,  505, 
597,  598,  599,  600,  601,  602,  603,  604, 
605,  606,  607,  608;  mechanical  tissue, 
699;  parasitic  fungi,  765;  reproductive 
organs,  878,  884,  885,  893,  898,  900,  902, 
903;  rhizoids,  516,  518;  roots,  505,  506, 
507,  508,  513;  root  hairs,  494,  495,  496; 
saprophytic  fungi,  759;  seeds,  917,  918; 
stems,  725,  726,  727,  729,  730,  731,  732, 
733,  735,  736,  739,  741,  744;  stomata, 
556,  557,  559,  561;  taxonomic,  486. 

Variegation,  523. 

Vascular  bundles,  530,  531,  533,  534,  53$, 
581,  617,  621,  680,  682,  683,  684,  686, 
687,  688,  695,  701,  702,  703;  elements, 
679,  680,  68 1 ;  plants,  609,  613  (see  also 
Dicotyls,  Ferns,  Monocotyls,  Pterido- 
phytes,  Seed  plants) ;  tissue,  513,  551, 
630,  638,  639,  678,  679,  680,  681,  682, 
683,  684,  685,  686,  687,  688,  689,  690, 
692,  693,  769. 

Vaucheria,  reproductive  variation,  880. 

Vegetative  periods,  885,  890;  reproduc- 
tion, 505,  509,  516,  591,  636,  637,  667, 
668,  669,  670,  671,  672,  673,  674,  675, 
676,  677,  678,  805,  806,  807,  808,  809. 

Veins,  521,  638,  639. 

Velamen,  511. 

Venation,  638. 

Ventral  scale,  516,  517 ;  wall,  555,  556,  562. 

Verbascum,  hairs,  572. 

Verbena,  hairs,  623. 

Vernation,  937. 

Veronica,  reproductive  variation,  893. 

Vertical  leaves,  546,  547,  549,  578. 


INDEX 


Vervain,  hairs,  623. 

Vessels,  679,  680,  68 1;   annular,  680,  681; 

latex,  721 ;  pitted,  68 i ;  reticulated,  680 ; 

scalariform,  67 Q,  680;  spiral,  680,  68 1. 
Vestibule,  555,  556,  574- 
Viburnum,  leaf  gall,  781;    shoots,  644. 
Vicia,  root  hairs,  491. 
Viola ;  violet,  leaf  section,  530. 
Virginia  creeper,  pendulous  stem,  657. 
Vitalism,  948. 
Vitality,  seed,  908,  923. 
Vitis,  gall,  576;  habit,  655. 
Vivipary,  930,  931. 

Wall,  dorsal,  555,  556,  55$,  562;  ventral, 
555,  556,  562. 

Wandering  Jew,  crystals,  625. 

Wasp,  fig,  86 1. 

Waste,  accumulation  of,  623,  624,  625,  626, 
718,  723,  724,  725. 

Water,  565  ;  absorption,  491,  493,  517,  518, 
565,  608,  614,  615;  accumulation,  627, 
628,  629,  630,  631,  632,  633,  718;  calyx, 
845  ;  dispersal,  922,  926;  exudation,  620, 
622  ;  hyacinth,  510;  leaf  form  and,  599; 
leaves,  590,  502,  503,  595,  837;  lenticels, 
663;  lily,  540,  561 ;  net,8io;  parasites, 
762,  771 ;  plants,  dog,  676,  678  (see  also 
Hydrophytes);  pollination,  837;  reten- 
tion, 627;  roots,  509,  5/0,  610;  roots 
and,  499,  502,  503,  504,  505,  506;  shield, 
623;  stem  elongation  and,  727  ;  stomata, 
620,  621;  synthesis  and,  527;  tissue, 
533,  629,  630,  631,  632;  vascular  de- 
velopment and,  686. 

Waterweed,  vascular  bundle,  687. 

Wax,  568,  570;  plant,  697. 

Wheat,  grain  section,  9/5 ,-  root  hairs,  492 ; 
rust,  813. 

Willow,  epidermis,  570;  habit,  584; 
polarity,  749;  roots,  503. 

Wind  dispersal,  920,  926;  pollination,  833, 
834,  835,  837,  838. 

Winged  fruits,  927. 


Winter  bud,  555,  572,  646,  678,  736,  936, 
938. 

Witches'  brooms,  783. 

Withered  leaves,  581. 

Wolffia,  678. 

Wood,  autumn,  689,  690 ;  fibers,  697 ; 
secondary,  684,  685,  689,  690;  spring, 
689,  690;  waste  accumulation  in,  725. 

Woolly  hairs,  572,  573,  574. 

Wound  cork,  707. 

Xanthic  colors,  845. 

Xanthium,  fruit,  924;  fruit  section,  932. 

Xanthophyll,  522. 

Xenogamy,  829  (see  also  Cross  pollination). 

Xerophile,  487. 

Xerophytes,  486,  939,  943 ;  absorption, 
613 ;  associations,  943,  944 ;  bog,  486, 
537,  942;  chlorenchyma,  532,  533;  con- 
genital, 951;  cutinization,  568;  dwarfs, 
733  ;  facultative,  951 ;  leaf  sections,  533, 
535,  567,  58i,  598,  629,  630,  631,  639, 
724;  leaves,  570,  578 ;  obligate,  951 ; 
reaction,  951;  roots,  505,  506,  507;  salt 
marsh,  486,  942  ;  stomata,  557,  558,  561 ; 
succulence,  627,  628,  629,  630,  631,  635 ; 
wax  coats,  570. 

Xylem,  530,  630,  680,  682,  683,  684,  685; 
secondary,  684,  685. 

Xylocentric  bundles,  683. 

Yarrow,  flowers,  846. 

Yucca,  flowers,  871;  habit,  588;  pollina- 
tion, 864. 

Zea,   prohydrotropism,   499;    prop  roots, 

514;   roots,  494. 
Zebrina,  crystals,  625. 
Zoogloea,  787. 
Zoospores,  810,  817. 
Zygadenus,  roots,  505. 
Zygomorphy,  841,  843,  846,  853,  859. 
Zygospore,  816. 


GRAY'S    NEW    MANUAL 

OF  BOTANY — SEVENTH 

EDITION 

Thoroughly  revised  and  largely  rewritten  by  BENJAMIN 
LINCOLN  ROBINSON,  Ph.D.,  Asa  Gray  Professor 
of  Systematic  Botany,  and  MERRITT  LYNDON 
FERNALD,  S.B.,  Assistant  Professor  of  Botany,  Harvard 
University,  assisted  by  specialists  in  certain  groups. 


Regular  edition.      Cloth,  926  pages $*-S° 

Tourist's  edition.      Flexible  leather,  926  pages 3.00 


A  MERICAN  botanists,  who  had  been  impatiently  await- 
^/X  ing  the  revision  of  this  indispensable  work,  will  be  de- 
lighted to  know  that  a  seventh,  completely  revised,  and 
copiously  illustrated  edition  has  now  been  issued.  The  re- 
vision has  entailed  years  of  work  by  skilled  specialists.  No 
effort  has  been  spared  to  attain  the  highest  degree  of  clearness, 
terseness,  and  accuracy.  The  plant  families  have  been  re- 
arranged in  a  manner  to  show  the  latest  view  of  their  affin- 
ities, and  hundreds  of  species  have  been  added.  The  synonomy 
is  copious,  and  the  ranges  are  stated  in  considerable  detail. 
^|  The  nomenclature  has  been  brought  into  thorough  accord 
with  the  important  international  rules  recently  established — a 
feature  of  great  significance.  Indeed,  the  Manual  is  the  only 
work  of  its  scope  which  in  the  matter  of  nomenclature  is  free 
from  provincialism  and  rests  upon  a  cosmopolitan  basis  of  in- 
ternational agreement.  Nearly  a  thousand  figures,  specially 
designed  for  this  edition,  have  been  added,  and  scores  of  brief 
and  lucid  keys  have  been  introduced  in  a  manner  which 
greatly  simplifies  the  problem  of  plant  identification.  The 
work  has  been  extended  to  include  Ontario,  Quebec,  and  the 
maritime  provinces  of  Canada. 


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INTRODUCTION    TO   POLITICAL 
SCIENCE 

By  JAMES  WILFORD   GARNER,  Ph.  D.,  Professor  of 
Political  Science,  University  of  Illinois 


THIS  systematic  treatise  on  the  science  of  government 
covers  a  wider  range  of  topics  on  the  nature,  origin, 
organization,  and  functions  of  the  state  than  is  found 
in  any  other  college  textbook  published  in  the  English  lan- 
guage. The  unusually  comprehensive  treatment  of  the  various 
topics  is  based  on  a  wide  reading  of  the  best  literature  on  the 
subject  in  English,  German,  French,  and  Italian,  and  the 
student  has  opportunity  to  profit  by  this  research  work  through 
the  bibliographies  placed  at  the  head  of  each  chapter,  as  well 
as  by  means  of  many  additional  references  in  the  footnotes. 
^[  An  introductory  chapter  is  followed  by  chapters  on  the 
nature  and  essential  elements  of  the  state;  on  the  various 
theories  concerning  the  origin  of  the  state ;  on  the  forms  of 
the  state ;  on  the  forms  of  government,  including  a  discussion 
of  the  elements  of  strength  and  weakness  of  each ;  on  sov- 
ereignty, its  nature,  its  essential  characteristics,  and  its  abiding 
place  in  the  state;  on  the  functions  and  sphere  of  the  state, 
including  the  various  theories  of  state  activity ;  and  on  the 
organization  of  the  state.  In  addition  there  are  chapters  on 
constitutions,  their  nature,  forms,  and  development;  on  the 
distribution  of  the  powers  of  government;  on  the  electorate; 
and  on  citizenship  and  nationality. 

^[  Before  stating  his  own  conclusions  the  author  gives  an  im- 
partial discussion  of  the  more  important  theories  of  the  origin, 
nature,  and  functions  of  the  state,  and  analyzes  and  criticises 
them  in  the  light  of  the  best  scientific  thought  and  practice. 
Thus  the  pupil  becomes  familiar  with  the  history  of  the  science 
as  well  as  with  its  principles  as  recognized'  to-day. 


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THE  FOUNDATIONS  OF  STEREO-CHEMISTRY.  Memoirs  by  Pasteur,  Le  Bel, 
and  Van't  Hoff,  together  with  selections  from  later  memoirs  by 
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RADIATION  AND  ABSORPTION.  Memoirs  by  Prevost,  Balfour  Stewart, 
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Brace.  $1.00. 


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